From 21130ce1a9c4fcbfce4c614be9e5408b43092bf0 Mon Sep 17 00:00:00 2001
From: Sergey Fedoseev <fedoseev.sergey@gmail.com>
Date: Tue, 7 Jun 2016 13:55:27 +0500
Subject: [PATCH] Fixed #26718 -- Added system check for existence of the
 fields specified by ForeignKey.to_field.

---
 django/db/models/fields/related.py            | 23 +++++++++
 docs/ref/checks.txt                           |  2 +
 .../test_relative_fields.py                   | 50 +++++++++++++++++++
 3 files changed, 75 insertions(+)

diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 44e053847a..7caba63c3e 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -463,9 +463,32 @@ class ForeignObject(RelatedField):
 
     def check(self, **kwargs):
         errors = super(ForeignObject, self).check(**kwargs)
+        errors.extend(self._check_to_fields_exist())
         errors.extend(self._check_unique_target())
         return errors
 
+    def _check_to_fields_exist(self):
+        # Skip nonexistent models.
+        if isinstance(self.remote_field.model, six.string_types):
+            return []
+
+        errors = []
+        for to_field in self.to_fields:
+            if to_field:
+                try:
+                    self.remote_field.model._meta.get_field(to_field)
+                except exceptions.FieldDoesNotExist:
+                    errors.append(
+                        checks.Error(
+                            "The to_field '%s' doesn't exist on the related "
+                            "model '%s'."
+                            % (to_field, self.remote_field.model._meta.label),
+                            obj=self,
+                            id='fields.E312',
+                        )
+                    )
+        return errors
+
     def _check_unique_target(self):
         rel_is_string = isinstance(self.remote_field.model, six.string_types)
         if rel_is_string or not self.requires_unique_target:
diff --git a/docs/ref/checks.txt b/docs/ref/checks.txt
index ac96fbf412..18b6d88add 100644
--- a/docs/ref/checks.txt
+++ b/docs/ref/checks.txt
@@ -211,6 +211,8 @@ Related Fields
   add at least a subset of them to a unique_together constraint.
 * **fields.E311**: ``<model>`` must set ``unique=True`` because it is
   referenced by a ``ForeignKey``.
+* **fields.E312**: The ``to_field`` ``<field name>`` doesn't exist on the
+  related model ``<app label>.<model>``.
 * **fields.E320**: Field specifies ``on_delete=SET_NULL``, but cannot be null.
 * **fields.E321**: The field specifies ``on_delete=SET_DEFAULT``, but has no
   default value.
diff --git a/tests/invalid_models_tests/test_relative_fields.py b/tests/invalid_models_tests/test_relative_fields.py
index 8b95cf3e18..2534b345c2 100644
--- a/tests/invalid_models_tests/test_relative_fields.py
+++ b/tests/invalid_models_tests/test_relative_fields.py
@@ -763,6 +763,56 @@ class RelativeFieldTests(SimpleTestCase):
             errors = Child.check()
             self.assertFalse(errors)
 
+    def test_to_fields_exist(self):
+        class Parent(models.Model):
+            pass
+
+        class Child(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            parent = ForeignObject(
+                Parent,
+                on_delete=models.SET_NULL,
+                from_fields=('a', 'b'),
+                to_fields=('a', 'b'),
+            )
+
+        field = Child._meta.get_field('parent')
+        expected = [
+            Error(
+                "The to_field 'a' doesn't exist on the related model 'invalid_models_tests.Parent'.",
+                obj=field,
+                id='fields.E312',
+            ),
+            Error(
+                "The to_field 'b' doesn't exist on the related model 'invalid_models_tests.Parent'.",
+                obj=field,
+                id='fields.E312',
+            ),
+        ]
+        self.assertEqual(field.check(), expected)
+
+    def test_to_fields_not_checked_if_related_model_doesnt_exist(self):
+        class Child(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            parent = ForeignObject(
+                'invalid_models_tests.Parent',
+                on_delete=models.SET_NULL,
+                from_fields=('a', 'b'),
+                to_fields=('a', 'b'),
+            )
+
+        field = Child._meta.get_field('parent')
+        self.assertEqual(field.check(), [
+            Error(
+                "Field defines a relation with model 'invalid_models_tests.Parent', "
+                "which is either not installed, or is abstract.",
+                id='fields.E300',
+                obj=field,
+            ),
+        ])
+
 
 @isolate_apps('invalid_models_tests')
 class AccessorClashTests(SimpleTestCase):