Resolve any settings.AUTH_USER_MODEL used as to in relation (#1746)

e.g.

```python
OneToOneField(settings.AUTH_USER_MODEL, ...)
ForeignKey(settings.AUTH_USER_MODEL, ...)
ManyToManyField(settings.AUTH_USER_MODEL, ...)
```

mypy_django_plugin/lib/fullnames.py

@@ -12,6 +12,7 @@
 ONETOONE_FIELD_FULLNAME = "django.db.models.fields.related.OneToOneField"
 MANYTOMANY_FIELD_FULLNAME = "django.db.models.fields.related.ManyToManyField"
 DUMMY_SETTINGS_BASE_CLASS = "django.conf._DjangoConfLazyObject"
+AUTH_USER_MODEL_FULLNAME = "django.conf.settings.AUTH_USER_MODEL"
 
 QUERYSET_CLASS_FULLNAME = "django.db.models.query._QuerySet"
 BASE_MANAGER_CLASS_FULLNAME = "django.db.models.manager.BaseManager"

mypy_django_plugin/transformers/manytomany.py

@@ -1,7 +1,7 @@
 from typing import NamedTuple, Optional, Union
 
 from mypy.checker import TypeChecker
-from mypy.nodes import AssignmentStmt, Expression, NameExpr, StrExpr, TypeInfo
+from mypy.nodes import AssignmentStmt, Expression, MemberExpr, NameExpr, StrExpr, TypeInfo
 from mypy.plugin import FunctionContext
 from mypy.semanal import SemanticAnalyzer
 from mypy.types import Instance, ProperType, UninhabitedType
@@ -129,16 +129,25 @@ def get_model_from_expression(
     Attempts to resolve an expression to a 'TypeInfo' instance. Any lazy reference
     argument(e.g. "<app_label>.<object_name>") to a Django model is also attempted.
     """
-    # TODO: Handle settings.AUTH_USER_MODEL?
     if isinstance(expr, NameExpr) and isinstance(expr.node, TypeInfo):
         if (
             expr.node.metaclass_type is not None
             and expr.node.metaclass_type.type.fullname == fullnames.MODEL_METACLASS_FULLNAME
         ):
             return Instance(expr.node, [])
-    elif isinstance(expr, StrExpr):
-        model_info = helpers.resolve_lazy_reference(expr.value, api=api, django_context=django_context, ctx=expr)
+
+    lazy_reference = None
+    if isinstance(expr, StrExpr):
+        lazy_reference = expr.value
+    elif (
+        isinstance(expr, MemberExpr)
+        and isinstance(expr.expr, NameExpr)
+        and f"{expr.expr.fullname}.{expr.name}" == fullnames.AUTH_USER_MODEL_FULLNAME
+    ):
+        lazy_reference = django_context.settings.AUTH_USER_MODEL
+
+    if lazy_reference is not None:
+        model_info = helpers.resolve_lazy_reference(lazy_reference, api=api, django_context=django_context, ctx=expr)
         if model_info is not None:
             return Instance(model_info, [])
-
     return None

tests/typecheck/models/test_contrib_models.yml

@@ -91,3 +91,71 @@
                   class Meta:
                       abstract = False
                       db_table = "auth_user"
+
+-   case: test_relation_specified_by_auth_user_model
+    main: |
+        from other.models import Other
+        reveal_type(Other().users.get())
+        reveal_type(Other.users.through)
+        reveal_type(Other.users.through().myuser)
+        reveal_type(Other.users.through.objects.get().myuser)
+
+        reveal_type(Other().user)
+
+        reveal_type(Other().unq_user)
+    out: |
+        main:2: note: Revealed type is "myapp.models.MyUser"
+        main:3: note: Revealed type is "Type[other.models.Other_users]"
+        main:4: note: Revealed type is "myapp.models.MyUser"
+        main:5: note: Revealed type is "myapp.models.MyUser"
+
+        main:7: note: Revealed type is "myapp.models.MyUser"
+
+        main:9: note: Revealed type is "myapp.models.MyUser"
+    custom_settings: |
+        INSTALLED_APPS = ('django.contrib.contenttypes', 'django.contrib.auth', 'myapp', 'other')
+        AUTH_USER_MODEL='myapp.MyUser'
+    files:
+        - path: myapp/__init__.py
+        - path: myapp/models.py
+          content: |
+              from django.db import models
+
+              class MyUser(models.Model):
+                  ...
+        - path: other/__init__.py
+        - path: other/models.py
+          content: |
+              from django.conf import settings
+              from django.db import models
+
+              class Other(models.Model):
+                  users = models.ManyToManyField(settings.AUTH_USER_MODEL)
+                  user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
+                  unq_user = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
+
+-   case: test_relate_to_auth_user_model_when_auth_not_installed
+    main: |
+        from other.models import Other
+        reveal_type(Other().user)
+    out: |
+        main:2: note: Revealed type is "myapp.models.MyUser"
+    custom_settings: |
+        INSTALLED_APPS = ('django.contrib.contenttypes', 'myapp', 'other')
+        AUTH_USER_MODEL='myapp.MyUser'
+    files:
+        - path: myapp/__init__.py
+        - path: myapp/models.py
+          content: |
+              from django.db import models
+
+              class MyUser(models.Model):
+                  ...
+        - path: other/__init__.py
+        - path: other/models.py
+          content: |
+              from django.conf import settings
+              from django.db import models
+
+              class Other(models.Model):
+                  user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)