Fixed #25746 -- Isolated inlined test models registration.

Thanks to Tim for the review.

29 files changed, 283 insertions, 369 deletions

diff --git a/django/contrib/auth/checks.py b/django/contrib/auth/checks.py
index 2d47377c9a..75f9542346 100644
--- a/django/contrib/auth/checks.py
+++ b/django/contrib/auth/checks.py
@@ -6,10 +6,22 @@ from django.conf import settings
 from django.core import checks
 
 
-def check_user_model(**kwargs):
-    errors = []
+def check_user_model(app_configs=None, **kwargs):
+    if app_configs is None:
+        cls = apps.get_model(settings.AUTH_USER_MODEL)
+    else:
+        app_label, model_name = settings.AUTH_USER_MODEL.split('.')
+        for app_config in app_configs:
+            if app_config.label == app_label:
+                cls = app_config.get_model(model_name)
+                break
+        else:
+            # Checks might be run against a set of app configs that don't
+            # include the specified user model. In this case we simply don't
+            # perform the checks defined below.
+            return []
 
-    cls = apps.get_model(settings.AUTH_USER_MODEL)
+    errors = []
 
     # Check that REQUIRED_FIELDS is a list
     if not isinstance(cls.REQUIRED_FIELDS, (list, tuple)):
diff --git a/django/contrib/contenttypes/checks.py b/django/contrib/contenttypes/checks.py
index 1b3df21353..3db58cfdbe 100644
--- a/django/contrib/contenttypes/checks.py
+++ b/django/contrib/contenttypes/checks.py
@@ -1,17 +1,23 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+from itertools import chain
+
 from django.apps import apps
 from django.utils import six
 
 
-def check_generic_foreign_keys(**kwargs):
+def check_generic_foreign_keys(app_configs=None, **kwargs):
     from .fields import GenericForeignKey
 
+    if app_configs is None:
+        models = apps.get_models()
+    else:
+        models = chain.from_iterable(app_config.get_models() for app_config in app_configs)
     errors = []
     fields = (obj
-        for cls in apps.get_models()
-        for obj in six.itervalues(vars(cls))
+        for model in models
+        for obj in six.itervalues(vars(model))
         if isinstance(obj, GenericForeignKey))
     for field in fields:
         errors.extend(field.check())
diff --git a/django/core/checks/model_checks.py b/django/core/checks/model_checks.py
index 3d0d56af58..b312602c6c 100644
--- a/django/core/checks/model_checks.py
+++ b/django/core/checks/model_checks.py
@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 
 import inspect
 import types
+from itertools import chain
 
 from django.apps import apps
 from django.core.checks import Error, Tags, register
@@ -11,21 +12,24 @@ from django.core.checks import Error, Tags, register
 @register(Tags.models)
 def check_all_models(app_configs=None, **kwargs):
     errors = []
-    for model in apps.get_models():
-        if app_configs is None or model._meta.app_config in app_configs:
-            if not inspect.ismethod(model.check):
-                errors.append(
-                    Error(
-                        "The '%s.check()' class method is "
-                        "currently overridden by %r." % (
-                            model.__name__, model.check),
-                        hint=None,
-                        obj=model,
-                        id='models.E020'
-                    )
+    if app_configs is None:
+        models = apps.get_models()
+    else:
+        models = chain.from_iterable(app_config.get_models() for app_config in app_configs)
+    for model in models:
+        if not inspect.ismethod(model.check):
+            errors.append(
+                Error(
+                    "The '%s.check()' class method is "
+                    "currently overridden by %r." % (
+                        model.__name__, model.check),
+                    hint=None,
+                    obj=model,
+                    id='models.E020'
                 )
-            else:
-                errors.extend(model.check(**kwargs))
+            )
+        else:
+            errors.extend(model.check(**kwargs))
     return errors
 
 
diff --git a/tests/absolute_url_overrides/tests.py b/tests/absolute_url_overrides/tests.py
index ab4ad0d005..7187456112 100644
--- a/tests/absolute_url_overrides/tests.py
+++ b/tests/absolute_url_overrides/tests.py
@@ -1,7 +1,9 @@
 from django.db import models
 from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
 
 
+@isolate_apps('absolute_url_overrides')
 class AbsoluteUrlOverrideTests(SimpleTestCase):
 
     def test_get_absolute_url(self):
diff --git a/tests/apps/tests.py b/tests/apps/tests.py
index 9dad3cfb86..198b0a92c4 100644
--- a/tests/apps/tests.py
+++ b/tests/apps/tests.py
@@ -9,7 +9,7 @@ from django.contrib.admin.models import LogEntry
 from django.core.exceptions import AppRegistryNotReady, ImproperlyConfigured
 from django.db import models
 from django.test import SimpleTestCase, override_settings
-from django.test.utils import extend_sys_path
+from django.test.utils import extend_sys_path, isolate_apps
 from django.utils import six
 from django.utils._os import upath
 
@@ -260,7 +260,8 @@ class AppsTests(SimpleTestCase):
         finally:
             apps.apps_ready = True
 
-    def test_lazy_model_operation(self):
+    @isolate_apps('apps', kwarg_name='apps')
+    def test_lazy_model_operation(self, apps):
         """
         Tests apps.lazy_model_operation().
         """
diff --git a/tests/auth_tests/models/__init__.py b/tests/auth_tests/models/__init__.py
index ec6b6099db..3f3bd89793 100644
--- a/tests/auth_tests/models/__init__.py
+++ b/tests/auth_tests/models/__init__.py
@@ -1,14 +1,10 @@
 from .custom_permissions import CustomPermissionsUser
-from .invalid_models import (
-    CustomUserBadRequiredFields, CustomUserNonListRequiredFields,
-    CustomUserNonUniqueUsername,
-)
+from .invalid_models import CustomUserNonUniqueUsername
 from .is_active import IsActiveTestUser1
 from .uuid_pk import UUIDUser
 from .with_foreign_key import CustomUserWithFK, Email
 
