Fixed #31499 -- Stored ModelState.fields into a dict.

This allows the removal of its O(n) .get_field_by_name method and many
other awkward access patterns.

While fields were initially stored in a list to preserve the initial
model definiton field ordering the auto-detector doesn't take field
ordering into account and no operations exists to reorder fields of a
model.

This makes the preservation of the field ordering completely superflous
because field reorganization after the creation of the model state
wouldn't be taken into account.

9 files changed, 104 insertions, 112 deletions

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index f01c9feffd..89b95b19a9 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -93,7 +93,7 @@ class MigrationAutodetector:
         of course, the related fields change during renames).
         """
         fields_def = []
-        for name, field in sorted(fields):
+        for name, field in sorted(fields.items()):
             deconstruction = self.deep_deconstruct(field)
             if field.remote_field and field.remote_field.model:
                 del deconstruction[2]['to']
@@ -208,17 +208,17 @@ class MigrationAutodetector:
         self.kept_unmanaged_keys = self.old_unmanaged_keys & self.new_unmanaged_keys
         self.through_users = {}
         self.old_field_keys = {
-            (app_label, model_name, x)
+            (app_label, model_name, field_name)
             for app_label, model_name in self.kept_model_keys
-            for x, y in self.from_state.models[
+            for field_name in self.from_state.models[
                 app_label,
                 self.renamed_models.get((app_label, model_name), model_name)
             ].fields
         }
         self.new_field_keys = {
-            (app_label, model_name, x)
+            (app_label, model_name, field_name)
             for app_label, model_name in self.kept_model_keys
-            for x, y in self.to_state.models[app_label, model_name].fields
+            for field_name in self.to_state.models[app_label, model_name].fields
         }
 
     def _generate_through_model_map(self):
@@ -226,7 +226,7 @@ class MigrationAutodetector:
         for app_label, model_name in sorted(self.old_model_keys):
             old_model_name = self.renamed_models.get((app_label, model_name), model_name)
             old_model_state = self.from_state.models[app_label, old_model_name]
-            for field_name, field in old_model_state.fields:
+            for field_name in old_model_state.fields:
                 old_field = self.old_apps.get_model(app_label, old_model_name)._meta.get_field(field_name)
                 if (hasattr(old_field, "remote_field") and getattr(old_field.remote_field, "through", None) and
                         not old_field.remote_field.through._meta.auto_created):
@@ -576,7 +576,7 @@ class MigrationAutodetector:
                 app_label,
                 operations.CreateModel(
                     name=model_state.name,
-                    fields=[d for d in model_state.fields if d[0] not in related_fields],
+                    fields=[d for d in model_state.fields.items() if d[0] not in related_fields],
                     options=model_state.options,
                     bases=model_state.bases,
                     managers=model_state.managers,
@@ -820,7 +820,7 @@ class MigrationAutodetector:
             field_dec = self.deep_deconstruct(field)
             for rem_app_label, rem_model_name, rem_field_name in sorted(self.old_field_keys - self.new_field_keys):
                 if rem_app_label == app_label and rem_model_name == model_name:
-                    old_field = old_model_state.get_field_by_name(rem_field_name)
+                    old_field = old_model_state.fields[rem_field_name]
                     old_field_dec = self.deep_deconstruct(old_field)
                     if field.remote_field and field.remote_field.model and 'to' in old_field_dec[2]:
                         old_rel_to = old_field_dec[2]['to']
diff --git a/django/db/migrations/operations/fields.py b/django/db/migrations/operations/fields.py
index ecb2af8a28..e4f78e365e 100644
--- a/django/db/migrations/operations/fields.py
+++ b/django/db/migrations/operations/fields.py
@@ -89,7 +89,7 @@ class AddField(FieldOperation):
             field.default = NOT_PROVIDED
         else:
             field = self.field
-        state.models[app_label, self.model_name_lower].fields.append((self.name, field))
+        state.models[app_label, self.model_name_lower].fields[self.name] = field
         # Delay rendering of relationships if it's not a relational field
         delay = not field.is_relation
         state.reload_model(app_label, self.model_name_lower, delay=delay)
@@ -154,14 +154,8 @@ class RemoveField(FieldOperation):
         )
 
