[1.9.x] Fixed #25723 -- Made related field checks lookup against their local apps.

Backport of c550beb0cc from master

django/db/models/fields/related.py

@@ -149,7 +149,7 @@ class RelatedField(Field):
         return []
 
     def _check_relation_model_exists(self):
-        rel_is_missing = self.remote_field.model not in apps.get_models()
+        rel_is_missing = self.remote_field.model not in self.opts.apps.get_models()
         rel_is_string = isinstance(self.remote_field.model, six.string_types)
         model_name = self.remote_field.model if rel_is_string else self.remote_field.model._meta.object_name
         if rel_is_missing and (rel_is_string or not self.remote_field.model._meta.swapped):
@@ -165,7 +165,7 @@ class RelatedField(Field):
         return []
 
     def _check_referencing_to_swapped_model(self):
-        if (self.remote_field.model not in apps.get_models() and
+        if (self.remote_field.model not in self.opts.apps.get_models() and
                 not isinstance(self.remote_field.model, six.string_types) and
                 self.remote_field.model._meta.swapped):
             model = "%s.%s" % (
@@ -1206,7 +1206,7 @@ class ManyToManyField(RelatedField):
 
         errors = []
 
-        if self.remote_field.through not in apps.get_models(include_auto_created=True):
+        if self.remote_field.through not in self.opts.apps.get_models(include_auto_created=True):
             # The relationship model is not installed.
             errors.append(
                 checks.Error(

tests/invalid_models_tests/test_relative_fields.py

@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 
 import warnings
 
+from django.apps.registry import Apps
 from django.core.checks import Error, Warning as DjangoWarning
 from django.db import models
 from django.db.models.fields.related import ForeignObject
@@ -132,6 +133,26 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    def test_foreign_key_to_isolated_apps_model(self):
+        """
+        #25723 - Referenced model registration lookup should be run against the
+        field's model registry.
+        """
+        test_apps = Apps(['invalid_models_tests'])
+
+        class OtherModel(models.Model):
+            class Meta:
+                apps = test_apps
+
+        class Model(models.Model):
+            foreign_key = models.ForeignKey('OtherModel', models.CASCADE)
+
+            class Meta:
+                apps = test_apps
+
+        field = Model._meta.get_field('foreign_key')
+        self.assertEqual(field.check(from_model=Model), [])
+
     def test_many_to_many_to_missing_model(self):
         class Model(models.Model):
             m2m = models.ManyToManyField("Rel2")
@@ -149,6 +170,26 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    def test_many_to_many_to_isolated_apps_model(self):
+        """
+        #25723 - Referenced model registration lookup should be run against the
+        field's model registry.
+        """
+        test_apps = Apps(['invalid_models_tests'])
+
+        class OtherModel(models.Model):
+            class Meta:
+                apps = test_apps
+
+        class Model(models.Model):
+            m2m = models.ManyToManyField('OtherModel')
+
+            class Meta:
+                apps = test_apps
+
+        field = Model._meta.get_field('m2m')
+        self.assertEqual(field.check(from_model=Model), [])
+
     def test_many_to_many_with_useless_options(self):
         class Model(models.Model):
             name = models.CharField(max_length=20)
@@ -288,6 +329,33 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    def test_many_to_many_through_isolated_apps_model(self):
+        """
+        #25723 - Through model registration lookup should be run against the
+        field's model registry.
+        """
+        test_apps = Apps(['invalid_models_tests'])
+
+        class GroupMember(models.Model):
+            person = models.ForeignKey('Person', models.CASCADE)
+            group = models.ForeignKey('Group', models.CASCADE)
+
+            class Meta:
+                apps = test_apps
+
+        class Person(models.Model):
+            class Meta:
+                apps = test_apps
+
+        class Group(models.Model):
+            members = models.ManyToManyField('Person', through='GroupMember')
+
+            class Meta:
+                apps = test_apps
+
+        field = Group._meta.get_field('members')
+        self.assertEqual(field.check(from_model=Group), [])
+
     def test_symmetrical_self_referential_field(self):
         class Person(models.Model):
             # Implicit symmetrical=False.