 __all__ = (
-    'CustomPermissionsUser', 'CustomUserNonUniqueUsername',
-    'CustomUserNonListRequiredFields', 'CustomUserBadRequiredFields',
-    'CustomUserWithFK', 'Email', 'IsActiveTestUser1', 'UUIDUser',
+    'CustomPermissionsUser', 'CustomUserWithFK', 'Email',
+    'IsActiveTestUser1', 'UUIDUser', 'CustomUserNonUniqueUsername',
 )
diff --git a/tests/auth_tests/models/invalid_models.py b/tests/auth_tests/models/invalid_models.py
index 4e2307ea59..123ed4d865 100644
--- a/tests/auth_tests/models/invalid_models.py
+++ b/tests/auth_tests/models/invalid_models.py
@@ -18,21 +18,3 @@ class CustomUserNonUniqueUsername(AbstractBaseUser):
     REQUIRED_FIELDS = ['email']
 
     objects = UserManager()
-
-
-class CustomUserNonListRequiredFields(AbstractBaseUser):
-    "A user with a non-list REQUIRED_FIELDS"
-    username = models.CharField(max_length=30, unique=True)
-    date_of_birth = models.DateField()
-
-    USERNAME_FIELD = 'username'
-    REQUIRED_FIELDS = 'date_of_birth'
-
-
-class CustomUserBadRequiredFields(AbstractBaseUser):
-    "A user with a USERNAME_FIELD that appears in REQUIRED_FIELDS (invalid)"
-    username = models.CharField(max_length=30, unique=True)
-    date_of_birth = models.DateField()
-
-    USERNAME_FIELD = 'username'
-    REQUIRED_FIELDS = ['username', 'date_of_birth']
diff --git a/tests/auth_tests/test_management.py b/tests/auth_tests/test_management.py
index e933caa027..a941847474 100644
--- a/tests/auth_tests/test_management.py
+++ b/tests/auth_tests/test_management.py
@@ -11,23 +11,24 @@ from django.contrib.auth.management import create_permissions
 from django.contrib.auth.management.commands import (
     changepassword, createsuperuser,
 )
-from django.contrib.auth.models import Group, Permission, User
+from django.contrib.auth.models import (
+    AbstractBaseUser, Group, Permission, User,
+)
 from django.contrib.auth.tests.custom_user import CustomUser
 from django.contrib.contenttypes.models import ContentType
 from django.core import checks, exceptions
 from django.core.management import call_command
 from django.core.management.base import CommandError
+from django.db import models
 from django.test import (
     SimpleTestCase, TestCase, override_settings, override_system_checks,
 )
+from django.test.utils import isolate_apps
 from django.utils import six
 from django.utils.encoding import force_str
 from django.utils.translation import ugettext_lazy as _
 
-from .models import (
-    CustomUserBadRequiredFields, CustomUserNonListRequiredFields,
-    CustomUserNonUniqueUsername, CustomUserWithFK, Email,
-)
+from .models import CustomUserNonUniqueUsername, CustomUserWithFK, Email
 
 
 def mock_inputs(inputs):
@@ -568,9 +569,17 @@ class CreatesuperuserManagementCommandTestCase(TestCase):
 class CustomUserModelValidationTestCase(SimpleTestCase):
     @override_settings(AUTH_USER_MODEL='auth_tests.CustomUserNonListRequiredFields')
     @override_system_checks([check_user_model])
-    def test_required_fields_is_list(self):
-        "REQUIRED_FIELDS should be a list."
-        errors = checks.run_checks()
+    @isolate_apps('auth_tests', kwarg_name='apps')
+    def test_required_fields_is_list(self, apps):
+        """REQUIRED_FIELDS should be a list."""
+        class CustomUserNonListRequiredFields(AbstractBaseUser):
+            username = models.CharField(max_length=30, unique=True)
+            date_of_birth = models.DateField()
+
+            USERNAME_FIELD = 'username'
+            REQUIRED_FIELDS = 'date_of_birth'
+
+        errors = checks.run_checks(app_configs=apps.get_app_configs())
         expected = [
             checks.Error(
                 "'REQUIRED_FIELDS' must be a list or tuple.",
@@ -583,9 +592,17 @@ class CustomUserModelValidationTestCase(SimpleTestCase):
 
     @override_settings(AUTH_USER_MODEL='auth_tests.CustomUserBadRequiredFields')
     @override_system_checks([check_user_model])
-    def test_username_not_in_required_fields(self):
-        "USERNAME_FIELD should not appear in REQUIRED_FIELDS."
-        errors = checks.run_checks()
+    @isolate_apps('auth_tests', kwarg_name='apps')
+    def test_username_not_in_required_fields(self, apps):
+        """USERNAME_FIELD should not appear in REQUIRED_FIELDS."""
+        class CustomUserBadRequiredFields(AbstractBaseUser):
+            username = models.CharField(max_length=30, unique=True)
+            date_of_birth = models.DateField()
+
+            USERNAME_FIELD = 'username'
+            REQUIRED_FIELDS = ['username', 'date_of_birth']
+
+        errors = checks.run_checks(apps.get_app_configs())
         expected = [
             checks.Error(
                 ("The field named as the 'USERNAME_FIELD' for a custom user model "
@@ -600,7 +617,10 @@ class CustomUserModelValidationTestCase(SimpleTestCase):
     @override_settings(AUTH_USER_MODEL='auth_tests.CustomUserNonUniqueUsername')
     @override_system_checks([check_user_model])
     def test_username_non_unique(self):
-        "A non-unique USERNAME_FIELD should raise a model validation error."
+        """
+        A non-unique USERNAME_FIELD should raise an error only if we use the
+        default authentication backend. Otherwise, an warning should be raised.
+        """
         errors = checks.run_checks()
         expected = [
             checks.Error(
@@ -612,28 +632,19 @@ class CustomUserModelValidationTestCase(SimpleTestCase):
             ),
         ]
         self.assertEqual(errors, expected)
-
-    @override_settings(AUTH_USER_MODEL='auth_tests.CustomUserNonUniqueUsername',
-                       AUTHENTICATION_BACKENDS=[
-                           'my.custom.backend',
-                       ])
-    @override_system_checks([check_user_model])
-    def test_username_non_unique_with_custom_backend(self):
-        """ A non-unique USERNAME_FIELD should raise an error only if we use the
-        default authentication backend. Otherwise, an warning should be raised.
-        """
-        errors = checks.run_checks()
-        expected = [
-            checks.Warning(
-                ("'CustomUserNonUniqueUsername.username' is named as "
-                 "the 'USERNAME_FIELD', but it is not unique."),
-                hint=('Ensure that your authentication backend(s) can handle '
-                      'non-unique usernames.'),
-                obj=CustomUserNonUniqueUsername,
-                id='auth.W004',
-            )
-        ]
-        self.assertEqual(errors, expected)
+        with self.settings(AUTHENTICATION_BACKENDS=['my.custom.backend']):
+            errors = checks.run_checks()
+            expected = [
+                checks.Warning(
+                    ("'CustomUserNonUniqueUsername.username' is named as "
+                     "the 'USERNAME_FIELD', but it is not unique."),
+                    hint=('Ensure that your authentication backend(s) can handle '
+                          'non-unique usernames.'),
+                    obj=CustomUserNonUniqueUsername,
+                    id='auth.W004',
+                )
+            ]
+            self.assertEqual(errors, expected)
 
 
 class PermissionTestCase(TestCase):
diff --git a/tests/check_framework/test_model_field_deprecation.py b/tests/check_framework/test_model_field_deprecation.py
index ce30803a61..21ece86bd4 100644
--- a/tests/check_framework/test_model_field_deprecation.py
+++ b/tests/check_framework/test_model_field_deprecation.py
@@ -1,11 +1,11 @@
 from django.core import checks
 from django.db import models
 from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
 