     def state_forwards(self, app_label, state):
-        new_fields = []
-        old_field = None
-        for name, instance in state.models[app_label, self.model_name_lower].fields:
-            if name != self.name:
-                new_fields.append((name, instance))
-            else:
-                old_field = instance
-        state.models[app_label, self.model_name_lower].fields = new_fields
+        model_state = state.models[app_label, self.model_name_lower]
+        old_field = model_state.fields.pop(self.name)
         # Delay rendering of relationships if it's not a relational field
         delay = not old_field.is_relation
         state.reload_model(app_label, self.model_name_lower, delay=delay)
@@ -217,11 +211,8 @@ class AlterField(FieldOperation):
             field.default = NOT_PROVIDED
         else:
             field = self.field
-        state.models[app_label, self.model_name_lower].fields = [
-            (n, field if n == self.name else f)
-            for n, f in
-            state.models[app_label, self.model_name_lower].fields
-        ]
+        model_state = state.models[app_label, self.model_name_lower]
+        model_state.fields[self.name] = field
         # TODO: investigate if old relational fields must be reloaded or if it's
         # sufficient if the new field is (#27737).
         # Delay rendering of relationships if it's not a relational field and
@@ -299,11 +290,14 @@ class RenameField(FieldOperation):
         model_state = state.models[app_label, self.model_name_lower]
         # Rename the field
         fields = model_state.fields
-        found = None
-        for index, (name, field) in enumerate(fields):
-            if not found and name == self.old_name:
-                fields[index] = (self.new_name, field)
-                found = field
+        try:
+            found = fields.pop(self.old_name)
+        except KeyError:
+            raise FieldDoesNotExist(
+                "%s.%s has no field named '%s'" % (app_label, self.model_name, self.old_name)
+            )
+        fields[self.new_name] = found
+        for field in fields.values():
             # Fix from_fields to refer to the new field.
             from_fields = getattr(field, 'from_fields', None)
             if from_fields:
@@ -311,10 +305,6 @@ class RenameField(FieldOperation):
                     self.new_name if from_field_name == self.old_name else from_field_name
                     for from_field_name in from_fields
                 ])
-        if found is None:
-            raise FieldDoesNotExist(
-                "%s.%s has no field named '%s'" % (app_label, self.model_name, self.old_name)
-            )
         # Fix index/unique_together to refer to the new field
         options = model_state.options
         for option in ('index_together', 'unique_together'):
diff --git a/django/db/migrations/operations/models.py b/django/db/migrations/operations/models.py
index baa902089f..8c06c4d157 100644
--- a/django/db/migrations/operations/models.py
+++ b/django/db/migrations/operations/models.py
@@ -310,7 +310,7 @@ class RenameModel(ModelOperation):
         old_model_tuple = (app_label, self.old_name_lower)
         new_remote_model = '%s.%s' % (app_label, self.new_name)
         to_reload = set()
-        for model_state, index, name, field, reference in get_references(state, old_model_tuple):
+        for model_state, name, field, reference in get_references(state, old_model_tuple):
             changed_field = None
             if reference.to:
                 changed_field = field.clone()
@@ -320,7 +320,7 @@ class RenameModel(ModelOperation):
                     changed_field = field.clone()
                 changed_field.remote_field.through = new_remote_model
             if changed_field:
-                model_state.fields[index] = name, changed_field
+                model_state.fields[name] = changed_field
                 to_reload.add((model_state.app_label, model_state.name_lower))
         # Reload models related to old model before removing the old model.
         state.reload_models(to_reload, delay=True)
diff --git a/django/db/migrations/operations/utils.py b/django/db/migrations/operations/utils.py
index 0295d60af9..6e2bacd549 100644
--- a/django/db/migrations/operations/utils.py
+++ b/django/db/migrations/operations/utils.py
@@ -83,17 +83,17 @@ def field_references(
 
 def get_references(state, model_tuple, field_tuple=()):
     """
-    Generator of (model_state, index, name, field, reference) referencing
+    Generator of (model_state, name, field, reference) referencing
     provided context.
 
