Fixed #24590 -- Cached calls to swappable_setting.

Moved the lookup in Field.swappable_setting to Apps, and added
an lru_cache to cache the results.

Refs #24743

Thanks Marten Kenbeek for the initial work on the patch. Thanks Aymeric
Augustin and Tim Graham for the review.

4 files changed, 36 insertions, 13 deletions

diff --git a/django/apps/registry.py b/django/apps/registry.py
index 269787545d..46c10ef4fb 100644
--- a/django/apps/registry.py
+++ b/django/apps/registry.py
@@ -260,6 +260,28 @@ class Apps(object):
                 "Model '%s.%s' not registered." % (app_label, model_name))
         return model
 
+    @lru_cache.lru_cache(maxsize=None)
+    def get_swappable_settings_name(self, to_string):
+        """
+        For a given model string (e.g. "auth.User"), return the name of the
+        corresponding settings name if it refers to a swappable model. If the
+        referred model is not swappable, return None.
+
+        This method is decorated with lru_cache because it's performance
+        critical when it comes to migrations. Since the swappable settings don't
+        change after Django has loaded the settings, there is no reason to get
+        the respective settings attribute over and over again.
+        """
+        for model in self.get_models(include_swapped=True):
+            swapped = model._meta.swapped
+            # Is this model swapped out for the model given by to_string?
+            if swapped and swapped == to_string:
+                return model._meta.swappable
+            # Is this model swappable and the one given by to_string?
+            if model._meta.swappable and model._meta.label == to_string:
+                return model._meta.swappable
+        return None
+
     def set_available_apps(self, available):
         """
         Restricts the set of installed apps used by get_app_config[s].
diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index f61e557aac..c433ce239d 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -314,17 +314,8 @@ class RelatedField(Field):
             if isinstance(self.remote_field.model, six.string_types):
                 to_string = self.remote_field.model
             else:
-                to_string = "%s.%s" % (
-                    self.remote_field.model._meta.app_label,
-                    self.remote_field.model._meta.object_name,
-                )
-            # See if anything swapped/swappable matches
-            for model in apps.get_models(include_swapped=True):
-                if model._meta.swapped:
-                    if model._meta.swapped == to_string:
-                        return model._meta.swappable
-                if ("%s.%s" % (model._meta.app_label, model._meta.object_name)) == to_string and model._meta.swappable:
-                    return model._meta.swappable
+                to_string = self.remote_field.model._meta.label
+            return apps.get_swappable_settings_name(to_string)
         return None
 
     def set_attributes_from_rel(self):
diff --git a/django/db/models/options.py b/django/db/models/options.py
index 1406ab9441..bc5a95e750 100644
--- a/django/db/models/options.py
+++ b/django/db/models/options.py
@@ -396,7 +396,7 @@ class Options(object):
                     # or as part of validation.
                     return swapped_for
 
-                if '%s.%s' % (swapped_label, swapped_object.lower()) not in (None, self.label_lower):
+                if '%s.%s' % (swapped_label, swapped_object.lower()) != self.label_lower:
                     return swapped_for
         return None
 
diff --git a/django/test/utils.py b/django/test/utils.py
index da77d8ef4e..415f17237b 100644
--- a/django/test/utils.py
+++ b/django/test/utils.py
@@ -29,7 +29,7 @@ except ImportError:
 
 
 __all__ = (
-    'Approximate', 'ContextList', 'get_runner',
+    'Approximate', 'ContextList', 'isolate_lru_cache', 'get_runner',
     'modify_settings', 'override_settings',
     'requires_tz_support',
     'setup_test_environment', 'teardown_test_environment',
@@ -504,6 +504,16 @@ def extend_sys_path(*paths):
 
 
 @contextmanager
+def isolate_lru_cache(lru_cache_object):
+    """Clear the cache of an LRU cache object on entering and exiting."""
+    lru_cache_object.cache_clear()
+    try:
+        yield
+    finally:
+        lru_cache_object.cache_clear()
+
+
+@contextmanager
 def captured_output(stream_name):
     """Return a context manager used by captured_stdout/stdin/stderr
     that temporarily replaces the sys stream *stream_name* with a StringIO.