-from .tests import IsolateModelsMixin
 
-
-class TestDeprecatedField(IsolateModelsMixin, SimpleTestCase):
+@isolate_apps('check_framework')
+class TestDeprecatedField(SimpleTestCase):
     def test_default_details(self):
         class MyField(models.Field):
             system_check_deprecated_details = {}
@@ -45,7 +45,8 @@ class TestDeprecatedField(IsolateModelsMixin, SimpleTestCase):
         ])
 
 
-class TestRemovedField(IsolateModelsMixin, SimpleTestCase):
+@isolate_apps('check_framework')
+class TestRemovedField(SimpleTestCase):
     def test_default_details(self):
         class MyField(models.Field):
             system_check_removed_details = {}
diff --git a/tests/check_framework/test_multi_db.py b/tests/check_framework/test_multi_db.py
index 168b2ea33f..bfbeab0029 100644
--- a/tests/check_framework/test_multi_db.py
+++ b/tests/check_framework/test_multi_db.py
@@ -1,8 +1,6 @@
 from django.db import connections, models
 from django.test import TestCase, mock
-from django.test.utils import override_settings
-
-from .tests import IsolateModelsMixin
+from django.test.utils import isolate_apps, override_settings
 
 
 class TestRouter(object):
@@ -14,7 +12,8 @@ class TestRouter(object):
 
 
 @override_settings(DATABASE_ROUTERS=[TestRouter()])
-class TestMultiDBChecks(IsolateModelsMixin, TestCase):
+@isolate_apps('check_framework')
+class TestMultiDBChecks(TestCase):
     multi_db = True
 
     def _patch_check_field_on(self, db):
diff --git a/tests/check_framework/tests.py b/tests/check_framework/tests.py
index 7ae7b633e0..90b8a944f0 100644
--- a/tests/check_framework/tests.py
+++ b/tests/check_framework/tests.py
@@ -11,7 +11,9 @@ from django.core.management import call_command
 from django.core.management.base import CommandError
 from django.db import models
 from django.test import SimpleTestCase
-from django.test.utils import override_settings, override_system_checks
+from django.test.utils import (
+    isolate_apps, override_settings, override_system_checks,
+)
 from django.utils.encoding import force_text
 from django.utils.six import StringIO
 
@@ -254,23 +256,10 @@ class SilencingCheckTests(SimpleTestCase):
         self.assertEqual(err.getvalue(), '')
 
 
-class IsolateModelsMixin(object):
-    def setUp(self):
-        self.current_models = apps.all_models[__package__]
-        self.saved_models = set(self.current_models)
-
-    def tearDown(self):
-        for model in (set(self.current_models) - self.saved_models):
-            del self.current_models[model]
-        apps.clear_cache()
-
-
-class CheckFrameworkReservedNamesTests(IsolateModelsMixin, SimpleTestCase):
-    @override_settings(
-        SILENCED_SYSTEM_CHECKS=['models.E20', 'fields.W342'],  # ForeignKey(unique=True)
-        INSTALLED_APPS=['django.contrib.auth', 'django.contrib.contenttypes', 'check_framework']
-    )
-    def test_model_check_method_not_shadowed(self):
+class CheckFrameworkReservedNamesTests(SimpleTestCase):
+    @isolate_apps('check_framework', kwarg_name='apps')
+    @override_system_checks([checks.model_checks.check_all_models])
+    def test_model_check_method_not_shadowed(self, apps):
         class ModelWithAttributeCalledCheck(models.Model):
             check = 42
 
@@ -288,7 +277,7 @@ class CheckFrameworkReservedNamesTests(IsolateModelsMixin, SimpleTestCase):
                 related_name='check',
             )
 
-        errors = checks.run_checks()
+        errors = checks.run_checks(app_configs=apps.get_app_configs())
         expected = [
             Error(
                 "The 'ModelWithAttributeCalledCheck.check()' class method is "
diff --git a/tests/contenttypes_tests/tests.py b/tests/contenttypes_tests/tests.py
index dd14eaad55..e5cf3d2010 100644
--- a/tests/contenttypes_tests/tests.py
+++ b/tests/contenttypes_tests/tests.py
@@ -12,8 +12,8 @@ from django.contrib.contenttypes.models import ContentType
 from django.contrib.sites.models import Site
 from django.core import checks
 from django.db import connections, models
-from django.test import TestCase, override_settings
-from django.test.utils import captured_stdout
+from django.test import SimpleTestCase, TestCase, override_settings
+from django.test.utils import captured_stdout, isolate_apps
 from django.utils.encoding import force_str, force_text
 
 from .models import Article, Author, SchemeIncludedURL
@@ -114,20 +114,9 @@ class ContentTypesViewsTests(TestCase):
         self.assertEqual(force_text(ct), 'modelcreatedonthefly')
 
 
-class IsolatedModelsTestCase(TestCase):
-    def setUp(self):
-        # The unmanaged models need to be removed after the test in order to
-        # prevent bad interactions with the flush operation in other tests.
-        self._old_models = apps.app_configs['contenttypes_tests'].models.copy()
-
-    def tearDown(self):
-        apps.app_configs['contenttypes_tests'].models = self._old_models
-        apps.all_models['contenttypes_tests'] = self._old_models
-        apps.clear_cache()
-
-
 @override_settings(SILENCED_SYSTEM_CHECKS=['fields.W342'])  # ForeignKey(unique=True)
-class GenericForeignKeyTests(IsolatedModelsTestCase):
+@isolate_apps('contenttypes_tests', attr_name='apps')
+class GenericForeignKeyTests(SimpleTestCase):
 
     def test_str(self):
         class Model(models.Model):
@@ -239,11 +228,12 @@ class GenericForeignKeyTests(IsolatedModelsTestCase):
         class Model(models.Model):
             content_object = MyGenericForeignKey()
 
-        errors = checks.run_checks()
+        errors = checks.run_checks(app_configs=self.apps.get_app_configs())
         self.assertEqual(errors, ['performed!'])
 