     If field_tuple is provided only references to this particular field of
     model_tuple will be generated.
     """
     for state_model_tuple, model_state in state.models.items():
-        for index, (name, field) in enumerate(model_state.fields):
+        for name, field in model_state.fields.items():
             reference = field_references(state_model_tuple, field, model_tuple, *field_tuple)
             if reference:
-                yield model_state, index, name, field, reference
+                yield model_state, name, field, reference
 
 
 def field_is_referenced(state, model_tuple, field_tuple):
diff --git a/django/db/migrations/state.py b/django/db/migrations/state.py
index 9280f5cd26..c8cb704aee 100644
--- a/django/db/migrations/state.py
+++ b/django/db/migrations/state.py
@@ -125,7 +125,7 @@ class ProjectState:
         # Directly related models are the models pointed to by ForeignKeys,
         # OneToOneFields, and ManyToManyFields.
         direct_related_models = set()
-        for name, field in model_state.fields:
+        for field in model_state.fields.values():
             if field.is_relation:
                 if field.remote_field.model == RECURSIVE_RELATIONSHIP_CONSTANT:
                     continue
@@ -359,16 +359,13 @@ class ModelState:
     def __init__(self, app_label, name, fields, options=None, bases=None, managers=None):
         self.app_label = app_label
         self.name = name
-        self.fields = fields
+        self.fields = dict(fields)
         self.options = options or {}
         self.options.setdefault('indexes', [])
         self.options.setdefault('constraints', [])
         self.bases = bases or (models.Model,)
         self.managers = managers or []
-        # Sanity-check that fields is NOT a dict. It must be ordered.
-        if isinstance(self.fields, dict):
-            raise ValueError("ModelState.fields cannot be a dict - it must be a list of 2-tuples.")
-        for name, field in fields:
+        for name, field in self.fields.items():
             # Sanity-check that fields are NOT already bound to a model.
             if hasattr(field, 'model'):
                 raise ValueError(
@@ -544,7 +541,7 @@ class ModelState:
         return self.__class__(
             app_label=self.app_label,
             name=self.name,
-            fields=list(self.fields),
+            fields=dict(self.fields),
             # Since options are shallow-copied here, operations such as
             # AddIndex must replace their option (e.g 'indexes') rather
             # than mutating it.
@@ -566,8 +563,8 @@ class ModelState:
             )
         except LookupError:
             raise InvalidBasesError("Cannot resolve one or more bases from %r" % (self.bases,))
-        # Turn fields into a dict for the body, add other bits
-        body = {name: field.clone() for name, field in self.fields}
+        # Clone fields for the body, add other bits.
+        body = {name: field.clone() for name, field in self.fields.items()}
         body['Meta'] = meta
         body['__module__'] = "__fake__"
 
@@ -576,12 +573,6 @@ class ModelState:
         # Then, make a Model object (apps.register_model is called in __new__)
         return type(self.name, bases, body)
 
