Fixed #14204 -- Enforced SQLite foreign key constraints.

Thanks Tim Graham for contributing to the patch and
Simon Charette for advice and review.

10 files changed, 95 insertions, 26 deletions

diff --git a/django/core/management/commands/flush.py b/django/core/management/commands/flush.py
index 3e9d9fefc9..0df5f67e39 100644
--- a/django/core/management/commands/flush.py
+++ b/django/core/management/commands/flush.py
@@ -5,7 +5,7 @@ from django.apps import apps
 from django.core.management.base import BaseCommand, CommandError
 from django.core.management.color import no_style
 from django.core.management.sql import emit_post_migrate_signal, sql_flush
-from django.db import DEFAULT_DB_ALIAS, connections, transaction
+from django.db import DEFAULT_DB_ALIAS, connections
 
 
 class Command(BaseCommand):
@@ -59,11 +59,7 @@ Are you sure you want to do this?
 
         if confirm == 'yes':
             try:
-                with transaction.atomic(using=database,
-                                        savepoint=connection.features.can_rollback_ddl):
-                    with connection.cursor() as cursor:
-                        for sql in sql_list:
-                            cursor.execute(sql)
+                connection.ops.execute_sql_flush(database, sql_list)
             except Exception as exc:
                 raise CommandError(
                     "Database %s couldn't be flushed. Possible reasons:\n"
diff --git a/django/db/backends/base/operations.py b/django/db/backends/base/operations.py
index cf6b5f9166..3e00f0cce0 100644
--- a/django/db/backends/base/operations.py
+++ b/django/db/backends/base/operations.py
@@ -4,6 +4,7 @@ from importlib import import_module
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
+from django.db import transaction
 from django.db.backends import utils
 from django.utils import timezone
 from django.utils.dateparse import parse_duration
@@ -366,6 +367,13 @@ class BaseDatabaseOperations:
         """
         raise NotImplementedError('subclasses of BaseDatabaseOperations must provide an sql_flush() method')
 
+    def execute_sql_flush(self, using, sql_list):
+        """Execute a list of SQL statements to flush the database."""
+        with transaction.atomic(using=using, savepoint=self.connection.features.can_rollback_ddl):
+            with self.connection.cursor() as cursor:
+                for sql in sql_list:
+                    cursor.execute(sql)
+
     def sequence_reset_by_name_sql(self, style, sequences):
         """
         Return a list of the SQL statements required to reset sequences
diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index ed5f2e869f..2d468a8c66 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -260,13 +260,13 @@ class BaseDatabaseSchemaEditor:
             if field.remote_field and field.db_constraint:
                 to_table = field.remote_field.model._meta.db_table
                 to_column = field.remote_field.model._meta.get_field(field.remote_field.field_name).column
-                if self.connection.features.supports_foreign_keys:
-                    self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
-                elif self.sql_create_inline_fk:
+                if self.sql_create_inline_fk:
                     definition += " " + self.sql_create_inline_fk % {
                         "to_table": self.quote_name(to_table),
                         "to_column": self.quote_name(to_column),
                     }
+                elif self.connection.features.supports_foreign_keys:
+                    self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
             # Add the SQL to our big list
             column_sqls.append("%s %s" % (
                 self.quote_name(field.column),
diff --git a/django/db/backends/sqlite3/base.py b/django/db/backends/sqlite3/base.py
index faa8b16f1f..e820c6bd2e 100644
--- a/django/db/backends/sqlite3/base.py
+++ b/django/db/backends/sqlite3/base.py
@@ -173,6 +173,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         conn.create_function("regexp", 2, _sqlite_regexp)
         conn.create_function("django_format_dtdelta", 3, _sqlite_format_dtdelta)
         conn.create_function("django_power", 2, _sqlite_power)
+        conn.execute('PRAGMA foreign_keys = ON')
         return conn
 
     def init_connection_state(self):
@@ -213,6 +214,16 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         with self.wrap_database_errors:
             self.connection.isolation_level = level
 