 
-class GenericRelationshipTests(IsolatedModelsTestCase):
+@isolate_apps('contenttypes_tests')
+class GenericRelationshipTests(SimpleTestCase):
 
     def test_valid_generic_relationship(self):
         class TaggedItem(models.Model):
diff --git a/tests/defer_regress/tests.py b/tests/defer_regress/tests.py
index cc1658ce58..ffb0b6e775 100644
--- a/tests/defer_regress/tests.py
+++ b/tests/defer_regress/tests.py
@@ -3,7 +3,6 @@ from __future__ import unicode_literals
 from operator import attrgetter
 
 from django.apps import apps
-from django.apps.registry import Apps
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.sessions.backends.db import SessionStore
 from django.db import models
@@ -12,6 +11,7 @@ from django.db.models.query_utils import (
     DeferredAttribute, deferred_class_factory,
 )
 from django.test import TestCase, override_settings
+from django.test.utils import isolate_apps
 
 from .models import (
     Base, Child, Derived, Feature, Item, ItemAndSimpleItem, Leaf, Location,
@@ -265,22 +265,20 @@ class DeferRegressionTest(TestCase):
         deferred_cls = deferred_class_factory(Item, ())
         self.assertFalse(deferred_cls._deferred)
 
-    def test_deferred_class_factory_apps_reuse(self):
+    @isolate_apps('defer_regress', kwarg_name='apps')
+    def test_deferred_class_factory_apps_reuse(self, apps):
         """
         #25563 - model._meta.apps should be used for caching and
         retrieval of the created proxy class.
         """
-        isolated_apps = Apps(['defer_regress'])
-
         class BaseModel(models.Model):
             field = models.BooleanField()
 
             class Meta:
-                apps = isolated_apps
                 app_label = 'defer_regress'
 
         deferred_model = deferred_class_factory(BaseModel, ['field'])
-        self.assertIs(deferred_model._meta.apps, isolated_apps)
+        self.assertIs(deferred_model._meta.apps, apps)
         self.assertIs(deferred_class_factory(BaseModel, ['field']), deferred_model)
 
 
diff --git a/tests/foreign_object/tests.py b/tests/foreign_object/tests.py
index bcdadc6d6d..f83698175e 100644
--- a/tests/foreign_object/tests.py
+++ b/tests/foreign_object/tests.py
@@ -1,11 +1,11 @@
 import datetime
 from operator import attrgetter
 
-from django.apps.registry import Apps
 from django.core.exceptions import FieldError
 from django.db import models
 from django.db.models.fields.related import ForeignObject
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
+from django.test.utils import isolate_apps
 from django.utils import translation
 
 from .models import (
@@ -410,15 +410,13 @@ class MultiColumnFKTests(TestCase):
 
 class TestModelCheckTests(SimpleTestCase):
 
+    @isolate_apps('foreign_object')
     def test_check_composite_foreign_object(self):
-        test_apps = Apps(['foreign_object'])
-
         class Parent(models.Model):
             a = models.PositiveIntegerField()
             b = models.PositiveIntegerField()
 
             class Meta:
-                apps = test_apps
                 unique_together = (('a', 'b'),)
 
         class Child(models.Model):
@@ -433,21 +431,16 @@ class TestModelCheckTests(SimpleTestCase):
                 related_name='children',
             )
 
-            class Meta:
-                apps = test_apps
-
         self.assertEqual(Child._meta.get_field('parent').check(from_model=Child), [])
 
+    @isolate_apps('foreign_object')
     def test_check_subset_composite_foreign_object(self):
-        test_apps = Apps(['foreign_object'])
-
         class Parent(models.Model):
             a = models.PositiveIntegerField()
             b = models.PositiveIntegerField()
             c = models.PositiveIntegerField()
 
             class Meta:
-                apps = test_apps
                 unique_together = (('a', 'b'),)
 
         class Child(models.Model):
@@ -463,7 +456,4 @@ class TestModelCheckTests(SimpleTestCase):
                 related_name='children',
             )
 
-            class Meta:
-                apps = test_apps
-
         self.assertEqual(Child._meta.get_field('parent').check(from_model=Child), [])
diff --git a/tests/invalid_models_tests/base.py b/tests/invalid_models_tests/base.py
deleted file mode 100644
index 9459b313d6..0000000000
--- a/tests/invalid_models_tests/base.py
+++ /dev/null
@@ -1,18 +0,0 @@
-# -*- encoding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.apps import apps
-from django.test import SimpleTestCase
-
-
-class IsolatedModelsTestCase(SimpleTestCase):
-
-    def setUp(self):
-        # The unmanaged models need to be removed after the test in order to
-        # prevent bad interactions with the flush operation in other tests.
-        self._old_models = apps.app_configs['invalid_models_tests'].models.copy()
-
-    def tearDown(self):
-        apps.app_configs['invalid_models_tests'].models = self._old_models
-        apps.all_models['invalid_models_tests'] = self._old_models
-        apps.clear_cache()
diff --git a/tests/invalid_models_tests/test_backend_specific.py b/tests/invalid_models_tests/test_backend_specific.py
index 5be1d6f691..fd73853a03 100644
--- a/tests/invalid_models_tests/test_backend_specific.py
+++ b/tests/invalid_models_tests/test_backend_specific.py
@@ -3,9 +3,8 @@ from __future__ import unicode_literals
 
 from django.core.checks import Error
 from django.db import connections, models
-from django.test import mock
-
-from .base import IsolatedModelsTestCase
+from django.test import SimpleTestCase, mock
+from django.test.utils import isolate_apps
 
 
 def dummy_allow_migrate(db, app_label, **hints):
@@ -14,7 +13,8 @@ def dummy_allow_migrate(db, app_label, **hints):
     return db == 'default'
 
 
-class BackendSpecificChecksTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class BackendSpecificChecksTests(SimpleTestCase):
 
     @mock.patch('django.db.models.fields.router.allow_migrate', new=dummy_allow_migrate)
     def test_check_field(self):
diff --git a/tests/invalid_models_tests/test_custom_fields.py b/tests/invalid_models_tests/test_custom_fields.py
index 03e65b599b..4c379e70f5 100644
--- a/tests/invalid_models_tests/test_custom_fields.py
+++ b/tests/invalid_models_tests/test_custom_fields.py
@@ -1,9 +1,10 @@
 from django.db import models
+from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
 
-from .base import IsolatedModelsTestCase
 
-
-class CustomFieldTest(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class CustomFieldTest(SimpleTestCase):
 
     def test_none_column(self):
         class NoColumnField(models.AutoField):
diff --git a/tests/invalid_models_tests/test_deprecated_fields.py b/tests/invalid_models_tests/test_deprecated_fields.py
index fa646515b3..7826596895 100644
--- a/tests/invalid_models_tests/test_deprecated_fields.py
+++ b/tests/invalid_models_tests/test_deprecated_fields.py
@@ -1,10 +1,11 @@
 from django.core import checks
 from django.db import models
+from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
 
-from .base import IsolatedModelsTestCase
 
-
-class DeprecatedFieldsTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class DeprecatedFieldsTests(SimpleTestCase):
     def test_IPAddressField_deprecated(self):
         class IPAddressModel(models.Model):
             ip = models.IPAddressField()
diff --git a/tests/invalid_models_tests/test_models.py b/tests/invalid_models_tests/test_models.py
index 8e1f515b13..5764764341 100644
--- a/tests/invalid_models_tests/test_models.py
+++ b/tests/invalid_models_tests/test_models.py
@@ -6,9 +6,8 @@ import unittest
 from django.conf import settings
 from django.core.checks import Error
 from django.db import connections, models
-from django.test.utils import override_settings
-
-from .base import IsolatedModelsTestCase
+from django.test import SimpleTestCase
+from django.test.utils import isolate_apps, override_settings
 
 
 def get_max_column_name_length():
@@ -31,7 +30,8 @@ def get_max_column_name_length():
     return (allowed_len, db_alias)
 