-    def get_field_by_name(self, name):
-        for fname, field in self.fields:
-            if fname == name:
-                return field
-        raise ValueError("No field called %s on model %s" % (name, self.name))
-
     def get_index_by_name(self, name):
         for index in self.options['indexes']:
             if index.name == name:
@@ -602,8 +593,13 @@ class ModelState:
             (self.app_label == other.app_label) and
             (self.name == other.name) and
             (len(self.fields) == len(other.fields)) and
-            all((k1 == k2 and (f1.deconstruct()[1:] == f2.deconstruct()[1:]))
-                for (k1, f1), (k2, f2) in zip(sorted(self.fields), sorted(other.fields))) and
+            all(
+                k1 == k2 and f1.deconstruct()[1:] == f2.deconstruct()[1:]
+                for (k1, f1), (k2, f2) in zip(
+                    sorted(self.fields.items()),
+                    sorted(other.fields.items()),
+                )
+            ) and
             (self.options == other.options) and
             (self.bases == other.bases) and
             (self.managers == other.managers)
diff --git a/tests/migrations/test_executor.py b/tests/migrations/test_executor.py
index 38391cdccf..5a122bc6b6 100644
--- a/tests/migrations/test_executor.py
+++ b/tests/migrations/test_executor.py
@@ -516,13 +516,13 @@ class ExecutorTests(MigrationTestBase):
         state = executor.migrate([
             ('mutate_state_a', '0001_initial'),
         ])
-        self.assertIn('added', dict(state.models['mutate_state_b', 'b'].fields))
+        self.assertIn('added', state.models['mutate_state_b', 'b'].fields)
         executor.loader.build_graph()
         # Migrate backward.
         state = executor.migrate([
             ('mutate_state_a', None),
         ])
-        self.assertIn('added', dict(state.models['mutate_state_b', 'b'].fields))
+        self.assertIn('added', state.models['mutate_state_b', 'b'].fields)
         executor.migrate([
             ('mutate_state_b', None),
         ])
diff --git a/tests/migrations/test_loader.py b/tests/migrations/test_loader.py
index 7bc9ccabd4..ce2f209188 100644
--- a/tests/migrations/test_loader.py
+++ b/tests/migrations/test_loader.py
@@ -73,15 +73,12 @@ class LoaderTests(TestCase):
 
         author_state = project_state.models["migrations", "author"]
         self.assertEqual(
-            [x for x, y in author_state.fields],
+            list(author_state.fields),
             ["id", "name", "slug", "age", "rating"]
         )
 
         book_state = project_state.models["migrations", "book"]
-        self.assertEqual(
-            [x for x, y in book_state.fields],
-            ["id", "author"]
-        )
+        self.assertEqual(list(book_state.fields), ['id', 'author'])
 
         # Ensure we've included unmigrated apps in there too
         self.assertIn("basic", project_state.real_apps)
@@ -122,10 +119,7 @@ class LoaderTests(TestCase):
         self.assertEqual(len([m for a, m in project_state.models if a == "migrations"]), 1)
 
         book_state = project_state.models["migrations", "book"]
-        self.assertEqual(
-            [x for x, y in book_state.fields],
-            ["id", "user"]
-        )
+        self.assertEqual(list(book_state.fields), ['id', 'user'])
 
