Fixed #24447 -- Made migrations add FK constraints for existing columns

When altering from e.g. an IntegerField to a ForeignKey, Django didn't
add a constraint.

Backport of f4f0060feaee6bbd76a0d575487682bc541111e4 from master

django/db/backends/schema.py

@@ -675,9 +675,9 @@ class BaseDatabaseSchemaEditor(object):
                 }
             )
         # Does it have a foreign key?
-        if new_field.rel and \
-           (fks_dropped or (old_field.rel and not old_field.db_constraint)) and \
-           new_field.db_constraint:
+        if (new_field.rel and
+                (fks_dropped or not old_field.rel or not old_field.db_constraint) and
+                new_field.db_constraint):
             self.execute(self._create_fk_sql(model, new_field, "_fk_%(to_table)s_%(to_column)s"))
         # Rebuild FKs that pointed to us if we previously had to drop them
         if old_field.primary_key and new_field.primary_key and old_type != new_type:

docs/releases/1.7.6.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed crash when coercing ``ManyRelatedManager`` to a string
   (:ticket:`24352`).
+
+* Fixed a bug that prevented migrations from adding a FK constraint
+  for an existing column. (:ticket:`24447`).

tests/schema/models.py

@@ -87,6 +87,15 @@ class BookWithM2M(models.Model):
         apps = new_apps
 
 
+class BookWithoutFK(models.Model):
+    author = models.IntegerField()
+    title = models.CharField(max_length=100, db_index=True)
+    pub_date = models.DateTimeField()
+
+    class Meta:
+        apps = new_apps
+
+
 class TagThrough(models.Model):
     book = models.ForeignKey("schema.BookWithM2MThrough")
     tag = models.ForeignKey("schema.TagM2MTest")

tests/schema/tests.py

@@ -11,7 +11,7 @@ from .fields import CustomManyToManyField, InheritedManyToManyField
 from .models import (Author, AuthorWithDefaultHeight, AuthorWithM2M, Book, BookWithLongName,
     BookWithSlug, BookWithM2M, Tag, TagIndexed, TagM2MTest, TagUniqueRename,
     UniqueTest, Thing, TagThrough, BookWithM2MThrough, AuthorTag, AuthorWithM2MThrough,
-    AuthorWithEvenLongerName, BookWeak, Note, BookWithO2O)
+    AuthorWithEvenLongerName, BookWeak, Note, BookWithO2O, BookWithoutFK)
 
 
 class SchemaTests(TransactionTestCase):
@@ -29,7 +29,7 @@ class SchemaTests(TransactionTestCase):
         Author, AuthorWithM2M, Book, BookWithLongName, BookWithSlug,
         BookWithM2M, Tag, TagIndexed, TagM2MTest, TagUniqueRename, UniqueTest,
         Thing, TagThrough, BookWithM2MThrough, AuthorWithEvenLongerName,
-        BookWeak, BookWithO2O,
+        BookWeak, BookWithO2O, BookWithoutFK,
     ]
 
     # Utility functions
@@ -536,6 +536,38 @@ class SchemaTests(TransactionTestCase):
         else:
             self.fail("No FK constraint for author_id found")
 
+    @unittest.skipUnless(connection.features.supports_foreign_keys, "No FK support")
+    def test_alter_to_fk(self):
+        """
+        #24447 - Tests adding a FK constraint for an existing column
+        """
+        # Create the tables
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(BookWithoutFK)
+        # Ensure no FK constraint exists
+        constraints = self.get_constraints(BookWithoutFK._meta.db_table)
+        for name, details in constraints.items():
+            if details['foreign_key']:
+                self.fail('Found an unexpected FK constraint to %s' % details['columns'])
+        new_field = ForeignKey(Author)
+        new_field.set_attributes_from_name("author")
+        with connection.schema_editor() as editor:
+            editor.alter_field(
+                BookWithoutFK,
+                BookWithoutFK._meta.get_field_by_name("author")[0],
+                new_field,
+                strict=True,
+            )
+        constraints = self.get_constraints(BookWithoutFK._meta.db_table)
+        # Ensure FK constraint exists
+        for name, details in constraints.items():
+            if details['foreign_key'] and details['columns'] == ["author_id"]:
+                self.assertEqual(details['foreign_key'], ('schema_author', 'id'))
+                break
+        else:
+            self.fail("No FK constraint for author_id found")
+
     @unittest.skipUnless(connection.features.supports_foreign_keys, "No FK support")
     def test_alter_o2o_to_fk(self):
         """