 
-class IndexTogetherTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class IndexTogetherTests(SimpleTestCase):
 
     def test_non_iterable(self):
         class Model(models.Model):
@@ -146,7 +146,8 @@ class IndexTogetherTests(IsolatedModelsTestCase):
 
 
 # unique_together tests are very similar to index_together tests.
-class UniqueTogetherTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class UniqueTogetherTests(SimpleTestCase):
 
     def test_non_iterable(self):
         class Model(models.Model):
@@ -251,7 +252,8 @@ class UniqueTogetherTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class FieldNamesTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class FieldNamesTests(SimpleTestCase):
 
     def test_ending_with_underscore(self):
         class Model(models.Model):
@@ -434,7 +436,8 @@ class FieldNamesTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class ShadowingFieldsTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class ShadowingFieldsTests(SimpleTestCase):
 
     def test_field_name_clash_with_child_accessor(self):
         class Parent(models.Model):
@@ -558,7 +561,8 @@ class ShadowingFieldsTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class OtherModelTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class OtherModelTests(SimpleTestCase):
 
     def test_unique_primary_key(self):
         invalid_id = models.IntegerField(primary_key=False)
diff --git a/tests/invalid_models_tests/test_ordinary_fields.py b/tests/invalid_models_tests/test_ordinary_fields.py
index 033970aae4..2a7e912f8c 100644
--- a/tests/invalid_models_tests/test_ordinary_fields.py
+++ b/tests/invalid_models_tests/test_ordinary_fields.py
@@ -5,14 +5,13 @@ import unittest
 
 from django.core.checks import Error, Warning as DjangoWarning
 from django.db import connection, models
-from django.test import TestCase
-from django.test.utils import override_settings
+from django.test import SimpleTestCase, TestCase
+from django.test.utils import isolate_apps, override_settings
 from django.utils.timezone import now
 
-from .base import IsolatedModelsTestCase
 
-
-class AutoFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class AutoFieldTests(SimpleTestCase):
 
     def test_valid_case(self):
         class Model(models.Model):
@@ -46,7 +45,8 @@ class AutoFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class BooleanFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class BooleanFieldTests(SimpleTestCase):
 
     def test_nullable_boolean_field(self):
         class Model(models.Model):
@@ -65,7 +65,8 @@ class BooleanFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class CharFieldTests(IsolatedModelsTestCase, TestCase):
+@isolate_apps('invalid_models_tests')
+class CharFieldTests(TestCase):
 
     def test_valid_field(self):
         class Model(models.Model):
@@ -216,7 +217,8 @@ class CharFieldTests(IsolatedModelsTestCase, TestCase):
         self.assertEqual(errors, expected)
 
 
-class DateFieldTests(IsolatedModelsTestCase, TestCase):
+@isolate_apps('invalid_models_tests')
+class DateFieldTests(TestCase):
 
     def test_auto_now_and_auto_now_add_raise_error(self):
         class Model(models.Model):
@@ -282,7 +284,8 @@ class DateFieldTests(IsolatedModelsTestCase, TestCase):
         self.test_fix_default_value()
 
 
-class DateTimeFieldTests(IsolatedModelsTestCase, TestCase):
+@isolate_apps('invalid_models_tests')
+class DateTimeFieldTests(TestCase):
 
     def test_fix_default_value(self):
         class Model(models.Model):
@@ -326,7 +329,8 @@ class DateTimeFieldTests(IsolatedModelsTestCase, TestCase):
         self.test_fix_default_value()
 
 
-class DecimalFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class DecimalFieldTests(SimpleTestCase):
 
     def test_required_attributes(self):
         class Model(models.Model):
@@ -420,7 +424,8 @@ class DecimalFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class FileFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class FileFieldTests(SimpleTestCase):
 
     def test_valid_case(self):
         class Model(models.Model):
@@ -464,7 +469,8 @@ class FileFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class FilePathFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class FilePathFieldTests(SimpleTestCase):
 
     def test_forbidden_files_and_folders(self):
         class Model(models.Model):
@@ -483,7 +489,8 @@ class FilePathFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class GenericIPAddressFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class GenericIPAddressFieldTests(SimpleTestCase):
 
     def test_non_nullable_blank(self):
         class Model(models.Model):
@@ -503,7 +510,8 @@ class GenericIPAddressFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class ImageFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class ImageFieldTests(SimpleTestCase):
 
     def test_pillow_installed(self):
         try:
@@ -530,7 +538,8 @@ class ImageFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class IntegerFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class IntegerFieldTests(SimpleTestCase):
 
     def test_max_length_warning(self):
         class Model(models.Model):
@@ -549,7 +558,8 @@ class IntegerFieldTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class TimeFieldTests(IsolatedModelsTestCase, TestCase):
+@isolate_apps('invalid_models_tests')
+class TimeFieldTests(TestCase):
 
     def test_fix_default_value(self):
         class Model(models.Model):
diff --git a/tests/invalid_models_tests/test_relative_fields.py b/tests/invalid_models_tests/test_relative_fields.py
index f157f2beb8..c2895f681f 100644
--- a/tests/invalid_models_tests/test_relative_fields.py
+++ b/tests/invalid_models_tests/test_relative_fields.py
@@ -3,21 +3,19 @@ from __future__ import unicode_literals
 
 import warnings
 
-from django.apps.registry import Apps
 from django.core.checks import Error, Warning as DjangoWarning
 from django.db import models
 from django.db.models.fields.related import ForeignObject
 from django.test import ignore_warnings
-from django.test.testcases import skipIfDBFeature
-from django.test.utils import override_settings
+from django.test.testcases import SimpleTestCase, skipIfDBFeature
+from django.test.utils import isolate_apps, override_settings
 from django.utils import six
 from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.version import get_docs_version
 
-from .base import IsolatedModelsTestCase
 
-
-class RelativeFieldTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class RelativeFieldTests(SimpleTestCase):
 
     def test_valid_foreign_key_without_accessor(self):
         class Target(models.Model):
@@ -133,23 +131,18 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
-    def test_foreign_key_to_isolated_apps_model(self):
+    @isolate_apps('invalid_models_tests')
+    def test_foreign_key_to_isolate_apps_model(self):
         """
         #25723 - Referenced model registration lookup should be run against the
         field's model registry.
         """
-        test_apps = Apps(['invalid_models_tests'])
-
         class OtherModel(models.Model):
-            class Meta:
-                apps = test_apps
+            pass
 
         class Model(models.Model):
             foreign_key = models.ForeignKey('OtherModel', models.CASCADE)
 