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_run_before"})
     def test_run_before(self):
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index bb4b74e727..55c20213b0 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -521,7 +521,10 @@ class OperationTests(OperationTestBase):
         self.assertNotIn(("test_rnmo", "pony"), new_state.models)
         self.assertIn(("test_rnmo", "horse"), new_state.models)
         # RenameModel also repoints all incoming FKs and M2Ms
-        self.assertEqual("test_rnmo.Horse", new_state.models["test_rnmo", "rider"].fields[1][1].remote_field.model)
+        self.assertEqual(
+            new_state.models['test_rnmo', 'rider'].fields['pony'].remote_field.model,
+            'test_rnmo.Horse',
+        )
         self.assertTableNotExists("test_rnmo_pony")
         self.assertTableExists("test_rnmo_horse")
         if connection.features.supports_foreign_keys:
@@ -532,7 +535,10 @@ class OperationTests(OperationTestBase):
         # Test original state and database
         self.assertIn(("test_rnmo", "pony"), original_state.models)
         self.assertNotIn(("test_rnmo", "horse"), original_state.models)
-        self.assertEqual("Pony", original_state.models["test_rnmo", "rider"].fields[1][1].remote_field.model)
+        self.assertEqual(
+            original_state.models['test_rnmo', 'rider'].fields['pony'].remote_field.model,
+            'Pony',
+        )
         self.assertTableExists("test_rnmo_pony")
         self.assertTableNotExists("test_rnmo_horse")
         if connection.features.supports_foreign_keys:
@@ -579,7 +585,7 @@ class OperationTests(OperationTestBase):
         # Remember, RenameModel also repoints all incoming FKs and M2Ms
         self.assertEqual(
             'self',
-            new_state.models["test_rmwsrf", "horserider"].fields[2][1].remote_field.model
+            new_state.models["test_rmwsrf", "horserider"].fields['friend'].remote_field.model
         )
         HorseRider = new_state.apps.get_model('test_rmwsrf', 'horserider')
         self.assertIs(HorseRider._meta.get_field('horserider').remote_field.model, HorseRider)
@@ -621,8 +627,8 @@ class OperationTests(OperationTestBase):
         self.assertIn(("test_rmwsc", "littlehorse"), new_state.models)
         # RenameModel shouldn't repoint the superclass's relations, only local ones
         self.assertEqual(
-            project_state.models["test_rmwsc", "rider"].fields[1][1].remote_field.model,
-            new_state.models["test_rmwsc", "rider"].fields[1][1].remote_field.model
+            project_state.models['test_rmwsc', 'rider'].fields['pony'].remote_field.model,
+            new_state.models['test_rmwsc', 'rider'].fields['pony'].remote_field.model,
         )
         # Before running the migration we have a table for Shetland Pony, not Little Horse
         self.assertTableExists("test_rmwsc_shetlandpony")
@@ -797,10 +803,7 @@ class OperationTests(OperationTestBase):
         self.assertEqual(operation.describe(), "Add field height to Pony")
         project_state, new_state = self.make_test_state("test_adfl", operation)
         self.assertEqual(len(new_state.models["test_adfl", "pony"].fields), 4)
-        field = [
-            f for n, f in new_state.models["test_adfl", "pony"].fields
-            if n == "height"
-        ][0]
+        field = new_state.models['test_adfl', 'pony'].fields['height']
         self.assertEqual(field.default, 5)
         # Test the database alteration
         self.assertColumnNotExists("test_adfl_pony", "height")
@@ -974,10 +977,7 @@ class OperationTests(OperationTestBase):
         new_state = project_state.clone()
         operation.state_forwards("test_adflpd", new_state)
         self.assertEqual(len(new_state.models["test_adflpd", "pony"].fields), 4)
-        field = [
-            f for n, f in new_state.models["test_adflpd", "pony"].fields
-            if n == "height"
-        ][0]
+        field = new_state.models['test_adflpd', 'pony'].fields['height']
         self.assertEqual(field.default, models.NOT_PROVIDED)
         # Test the database alteration
         project_state.apps.get_model("test_adflpd", "pony").objects.create(
@@ -1234,8 +1234,8 @@ class OperationTests(OperationTestBase):
         self.assertEqual(operation.describe(), "Alter field pink on Pony")
         new_state = project_state.clone()
         operation.state_forwards("test_alfl", new_state)
-        self.assertIs(project_state.models["test_alfl", "pony"].get_field_by_name("pink").null, False)
-        self.assertIs(new_state.models["test_alfl", "pony"].get_field_by_name("pink").null, True)
+        self.assertIs(project_state.models['test_alfl', 'pony'].fields['pink'].null, False)
+        self.assertIs(new_state.models['test_alfl', 'pony'].fields['pink'].null, True)
         # Test the database alteration
         self.assertColumnNotNull("test_alfl_pony", "pink")
         with connection.schema_editor() as editor:
@@ -1260,8 +1260,14 @@ class OperationTests(OperationTestBase):
         operation = migrations.AlterField("Pony", "id", models.IntegerField(primary_key=True))
         new_state = project_state.clone()
         operation.state_forwards("test_alflpk", new_state)
-        self.assertIsInstance(project_state.models["test_alflpk", "pony"].get_field_by_name("id"), models.AutoField)
-        self.assertIsInstance(new_state.models["test_alflpk", "pony"].get_field_by_name("id"), models.IntegerField)
+        self.assertIsInstance(
+            project_state.models['test_alflpk', 'pony'].fields['id'],
+            models.AutoField,
+        )
+        self.assertIsInstance(
+            new_state.models['test_alflpk', 'pony'].fields['id'],
+            models.IntegerField,
+        )
         # Test the database alteration
         with connection.schema_editor() as editor:
             operation.database_forwards("test_alflpk", editor, project_state, new_state)
@@ -1289,8 +1295,14 @@ class OperationTests(OperationTestBase):
         operation = migrations.AlterField("Pony", "id", models.FloatField(primary_key=True))
         new_state = project_state.clone()
         operation.state_forwards("test_alflpkfk", new_state)
-        self.assertIsInstance(project_state.models["test_alflpkfk", "pony"].get_field_by_name("id"), models.AutoField)
-        self.assertIsInstance(new_state.models["test_alflpkfk", "pony"].get_field_by_name("id"), models.FloatField)
+        self.assertIsInstance(
+            project_state.models['test_alflpkfk', 'pony'].fields['id'],
+            models.AutoField,
+        )
+        self.assertIsInstance(
+            new_state.models['test_alflpkfk', 'pony'].fields['id'],
+            models.FloatField,
+        )
 
