Models of apps w/o migrations don't have relational fields to other apps w/o migrations when applying migrations

[Markus Holtermann]

Consider the three apps w_migrations, wo_migrations1 and wo_migrations2 with the following models:

w_migrations:

class With(models.Model):
    without1 = models.ForeignKey('wo_migrations1.Without1')

wo_migrations1:

class Without1(models.Model):
    without2 = models.ForeignKey('wo_migrations2.Without2')

wo_migrations2:

class Without2(models.Model):
    i = models.IntegerField()

When you create the migration for w_migrations and add an additional RunPython operation:

def forward(apps, schema_editor):
    print(apps.get_model('wo_migrations1', 'Without1')._meta._get_fields())


operations = [
    ...,
    migrations.RunPython(forward, migrations.RunPython.noop),
]

you will get the following output:

(<django.db.models.fields.AutoField: id>,)

However, one would expect to see the ForeignKey to wo_migrations2, too.

A fix should be rather simple by updating django.db.migrations.state.ModelState.from_model() similar to those lines:

django/db/migrations/state.py

@@ -219 +219 @@
         for app_label in real_apps:
             app = global_apps.get_app_config(app_label)
             for model in app.get_models():
-                self.real_models.append(ModelState.from_model(model, exclude_rels=True))
+                self.real_models.append(ModelState.from_model(model, exclude_rels=True, allow_rels_to=real_apps))
         # Populate the app registry with a stub for each application.
         app_labels = {model_state.app_label for model_state in models.values()}
         app_configs = [AppConfigStub(label) for label in sorted(real_apps + list(app_labels))]
@@ -346 +346 @@
         return self.name.lower()
 
     @classmethod
-    def from_model(cls, model, exclude_rels=False):
+    def from_model(cls, model, exclude_rels=False, allow_rels_to=[]):
         """
         Feed me a model, get a ModelState representing it out.
         """
         # Deconstruct the fields
         fields = []
         for field in model._meta.local_fields:
-            if getattr(field, "remote_field", None) and exclude_rels:
+            if (exclude_rels and field.is_relation and
+                    _get_app_label_and_model_name(field.related_model)[0] not in allow_rels_to):
                 continue
             if isinstance(field, OrderWrt):
                 continue
@@ -367 +368 @@
                     model._meta.label,
                     e,
                 ))
-        if not exclude_rels:
-            for field in model._meta.local_many_to_many:
-                name, path, args, kwargs = field.deconstruct()
-                field_class = import_string(path)
-                try:
-                    fields.append((name, field_class(*args, **kwargs)))
-                except TypeError as e:
-                    raise TypeError("Couldn't reconstruct m2m field %s on %s: %s" % (
-                        name,
-                        model._meta.object_name,
-                        e,
-                    ))
+        for field in model._meta.local_many_to_many:
+            if (exclude_rels and field.is_relation and
+                    _get_app_label_and_model_name(field.related_model)[0] not in allow_rels_to):
+                continue
+            name, path, args, kwargs = field.deconstruct()
+            field_class = import_string(path)
+            try:
+                fields.append((name, field_class(*args, **kwargs)))
+            except TypeError as e:
+                raise TypeError("Couldn't reconstruct m2m field %s on %s: %s" % (
+                    name,
+                    model._meta.object_name,
+                    e,
+                ))
         # Extract the options
         options = {}
         for name in DEFAULT_NAMES:

I believe this bug is also present on 1.8, haven't investigated though. If it is, it is I think it is a regression from 1.7 to 1.8.

[Tim Graham]

Don't you need to add a dependency to wo_migrations1 on w_migrations if you want to use its models? In which case, you have the case of migrated apps depending on unmigrated apps which isn't supported.

​https://docs.djangoproject.com/en/1.8/topics/migrations/#accessing-models-from-other-apps

See also #24853 and the docs added in that ticket.