Repoint ForeignKeys when their to= changes.

django/db/backends/postgresql_psycopg2/introspection.py

@@ -118,7 +118,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                     "columns": set(),
                     "primary_key": kind.lower() == "primary key",
                     "unique": kind.lower() in ["primary key", "unique"],
-                    "foreign_key": set([tuple(x.split(".", 1)) for x in used_cols]) if kind.lower() == "foreign key" else None,
+                    "foreign_key": tuple(used_cols[0].split(".", 1)) if kind.lower() == "foreign key" else None,
                     "check": False,
                     "index": False,
                 }

django/db/backends/schema.py

@@ -21,7 +21,6 @@ class BaseDatabaseSchemaEditor(object):
     commit() is called.
 
     TODO:
-        - Repointing of FKs
         - Repointing of M2Ms
         - Check constraints (PosIntField)
     """
@@ -401,6 +400,22 @@ class BaseDatabaseSchemaEditor(object):
                         "name": index_name,
                     }
                 )
+        # Drop any FK constraints, we'll remake them later
+        if getattr(old_field, "rel"):
+            fk_names = self._constraint_names(model, [old_field.column], foreign_key=True)
+            if strict and len(fk_names) != 1:
+                raise ValueError("Found wrong number (%s) of foreign key constraints for %s.%s" % (
+                    len(fk_names),
+                    model._meta.db_table,
+                    old_field.column,
+                ))
+            for fk_name in fk_names:
+                self.execute(
+                    self.sql_delete_fk % {
+                        "table": self.quote_name(model._meta.db_table),
+                        "name": fk_name,
+                    }
+                )
         # Have they renamed the column?
         if old_field.column != new_field.column:
             self.execute(self.sql_rename_column % {
@@ -516,6 +531,17 @@ class BaseDatabaseSchemaEditor(object):
                     "columns": self.quote_name(new_field.column),
                 }
             )
+        # Does it have a foreign key?
+        if getattr(new_field, "rel"):
+            self.execute(
+                self.sql_create_fk % {
+                    "table": self.quote_name(model._meta.db_table),
+                    "name": self._create_index_name(model, [new_field.column], suffix="_fk"),
+                    "column": self.quote_name(new_field.column),
+                    "to_table": self.quote_name(new_field.rel.to._meta.db_table),
+                    "to_column": self.quote_name(new_field.rel.get_related_field().column),
+                }
+            )
 
     def _type_for_alter(self, field):
         """
@@ -543,7 +569,7 @@ class BaseDatabaseSchemaEditor(object):
             index_name = '%s%s' % (table_name[:(self.connection.features.max_index_name_length - len(part))], part)
         return index_name
 
-    def _constraint_names(self, model, column_names=None, unique=None, primary_key=None, index=None):
+    def _constraint_names(self, model, column_names=None, unique=None, primary_key=None, index=None, foreign_key=None):
         "Returns all constraint names matching the columns and conditions"
         column_names = set(column_names) if column_names else None
         constraints = self.connection.introspection.get_constraints(self.connection.cursor(), model._meta.db_table)
@@ -556,5 +582,7 @@ class BaseDatabaseSchemaEditor(object):
                     continue
                 if index is not None and infodict['index'] != index:
                     continue
+                if foreign_key is not None and not infodict['foreign_key']:
+                    continue
                 result.append(name)
         return result

tests/modeltests/schema/tests.py

@@ -5,7 +5,7 @@ from django.test import TestCase
 from django.utils.unittest import skipUnless
 from django.db import connection, DatabaseError, IntegrityError
 from django.db.models.fields import IntegerField, TextField, CharField, SlugField
-from django.db.models.fields.related import ManyToManyField
+from django.db.models.fields.related import ManyToManyField, ForeignKey
 from django.db.models.loading import cache
 from .models import Author, Book, BookWithSlug, AuthorWithM2M, Tag, TagUniqueRename, UniqueTest
 
@@ -114,15 +114,16 @@ class SchemaTests(TestCase):
         )
 
     @skipUnless(connection.features.supports_foreign_keys, "No FK support")
-    def test_creation_fk(self):
-        "Tests that creating tables out of FK order works"
+    def test_fk(self):
+        "Tests that creating tables out of FK order, then repointing, works"
         # Create the table
         editor = connection.schema_editor()
         editor.start()
         editor.create_model(Book)
         editor.create_model(Author)
+        editor.create_model(Tag)
         editor.commit()
-        # Check that both tables are there
+        # Check that initial tables are there
         try:
             list(Author.objects.all())
         except DatabaseError, e:
@@ -139,6 +140,26 @@ class SchemaTests(TestCase):
                 pub_date = datetime.datetime.now(),
             )
             connection.commit()
+        # Repoint the FK constraint
+        new_field = ForeignKey(Tag)
+        new_field.set_attributes_from_name("author")
+        editor = connection.schema_editor()
+        editor.start()
+        editor.alter_field(
+            Book,
+            Book._meta.get_field_by_name("author")[0],
+            new_field,
+            strict=True,
+        )
+        editor.commit()
+        # Make sure the new FK constraint is present
+        constraints = connection.introspection.get_constraints(connection.cursor(), Book._meta.db_table)
+        for name, details in constraints.items():
+            if details['columns'] == set(["author_id"]) and details['foreign_key']:
+                self.assertEqual(details['foreign_key'], ('schema_tag', 'id'))
+                break
+        else:
+            self.fail("No FK constraint for author_id found")
 
     def test_create_field(self):
         """