-            class Meta:
-                apps = test_apps
-
         field = Model._meta.get_field('foreign_key')
         self.assertEqual(field.check(from_model=Model), [])
 
@@ -170,23 +163,18 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
-    def test_many_to_many_to_isolated_apps_model(self):
+    @isolate_apps('invalid_models_tests')
+    def test_many_to_many_to_isolate_apps_model(self):
         """
         #25723 - Referenced model registration lookup should be run against the
         field's model registry.
         """
-        test_apps = Apps(['invalid_models_tests'])
-
         class OtherModel(models.Model):
-            class Meta:
-                apps = test_apps
+            pass
 
         class Model(models.Model):
             m2m = models.ManyToManyField('OtherModel')
 
-            class Meta:
-                apps = test_apps
-
         field = Model._meta.get_field('m2m')
         self.assertEqual(field.check(from_model=Model), [])
 
@@ -329,30 +317,22 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
-    def test_many_to_many_through_isolated_apps_model(self):
+    @isolate_apps('invalid_models_tests')
+    def test_many_to_many_through_isolate_apps_model(self):
         """
         #25723 - Through model registration lookup should be run against the
         field's model registry.
         """
-        test_apps = Apps(['invalid_models_tests'])
-
         class GroupMember(models.Model):
             person = models.ForeignKey('Person', models.CASCADE)
             group = models.ForeignKey('Group', models.CASCADE)
 
-            class Meta:
-                apps = test_apps
-
         class Person(models.Model):
-            class Meta:
-                apps = test_apps
+            pass
 
         class Group(models.Model):
             members = models.ManyToManyField('Person', through='GroupMember')
 
-            class Meta:
-                apps = test_apps
-
         field = Group._meta.get_field('members')
         self.assertEqual(field.check(from_model=Group), [])
 
@@ -790,7 +770,8 @@ class RelativeFieldTests(IsolatedModelsTestCase):
             self.assertFalse(errors)
 
 
-class AccessorClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class AccessorClashTests(SimpleTestCase):
 
     def test_fk_to_integer(self):
         self._test_accessor_clash(
@@ -902,7 +883,8 @@ class AccessorClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class ReverseQueryNameClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class ReverseQueryNameClashTests(SimpleTestCase):
 
     def test_fk_to_integer(self):
         self._test_reverse_query_name_clash(
@@ -958,7 +940,8 @@ class ReverseQueryNameClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class ExplicitRelatedNameClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class ExplicitRelatedNameClashTests(SimpleTestCase):
 
     def test_fk_to_integer(self):
         self._test_explicit_related_name_clash(
@@ -1022,7 +1005,8 @@ class ExplicitRelatedNameClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class ExplicitRelatedQueryNameClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class ExplicitRelatedQueryNameClashTests(SimpleTestCase):
 
     def test_fk_to_integer(self):
         self._test_explicit_related_query_name_clash(
@@ -1086,7 +1070,8 @@ class ExplicitRelatedQueryNameClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class SelfReferentialM2MClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class SelfReferentialM2MClashTests(SimpleTestCase):
 
     def test_clash_between_accessors(self):
         class Model(models.Model):
@@ -1181,7 +1166,8 @@ class SelfReferentialM2MClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, [])
 
 
-class SelfReferentialFKClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class SelfReferentialFKClashTests(SimpleTestCase):
 
     def test_accessor_clash(self):
         class Model(models.Model):
@@ -1244,7 +1230,8 @@ class SelfReferentialFKClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class ComplexClashTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class ComplexClashTests(SimpleTestCase):
 
     # New tests should not be included here, because this is a single,
     # self-contained sanity check, not a test of everything.
@@ -1358,7 +1345,8 @@ class ComplexClashTests(IsolatedModelsTestCase):
         self.assertEqual(errors, expected)
 
 
-class M2mThroughFieldsTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class M2mThroughFieldsTests(SimpleTestCase):
     def test_m2m_field_argument_validation(self):
         """
         Tests that ManyToManyField accepts the ``through_fields`` kwarg
diff --git a/tests/managers_regress/tests.py b/tests/managers_regress/tests.py
index cd9898ae63..e05602c50d 100644
--- a/tests/managers_regress/tests.py
+++ b/tests/managers_regress/tests.py
@@ -1,9 +1,9 @@
 from __future__ import unicode_literals
 
-from django.apps import apps
 from django.db import models
 from django.template import Context, Template
 from django.test import TestCase, override_settings
+from django.test.utils import isolate_apps
 from django.utils.encoding import force_text
 
 from .models import (
@@ -91,82 +91,58 @@ class ManagersRegressionTests(TestCase):
             AbstractBase1.objects.all()
 
     @override_settings(TEST_SWAPPABLE_MODEL='managers_regress.Parent')
+    @isolate_apps('managers_regress')
     def test_swappable_manager(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['managers_regress'].models.copy()
+        class SwappableModel(models.Model):
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-        try:
-            class SwappableModel(models.Model):
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
-
-            # Accessing the manager on a swappable model should
-            # raise an attribute error with a helpful message
-            msg = (
-                "Manager isn't available; 'managers_regress.SwappableModel' "
-                "has been swapped for 'managers_regress.Parent'"
-            )
-            with self.assertRaisesMessage(AttributeError, msg):
-                SwappableModel.objects.all()
-        finally:
-            apps.app_configs['managers_regress'].models = _old_models
-            apps.all_models['managers_regress'] = _old_models
-            apps.clear_cache()
+        # Accessing the manager on a swappable model should
+        # raise an attribute error with a helpful message
+        msg = (
+            "Manager isn't available; 'managers_regress.SwappableModel' "
+            "has been swapped for 'managers_regress.Parent'"
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
+            SwappableModel.objects.all()
 
     @override_settings(TEST_SWAPPABLE_MODEL='managers_regress.Parent')
+    @isolate_apps('managers_regress')
     def test_custom_swappable_manager(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['managers_regress'].models.copy()
+        class SwappableModel(models.Model):
+            stuff = models.Manager()
 
-        try:
-            class SwappableModel(models.Model):
-                stuff = models.Manager()
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
-
-            # Accessing the manager on a swappable model with an
-            # explicit manager should raise an attribute error with a
-            # helpful message
-            msg = (
-                "Manager isn't available; 'managers_regress.SwappableModel' "
-                "has been swapped for 'managers_regress.Parent'"
-            )
-            with self.assertRaisesMessage(AttributeError, msg):
-                SwappableModel.stuff.all()
-        finally:
-            apps.app_configs['managers_regress'].models = _old_models
-            apps.all_models['managers_regress'] = _old_models
-            apps.clear_cache()
+        # Accessing the manager on a swappable model with an
+        # explicit manager should raise an attribute error with a
+        # helpful message
+        msg = (
+            "Manager isn't available; 'managers_regress.SwappableModel' "
+            "has been swapped for 'managers_regress.Parent'"
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
+            SwappableModel.stuff.all()
 
