Fixed #24591 -- Optimized cloning of ModelState objects.

Changed ModelState.clone() to create a shallow copy of self.fields
and self.managers.

django/db/migrations/state.py

@@ -312,12 +312,23 @@ class ModelState(object):
         # Sanity-check that fields is NOT a dict. It must be ordered.
         if isinstance(self.fields, dict):
             raise ValueError("ModelState.fields cannot be a dict - it must be a list of 2-tuples.")
-        # Sanity-check that fields are NOT already bound to a model.
         for name, field in fields:
+            # Sanity-check that fields are NOT already bound to a model.
             if hasattr(field, 'model'):
                 raise ValueError(
                     'ModelState.fields cannot be bound to a model - "%s" is.' % name
                 )
+            # Sanity-check that relation fields are NOT referring to a model class.
+            if field.is_relation and hasattr(field.related_model, '_meta'):
+                raise ValueError(
+                    'ModelState.fields cannot refer to a model class - "%s.to" does. '
+                    'Use a string reference instead.' % name
+                )
+            if field.many_to_many and hasattr(field.remote_field.through, '_meta'):
+                raise ValueError(
+                    'ModelState.fields cannot refer to a model class - "%s.through" does. '
+                    'Use a string reference instead.' % name
+                )
 
     @cached_property
     def name_lower(self):
@@ -502,10 +513,10 @@ class ModelState(object):
         return self.__class__(
             app_label=self.app_label,
             name=self.name,
-            fields=list(self.construct_fields()),
+            fields=list(self.fields),
             options=dict(self.options),
             bases=self.bases,
-            managers=list(self.construct_managers()),
+            managers=list(self.managers),
         )
 
     def render(self, apps):