+    def disable_constraint_checking(self):
+        if self.in_atomic_block:
+            # sqlite3 cannot disable constraint checking inside a transaction.
+            return False
+        self.cursor().execute('PRAGMA foreign_keys = OFF')
+        return True
+
+    def enable_constraint_checking(self):
+        self.cursor().execute('PRAGMA foreign_keys = ON')
+
     def check_constraints(self, table_names=None):
         """
         Check each table name in `table_names` for rows with invalid foreign
diff --git a/django/db/backends/sqlite3/features.py b/django/db/backends/sqlite3/features.py
index 2209adaa56..80af6068ff 100644
--- a/django/db/backends/sqlite3/features.py
+++ b/django/db/backends/sqlite3/features.py
@@ -17,7 +17,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     max_query_params = 999
     supports_mixed_date_datetime_comparisons = False
     has_bulk_insert = True
-    supports_foreign_keys = False
     supports_column_check_constraints = False
     autocommits_when_autocommit_is_off = True
     can_introspect_decimal_field = False
diff --git a/django/db/backends/sqlite3/introspection.py b/django/db/backends/sqlite3/introspection.py
index 0e87dcf0c6..49c2095a1c 100644
--- a/django/db/backends/sqlite3/introspection.py
+++ b/django/db/backends/sqlite3/introspection.py
@@ -289,4 +289,17 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                 "check": False,
                 "index": False,
             }
+        # Get foreign keys
+        cursor.execute('PRAGMA foreign_key_list(%s)' % self.connection.ops.quote_name(table_name))
+        for row in cursor.fetchall():
+            # Remaining on_update/on_delete/match values are of no interest here
+            id_, seq, table, from_, to = row[:5]
+            constraints['fk_%d' % id_] = {
+                'columns': [from_],
+                'primary_key': False,
+                'unique': False,
+                'foreign_key': (table, to),
+                'check': False,
+                'index': False,
+            }
         return constraints
diff --git a/django/db/backends/sqlite3/operations.py b/django/db/backends/sqlite3/operations.py
index f030c9ed99..28db849828 100644
--- a/django/db/backends/sqlite3/operations.py
+++ b/django/db/backends/sqlite3/operations.py
@@ -149,9 +149,6 @@ class DatabaseOperations(BaseDatabaseOperations):
         return -1
 
     def sql_flush(self, style, tables, sequences, allow_cascade=False):
-        # NB: The generated SQL below is specific to SQLite
-        # Note: The DELETE FROM... SQL generated below works for SQLite databases
-        # because constraints don't exist
         sql = ['%s %s %s;' % (
             style.SQL_KEYWORD('DELETE'),
             style.SQL_KEYWORD('FROM'),
@@ -161,6 +158,12 @@ class DatabaseOperations(BaseDatabaseOperations):
         # sql_flush() implementations). Just return SQL at this point
         return sql
 
+    def execute_sql_flush(self, using, sql_list):
+        # To prevent possible violation of foreign key constraints, deactivate
+        # constraints outside of the transaction created in super().
+        with self.connection.constraint_checks_disabled():
+            super().execute_sql_flush(using, sql_list)
+
     def adapt_datetimefield_value(self, value):
         if value is None:
             return None
diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index 3bf71e071a..93cf077dd2 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -11,26 +11,20 @@ from django.db.backends.ddl_references import Statement
 class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     sql_delete_table = "DROP TABLE %(table)s"
-    sql_create_inline_fk = "REFERENCES %(to_table)s (%(to_column)s)"
+    sql_create_fk = None
+    sql_create_inline_fk = "REFERENCES %(to_table)s (%(to_column)s) DEFERRABLE INITIALLY DEFERRED"
     sql_create_unique = "CREATE UNIQUE INDEX %(name)s ON %(table)s (%(columns)s)"
     sql_delete_unique = "DROP INDEX %(name)s"
 
