Adds better id and pk handling (#791)

tests/typecheck/models/test_primary_key.yml

@@ -0,0 +1,66 @@
+- case: test_access_to_id_field_through_self_if_no_primary_key_defined
+  main: |
+    from myapp.models import MyModel
+    x = MyModel.objects.get(id=1)
+    reveal_type(x.id)  # N: Revealed type is "builtins.int*"
+    reveal_type(x.pk)  # N: Revealed type is "builtins.int*"
+
+    MyModel.objects.get(pk=1)
+  installed_apps:
+    - myapp
+  files:
+    - path: myapp/__init__.py
+    - path: myapp/models.py
+      content: |
+        from django.db import models
+        class MyModel(models.Model):
+            def __str__(self):
+                reveal_type(self.id)  # N: Revealed type is "builtins.int*"
+                reveal_type(self.pk)  # N: Revealed type is "builtins.int*"
+
+
+- case: test_access_to_id_field_through_self_if_primary_key_is_defined
+  main: |
+    from myapp.models import MyModel
+    x = MyModel.objects.get(id='a')
+    reveal_type(x.id)  # N: Revealed type is "builtins.str*"
+    reveal_type(x.pk)  # N: Revealed type is "builtins.str*"
+
+    MyModel.objects.get(pk='a')
+  installed_apps:
+    - myapp
+  files:
+    - path: myapp/__init__.py
+    - path: myapp/models.py
+      content: |
+        from django.db import models
+        class MyModel(models.Model):
+            id = models.CharField(max_length=10, primary_key=True)
+            def __str__(self):
+                reveal_type(self.id)  # N: Revealed type is "builtins.str*"
+                reveal_type(self.pk)  # N: Revealed type is "builtins.str*"
+
+
+- case: test_access_to_id_field_through_self_if_primary_key_has_different_name
+  main: |
+    from myapp.models import MyModel
+    x = MyModel.objects.get(primary='a')
+    reveal_type(x.primary)  # N: Revealed type is "builtins.str*"
+    reveal_type(x.pk)  # N: Revealed type is "builtins.str*"
+    x.id  # E: "MyModel" has no attribute "id"
+
+    MyModel.objects.get(pk='a')
+    MyModel.objects.get(id='a')  # E: Cannot resolve keyword 'id' into field. Choices are: primary
+  installed_apps:
+    - myapp
+  files:
+    - path: myapp/__init__.py
+    - path: myapp/models.py
+      content: |
+        from django.db import models
+        class MyModel(models.Model):
+            primary = models.CharField(max_length=10, primary_key=True)
+            def __str__(self):
+                reveal_type(self.primary)  # N: Revealed type is "builtins.str*"
+                reveal_type(self.pk)  # N: Revealed type is "builtins.str*"
+                self.id  # E: "MyModel" has no attribute "id"