     @override_settings(TEST_SWAPPABLE_MODEL='managers_regress.Parent')
+    @isolate_apps('managers_regress')
     def test_explicit_swappable_manager(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['managers_regress'].models.copy()
-
-        try:
-            class SwappableModel(models.Model):
-                objects = models.Manager()
+        class SwappableModel(models.Model):
+            objects = models.Manager()
 
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-            # Accessing the manager on a swappable model with an
-            # explicit manager should raise an attribute error with a
-            # helpful message
-            msg = (
-                "Manager isn't available; 'managers_regress.SwappableModel' "
-                "has been swapped for 'managers_regress.Parent'"
-            )
-            with self.assertRaisesMessage(AttributeError, msg):
-                SwappableModel.objects.all()
-        finally:
-            apps.app_configs['managers_regress'].models = _old_models
-            apps.all_models['managers_regress'] = _old_models
-            apps.clear_cache()
+        # Accessing the manager on a swappable model with an
+        # explicit manager should raise an attribute error with a
+        # helpful message
+        msg = (
+            "Manager isn't available; 'managers_regress.SwappableModel' "
+            "has been swapped for 'managers_regress.Parent'"
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
+            SwappableModel.objects.all()
 
     def test_regress_3871(self):
         related = RelatedModel.objects.create()
diff --git a/tests/model_fields/tests.py b/tests/model_fields/tests.py
index 77361fae79..14b56139de 100644
--- a/tests/model_fields/tests.py
+++ b/tests/model_fields/tests.py
@@ -19,7 +19,7 @@ from django.db.models.fields import (
     TimeField, URLField,
 )
 from django.db.models.fields.files import FileField, ImageField
-from django.test.utils import requires_tz_support
+from django.test.utils import isolate_apps, requires_tz_support
 from django.utils import six, timezone
 from django.utils.encoding import force_str
 from django.utils.functional import lazy
@@ -207,7 +207,11 @@ class ForeignKeyTests(test.TestCase):
         fk_model_empty = FkToChar.objects.select_related('out').get(id=fk_model_empty.pk)
         self.assertEqual(fk_model_empty.out, char_model_empty)
 
+    @isolate_apps('model_fields')
     def test_warning_when_unique_true_on_fk(self):
+        class Foo(models.Model):
+            pass
+
         class FKUniqueTrue(models.Model):
             fk_field = models.ForeignKey(Foo, models.CASCADE, unique=True)
 
diff --git a/tests/order_with_respect_to/tests.py b/tests/order_with_respect_to/tests.py
index c22f9b4ea4..7788fe3368 100644
--- a/tests/order_with_respect_to/tests.py
+++ b/tests/order_with_respect_to/tests.py
@@ -2,9 +2,9 @@ from __future__ import unicode_literals
 
 from operator import attrgetter
 
-from django.apps.registry import Apps
 from django.db import models
 from django.test import TestCase
+from django.test.utils import isolate_apps
 
 from .base_tests import BaseOrderWithRespectToTests
 from .models import Answer, Dimension, Entity, Post, Question
@@ -18,12 +18,10 @@ class OrderWithRespectToBaseTests(BaseOrderWithRespectToTests, TestCase):
 
 class OrderWithRespectToTests(TestCase):
 
+    @isolate_apps('order_with_respect_to')
     def test_duplicate_order_field(self):
-        test_apps = Apps(['order_with_respect_to'])
-
         class Bar(models.Model):
             class Meta:
-                apps = test_apps
                 app_label = 'order_with_respect_to'
 
         class Foo(models.Model):
@@ -32,7 +30,6 @@ class OrderWithRespectToTests(TestCase):
 
             class Meta:
                 order_with_respect_to = 'bar'
-                apps = test_apps
                 app_label = 'order_with_respect_to'
 
         count = 0
diff --git a/tests/postgres_tests/test_array.py b/tests/postgres_tests/test_array.py
index e9cec6b5b5..24a47db1d7 100644
--- a/tests/postgres_tests/test_array.py
+++ b/tests/postgres_tests/test_array.py
@@ -4,11 +4,11 @@ import unittest
 import uuid
 
 from django import forms
-from django.apps.registry import Apps
 from django.core import exceptions, serializers, validators
 from django.core.management import call_command
 from django.db import IntegrityError, connection, models
 from django.test import TransactionTestCase, override_settings
+from django.test.utils import isolate_apps
 from django.utils import timezone
 
 from . import PostgreSQLTestCase
@@ -333,17 +333,13 @@ class TestOtherTypesExactQuerying(PostgreSQLTestCase):
         )
 
 
+@isolate_apps('postgres_tests')
 class TestChecks(PostgreSQLTestCase):
 
     def test_field_checks(self):
-        test_apps = Apps(['postgres_tests'])
-
         class MyModel(PostgreSQLModel):
             field = ArrayField(models.CharField())
 
-            class Meta:
-                apps = test_apps
-
         model = MyModel()
         errors = model.check()
         self.assertEqual(len(errors), 1)
@@ -352,14 +348,9 @@ class TestChecks(PostgreSQLTestCase):
         self.assertIn('max_length', errors[0].msg)
 
     def test_invalid_base_fields(self):
-        test_apps = Apps(['postgres_tests'])
-
         class MyModel(PostgreSQLModel):
             field = ArrayField(models.ManyToManyField('postgres_tests.IntegerArrayModel'))
 
-            class Meta:
-                apps = test_apps
-
         model = MyModel()
         errors = model.check()
         self.assertEqual(len(errors), 1)
@@ -369,14 +360,9 @@ class TestChecks(PostgreSQLTestCase):
         """
         Nested ArrayFields are permitted.
         """
-        test_apps = Apps(['postgres_tests'])
-
         class MyModel(PostgreSQLModel):
             field = ArrayField(ArrayField(models.CharField()))
 
-            class Meta:
-                apps = test_apps
-
         model = MyModel()
         errors = model.check()
         self.assertEqual(len(errors), 1)
diff --git a/tests/proxy_models/tests.py b/tests/proxy_models/tests.py
index 625925fcb5..874a2596f0 100644
--- a/tests/proxy_models/tests.py
+++ b/tests/proxy_models/tests.py
@@ -2,7 +2,6 @@ from __future__ import unicode_literals
 
 import datetime
 
-from django.apps import apps
 from django.contrib import admin
 from django.contrib.auth.models import User as AuthUser
 from django.contrib.contenttypes.models import ContentType
@@ -10,6 +9,7 @@ from django.core import checks, management
 from django.db import DEFAULT_DB_ALIAS, models
 from django.db.models import signals
 from django.test import TestCase, override_settings
+from django.test.utils import isolate_apps
 from django.urls import reverse
 
 from .admin import admin as force_admin_model_registration  # NOQA
@@ -129,6 +129,7 @@ class ProxyModelTests(TestCase):
                     proxy = True
         self.assertRaises(TypeError, build_abc)
 