     def __enter__(self):
-        with self.connection.cursor() as c:
-            # Some SQLite schema alterations need foreign key constraints to be
-            # disabled. This is the default in SQLite but can be changed with a
-            # build flag and might change in future, so can't be relied upon.
-            # Enforce it here for the duration of the transaction.
-            c.execute('PRAGMA foreign_keys')
-            self._initial_pragma_fk = c.fetchone()[0]
-            c.execute('PRAGMA foreign_keys = 0')
+        # Some SQLite schema alterations need foreign key constraints to be
+        # disabled. Enforce it here for the duration of the transaction.
+        self.connection.disable_constraint_checking()
         return super().__enter__()
 
     def __exit__(self, exc_type, exc_value, traceback):
         super().__exit__(exc_type, exc_value, traceback)
-        with self.connection.cursor() as c:
-            # Restore initial FK setting - PRAGMA values can't be parametrized
-            c.execute('PRAGMA foreign_keys = %s' % int(self._initial_pragma_fk))
+        self.connection.enable_constraint_checking()
 
     def quote_value(self, value):
         # The backend "mostly works" without this function and there are use
@@ -259,6 +253,11 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         """Perform a "physical" (non-ManyToMany) field update."""
         # Alter by remaking table
         self._remake_table(model, alter_field=(old_field, new_field))
+        # Rebuild tables with FKs pointing to this field if the PK type changed.
+        if old_field.primary_key and new_field.primary_key and old_type != new_type:
+            for rel in new_field.model._meta.related_objects:
+                if not rel.many_to_many:
+                    self._remake_table(rel.related_model)
 
     def _alter_many_to_many(self, model, old_field, new_field, strict):
         """Alter M2Ms to repoint their to= endpoints."""
diff --git a/docs/releases/2.0.txt b/docs/releases/2.0.txt
index e58c2588cc..03f700163c 100644
--- a/docs/releases/2.0.txt
+++ b/docs/releases/2.0.txt
@@ -435,6 +435,46 @@ raises an exception and should be replaced with::
 
     models.Index(fields=['headline', '-pub_date'], name='index_name')
 
+Foreign key constraints are now enabled on SQLite
+-------------------------------------------------
+
+This will appear as a backwards-incompatible change (``IntegrityError:
+FOREIGN KEY constraint failed``) if attempting to save an existing model
+instance that's violating a foreign key constraint.
+
+Foreign keys are now created with ``DEFERRABLE INITIALLY DEFERRED`` instead of
+``DEFERRABLE IMMEDIATE``. Thus, tables may need to be rebuilt to recreate
+foreign keys with the new definition, particularly if you're using a pattern
+like this::
+
+    from django.db import transaction
+
+    with transaction.atomic():
+        Book.objects.create(author_id=1)
+        Author.objects.create(id=1)
+
+If you don't recreate the foreign key as ``DEFERRED``, the first ``create()``
+would fail now that foreign key constraints are enforced.
+
+Backup your database first! After upgrading to Django 2.0, you can then
+rebuild tables using a script similar to this::
+
+    from django.apps import apps
+    from django.db import connection
+
+    for app in apps.get_app_configs():
+        for model in app.get_models(include_auto_created=True):
+            if model._meta.managed and not (model._meta.proxy or model._meta.swapped):
+                for base in model.__bases__:
+                    if hasattr(base, '_meta'):
+                        base._meta.local_many_to_many = []
+                model._meta.local_many_to_many = []
+                with connection.schema_editor() as editor:
+                    editor._remake_table(model)
+
+This script hasn't received extensive testing and needs adaption for various
+cases such as multiple databases. Feel free to contribute improvements.
+
 Miscellaneous
 -------------
 
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index e2b4f8ca6b..e7f47865d4 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -1953,7 +1953,7 @@ class SchemaTests(TransactionTestCase):
             editor.alter_field(model, get_field(unique=True), field, strict=True)
             self.assertNotIn(constraint_name, self.get_constraints(model._meta.db_table))
 
-            if connection.features.supports_foreign_keys:
+            if editor.sql_create_fk:
                 constraint_name = "CamelCaseFKConstraint"
                 editor.execute(
                     editor.sql_create_fk % {