         def assertIdTypeEqualsFkType():
             with connection.cursor() as cursor:
@@ -1479,8 +1491,8 @@ class OperationTests(OperationTestBase):
         self.assertEqual(operation.describe(), "Rename field pink on Pony to blue")
         new_state = project_state.clone()
         operation.state_forwards("test_rnfl", new_state)
-        self.assertIn("blue", [n for n, f in new_state.models["test_rnfl", "pony"].fields])
-        self.assertNotIn("pink", [n for n, f in new_state.models["test_rnfl", "pony"].fields])
+        self.assertIn("blue", new_state.models["test_rnfl", "pony"].fields)
+        self.assertNotIn("pink", new_state.models["test_rnfl", "pony"].fields)
         # Make sure the unique_together has the renamed column too
         self.assertIn("blue", new_state.models["test_rnfl", "pony"].options['unique_together'][0])
         self.assertNotIn("pink", new_state.models["test_rnfl", "pony"].options['unique_together'][0])
@@ -1536,19 +1548,19 @@ class OperationTests(OperationTestBase):
         operation = migrations.RenameField('Model', 'field', 'renamed')
         new_state = state.clone()
         operation.state_forwards('app', new_state)
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[1][1].remote_field.field_name, 'renamed')
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[1][1].from_fields, ['self'])
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[1][1].to_fields, ('renamed',))
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[2][1].from_fields, ('fk',))
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[2][1].to_fields, ('renamed',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fk'].remote_field.field_name, 'renamed')
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fk'].from_fields, ['self'])
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fk'].to_fields, ('renamed',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fo'].from_fields, ('fk',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fo'].to_fields, ('renamed',))
         operation = migrations.RenameField('OtherModel', 'fk', 'renamed_fk')
         new_state = state.clone()
         operation.state_forwards('app', new_state)
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[1][1].remote_field.field_name, 'renamed')
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[1][1].from_fields, ('self',))
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[1][1].to_fields, ('renamed',))
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[2][1].from_fields, ('renamed_fk',))
-        self.assertEqual(new_state.models['app', 'othermodel'].fields[2][1].to_fields, ('renamed',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['renamed_fk'].remote_field.field_name, 'renamed')
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['renamed_fk'].from_fields, ('self',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['renamed_fk'].to_fields, ('renamed',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fo'].from_fields, ('renamed_fk',))
+        self.assertEqual(new_state.models['app', 'othermodel'].fields['fo'].to_fields, ('renamed',))
 
     def test_alter_unique_together(self):
         """
diff --git a/tests/migrations/test_state.py b/tests/migrations/test_state.py
index 3b34376188..40277bf506 100644
--- a/tests/migrations/test_state.py
+++ b/tests/migrations/test_state.py
@@ -121,10 +121,10 @@ class StateTests(SimpleTestCase):
 
         self.assertEqual(author_state.app_label, "migrations")
         self.assertEqual(author_state.name, "Author")
-        self.assertEqual([x for x, y in author_state.fields], ["id", "name", "bio", "age"])
-        self.assertEqual(author_state.fields[1][1].max_length, 255)
-        self.assertIs(author_state.fields[2][1].null, False)
-        self.assertIs(author_state.fields[3][1].null, True)
+        self.assertEqual(list(author_state.fields), ["id", "name", "bio", "age"])
+        self.assertEqual(author_state.fields['name'].max_length, 255)
+        self.assertIs(author_state.fields['bio'].null, False)
+        self.assertIs(author_state.fields['age'].null, True)
         self.assertEqual(
             author_state.options,
             {
@@ -138,10 +138,10 @@ class StateTests(SimpleTestCase):
 