+    @isolate_apps('proxy_models')
     def test_no_cbc(self):
         """
         The proxy must actually have one concrete base class
@@ -139,6 +140,7 @@ class ProxyModelTests(TestCase):
                     proxy = True
         self.assertRaises(TypeError, build_no_cbc)
 
+    @isolate_apps('proxy_models')
     def test_no_base_classes(self):
         def build_no_base_classes():
             class NoBaseClasses(models.Model):
@@ -146,15 +148,13 @@ class ProxyModelTests(TestCase):
                     proxy = True
         self.assertRaises(TypeError, build_no_base_classes)
 
+    @isolate_apps('proxy_models')
     def test_new_fields(self):
         class NoNewFields(Person):
             newfield = models.BooleanField()
 
             class Meta:
                 proxy = True
-                # don't register this model in the app_cache for the current app,
-                # otherwise the check fails when other tests are being run.
-                app_label = 'no_such_app'
 
         errors = NoNewFields.check()
         expected = [
@@ -168,30 +168,22 @@ class ProxyModelTests(TestCase):
         self.assertEqual(errors, expected)
 
     @override_settings(TEST_SWAPPABLE_MODEL='proxy_models.AlternateModel')
+    @isolate_apps('proxy_models')
     def test_swappable(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['proxy_models'].models.copy()
+        class SwappableModel(models.Model):
 
-        try:
-            class SwappableModel(models.Model):
-
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-            class AlternateModel(models.Model):
-                pass
+        class AlternateModel(models.Model):
+            pass
 
-            # You can't proxy a swapped model
-            with self.assertRaises(TypeError):
-                class ProxyModel(SwappableModel):
+        # You can't proxy a swapped model
+        with self.assertRaises(TypeError):
+            class ProxyModel(SwappableModel):
 
-                    class Meta:
-                        proxy = True
-        finally:
-            apps.app_configs['proxy_models'].models = _old_models
-            apps.all_models['proxy_models'] = _old_models
-            apps.clear_cache()
+                class Meta:
+                    proxy = True
 
     def test_myperson_manager(self):
         Person.objects.create(name="fred")
diff --git a/tests/signals/tests.py b/tests/signals/tests.py
index ba99316bfb..f3bbc05f65 100644
--- a/tests/signals/tests.py
+++ b/tests/signals/tests.py
@@ -4,6 +4,7 @@ from django.db import models
 from django.db.models import signals
 from django.dispatch import receiver
 from django.test import TestCase
+from django.test.utils import isolate_apps
 from django.utils import six
 
 from .models import Author, Book, Car, Person
@@ -285,6 +286,7 @@ class LazyModelRefTest(BaseSignalTest):
         finally:
             signals.post_init.disconnect(self.receiver, sender=Book)
 
+    @isolate_apps('signals')
     def test_not_loaded_model(self):
         signals.post_init.connect(
             self.receiver, sender='signals.Created', weak=False
diff --git a/tests/sites_framework/tests.py b/tests/sites_framework/tests.py
index ca516388cf..576dbfc7eb 100644
--- a/tests/sites_framework/tests.py
+++ b/tests/sites_framework/tests.py
@@ -1,14 +1,12 @@
-from django.apps import apps
 from django.conf import settings
 from django.contrib.sites.managers import CurrentSiteManager
 from django.contrib.sites.models import Site
 from django.core import checks
 from django.db import models
-from django.test import TestCase
+from django.test import SimpleTestCase, TestCase
+from django.test.utils import isolate_apps
 
-from .models import (
-    AbstractArticle, CustomArticle, ExclusiveArticle, SyndicatedArticle,
-)
+from .models import CustomArticle, ExclusiveArticle, SyndicatedArticle
 
 
 class SitesFrameworkTestCase(TestCase):
@@ -16,13 +14,6 @@ class SitesFrameworkTestCase(TestCase):
         Site.objects.get_or_create(id=settings.SITE_ID, domain="example.com", name="example.com")
         Site.objects.create(id=settings.SITE_ID + 1, domain="example2.com", name="example2.com")
 
-        self._old_models = apps.app_configs['sites_framework'].models.copy()
-
-    def tearDown(self):
-        apps.app_configs['sites_framework'].models = self._old_models
-        apps.all_models['sites_framework'] = self._old_models
-        apps.clear_cache()
-
     def test_site_fk(self):
         article = ExclusiveArticle.objects.create(title="Breaking News!", site_id=settings.SITE_ID)
         self.assertEqual(ExclusiveArticle.on_site.all().get(), article)
@@ -42,12 +33,12 @@ class SitesFrameworkTestCase(TestCase):
         )
         self.assertEqual(CustomArticle.on_site.all().get(), article)
 
-    def test_invalid_name(self):
 
-        class InvalidArticle(AbstractArticle):
-            site = models.ForeignKey(Site, models.CASCADE)
+@isolate_apps('sites_framework')
+class CurrentSiteManagerChecksTests(SimpleTestCase):
 
-            objects = models.Manager()
+    def test_invalid_name(self):
+        class InvalidArticle(models.Model):
             on_site = CurrentSiteManager("places_this_article_should_appear")
 
         errors = InvalidArticle.check()
@@ -64,8 +55,9 @@ class SitesFrameworkTestCase(TestCase):
 
     def test_invalid_field_type(self):
 
-        class ConfusedArticle(AbstractArticle):
+        class ConfusedArticle(models.Model):
             site = models.IntegerField()
+            on_site = CurrentSiteManager()
 
         errors = ConfusedArticle.check()
         expected = [
diff --git a/tests/str/tests.py b/tests/str/tests.py
index 6467875a5c..08fd554e5e 100644
--- a/tests/str/tests.py
+++ b/tests/str/tests.py
@@ -4,9 +4,9 @@ from __future__ import unicode_literals
 import datetime
 from unittest import skipIf
 
-from django.apps.registry import Apps
 from django.db import models
 from django.test import TestCase
+from django.test.utils import isolate_apps
 from django.utils import six
 
 from .models import Article, InternationalArticle
@@ -36,16 +36,14 @@ class SimpleTests(TestCase):
             # python_2_unicode_compatible decorator is used.
             self.assertEqual(str(a), b'Girl wins \xe2\x82\xac12.500 in lottery')
 
+    @isolate_apps('str')
     def test_defaults(self):
         """
         The default implementation of __str__ and __repr__ should return
         instances of str.
         """
-        test_apps = Apps(['str'])
-
         class Default(models.Model):
-            class Meta:
-                apps = test_apps
+            pass
 
         obj = Default()
         # Explicit call to __str__/__repr__ to make sure str()/repr() don't