[1.10.x] Fixed #25292 -- Fixed crash in ManyToManyField.through_fields check.

Backport of baff4dd37d from master

django/db/models/base.py

@@ -1685,6 +1685,10 @@ class Model(six.with_metaclass(ModelBase)):
                 )
 
         for f in cls._meta.local_many_to_many:
+            # Skip nonexistent models.
+            if isinstance(f.remote_field.through, six.string_types):
+                continue
+
             # Check if auto-generated name for the M2M field is too long
             # for the database.
             for m2m in f.remote_field.through._meta.local_fields:

tests/invalid_models_tests/test_relative_fields.py

@@ -307,6 +307,22 @@ class RelativeFieldTests(SimpleTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    def test_missing_relationship_model_on_model_check(self):
+        class Person(models.Model):
+            pass
+
+        class Group(models.Model):
+            members = models.ManyToManyField('Person', through='MissingM2MModel')
+
+        self.assertEqual(Group.check(), [
+            Error(
+                "Field specifies a many-to-many relation through model "
+                "'MissingM2MModel', which has not been installed.",
+                obj=Group._meta.get_field('members'),
+                id='fields.E331',
+            ),
+        ])
+
     @isolate_apps('invalid_models_tests')
     def test_many_to_many_through_isolate_apps_model(self):
         """