         self.assertEqual(book_state.app_label, "migrations")
         self.assertEqual(book_state.name, "Book")
-        self.assertEqual([x for x, y in book_state.fields], ["id", "title", "author", "contributors"])
-        self.assertEqual(book_state.fields[1][1].max_length, 1000)
-        self.assertIs(book_state.fields[2][1].null, False)
-        self.assertEqual(book_state.fields[3][1].__class__.__name__, "ManyToManyField")
+        self.assertEqual(list(book_state.fields), ["id", "title", "author", "contributors"])
+        self.assertEqual(book_state.fields['title'].max_length, 1000)
+        self.assertIs(book_state.fields['author'].null, False)
+        self.assertEqual(book_state.fields['contributors'].__class__.__name__, 'ManyToManyField')
         self.assertEqual(
             book_state.options,
             {"verbose_name": "tome", "db_table": "test_tome", "indexes": [book_index], "constraints": []},
@@ -150,7 +150,7 @@ class StateTests(SimpleTestCase):
 
         self.assertEqual(author_proxy_state.app_label, "migrations")
         self.assertEqual(author_proxy_state.name, "AuthorProxy")
-        self.assertEqual(author_proxy_state.fields, [])
+        self.assertEqual(author_proxy_state.fields, {})
         self.assertEqual(
             author_proxy_state.options,
             {"proxy": True, "ordering": ["name"], "indexes": [], "constraints": []},
@@ -923,7 +923,7 @@ class StateTests(SimpleTestCase):
         project_state.add_model(ModelState.from_model(Author))
         project_state.add_model(ModelState.from_model(Book))
         self.assertEqual(
-            [name for name, field in project_state.models["migrations", "book"].fields],
+            list(project_state.models['migrations', 'book'].fields),
             ["id", "author"],
         )
 
@@ -1084,10 +1084,10 @@ class ModelStateTests(SimpleTestCase):
         author_state = ModelState.from_model(Author)
         self.assertEqual(author_state.app_label, 'migrations')
         self.assertEqual(author_state.name, 'Author')
-        self.assertEqual([x for x, y in author_state.fields], ['id', 'name', 'bio', 'age'])
-        self.assertEqual(author_state.fields[1][1].max_length, 255)
-        self.assertIs(author_state.fields[2][1].null, False)
-        self.assertIs(author_state.fields[3][1].null, True)
+        self.assertEqual(list(author_state.fields), ['id', 'name', 'bio', 'age'])
+        self.assertEqual(author_state.fields['name'].max_length, 255)
+        self.assertIs(author_state.fields['bio'].null, False)
+        self.assertIs(author_state.fields['age'].null, True)
         self.assertEqual(author_state.options, {'swappable': 'TEST_SWAPPABLE_MODEL', 'indexes': [], "constraints": []})
         self.assertEqual(author_state.bases, (models.Model,))
         self.assertEqual(author_state.managers, [])
@@ -1126,11 +1126,11 @@ class ModelStateTests(SimpleTestCase):
         self.assertEqual(station_state.app_label, 'migrations')
         self.assertEqual(station_state.name, 'BusStation')
         self.assertEqual(
-            [x for x, y in station_state.fields],
+            list(station_state.fields),
             ['searchablelocation_ptr', 'name', 'bus_routes', 'inbound']
         )
-        self.assertEqual(station_state.fields[1][1].max_length, 128)
-        self.assertIs(station_state.fields[2][1].null, False)
+        self.assertEqual(station_state.fields['name'].max_length, 128)
+        self.assertIs(station_state.fields['bus_routes'].null, False)
         self.assertEqual(
             station_state.options,
             {'abstract': False, 'swappable': 'TEST_SWAPPABLE_MODEL', 'indexes': [], 'constraints': []}