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

1 files changed, 3 insertions, 3 deletions

diff --git a/django/db/backends/schema.py b/django/db/backends/schema.py
index 54e6e06c4f..4a5637c8da 100644
--- a/django/db/backends/schema.py
+++ b/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: