Fixed #25535 -- Made ForeignObject checks less strict.

Check that the foreign object `from_fields` are a subset of any unique
constraints on the foreign model.

django/db/models/fields/related.py

@@ -464,22 +464,35 @@ class ForeignObject(RelatedField):
         if not self.foreign_related_fields:
             return []
 
-        has_unique_field = any(rel_field.unique
-            for rel_field in self.foreign_related_fields)
-        if not has_unique_field and len(self.foreign_related_fields) > 1:
+        unique_foreign_fields = {
+            frozenset([f.name])
+            for f in self.remote_field.model._meta.get_fields()
+            if getattr(f, 'unique', False)
+        }
+        unique_foreign_fields.update({
+            frozenset(ut)
+            for ut in self.remote_field.model._meta.unique_together
+        })
+        foreign_fields = {f.name for f in self.foreign_related_fields}
+
+        has_unique_constraint = any(u <= foreign_fields for u in unique_foreign_fields)
+
+        if not has_unique_constraint and len(self.foreign_related_fields) > 1:
             field_combination = ', '.join("'%s'" % rel_field.name
                 for rel_field in self.foreign_related_fields)
             model_name = self.remote_field.model.__name__
             return [
                 checks.Error(
-                    "None of the fields %s on model '%s' have a unique=True constraint."
+                    "No subset of the fields %s on model '%s' is unique"
                     % (field_combination, model_name),
-                    hint=None,
+                    hint="Add a unique=True on any of the fields %s of '%s', or add them all or a "
+                         "subset to a unique_together constraint."
+                         % (field_combination, model_name),
                     obj=self,
                     id='fields.E310',
                 )
             ]
-        elif not has_unique_field:
+        elif not has_unique_constraint:
             field_name = self.foreign_related_fields[0].name
             model_name = self.remote_field.model.__name__
             return [

tests/foreign_object/tests.py

@@ -2,6 +2,8 @@ import datetime
 from operator import attrgetter
 
 from django.core.exceptions import FieldError
+from django.db import models
+from django.db.models.fields.related import ForeignObject
 from django.test import TestCase, skipUnlessDBFeature
 from django.utils import translation
 
@@ -391,3 +393,61 @@ class MultiColumnFKTests(TestCase):
         """ See: https://code.djangoproject.com/ticket/21566 """
         objs = [Person(name="abcd_%s" % i, person_country=self.usa) for i in range(0, 5)]
         Person.objects.bulk_create(objs, 10)
+
+    def test_check_composite_foreign_object(self):
+        class Parent(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+
+            class Meta:
+                unique_together = (
+                    ('a', 'b'),
+                )
+
+        class Child(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            value = models.CharField(max_length=255)
+
+            parent = ForeignObject(
+                Parent,
+                on_delete=models.SET_NULL,
+                from_fields=('a', 'b'),
+                to_fields=('a', 'b'),
+                related_name='children',
+            )
+
+        field = Child._meta.get_field('parent')
+        errors = field.check(from_model=Child)
+
+        self.assertEqual(errors, [])
+
+    def test_check_subset_composite_foreign_object(self):
+        class Parent(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            c = models.PositiveIntegerField()
+
+            class Meta:
+                unique_together = (
+                    ('a', 'b'),
+                )
+
+        class Child(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            c = models.PositiveIntegerField()
+            d = models.CharField(max_length=255)
+
+            parent = ForeignObject(
+                Parent,
+                on_delete=models.SET_NULL,
+                from_fields=('a', 'b', 'c'),
+                to_fields=('a', 'b', 'c'),
+                related_name='children',
+            )
+
+        field = Child._meta.get_field('parent')
+        errors = field.check(from_model=Child)
+
+        self.assertEqual(errors, [])

tests/invalid_models_tests/test_relative_fields.py

@@ -5,6 +5,7 @@ import warnings
 
 from django.core.checks import Error, Warning as DjangoWarning
 from django.db import models
+from django.db.models.fields.related import ForeignObject
 from django.test import ignore_warnings
 from django.test.testcases import skipIfDBFeature
 from django.test.utils import override_settings
@@ -507,9 +508,9 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         errors = field.check()
         expected = [
             Error(
-                "None of the fields 'country_id', 'city_id' on model 'Person' "
-                "have a unique=True constraint.",
-                hint=None,
+                "No subset of the fields 'country_id', 'city_id' on model 'Person' is unique",
+                hint="Add a unique=True on any of the fields 'country_id', 'city_id' of 'Person', "
+                     "or add them all or a subset to a unique_together constraint.",
                 obj=field,
                 id='fields.E310',
             )
@@ -1395,3 +1396,79 @@ class M2mThroughFieldsTests(IsolatedModelsTestCase):
                 obj=field,
                 id='fields.E337')]
         self.assertEqual(expected, errors)
+
+    def test_superset_foreign_object(self):
+        class Parent(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            c = models.PositiveIntegerField()
+
+            class Meta:
+                unique_together = (
+                    ('a', 'b', 'c'),
+                )
+
+        class Child(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            value = models.CharField(max_length=255)
+
+            parent = ForeignObject(
+                Parent,
+                on_delete=models.SET_NULL,
+                from_fields=('a', 'b'),
+                to_fields=('a', 'b'),
+                related_name='children',
+            )
+
+        field = Child._meta.get_field('parent')
+        errors = field.check(from_model=Child)
+        expected = [
+            Error(
+                "No subset of the fields 'a', 'b' on model 'Parent' is unique",
+                hint="Add a unique=True on any of the fields 'a', 'b' of 'Parent', or add them "
+                     "all or a subset to a unique_together constraint.",
+                obj=field,
+                id='fields.E310',
+            ),
+        ]
+        self.assertEqual(expected, errors)
+
+    def test_insersection_foreign_object(self):
+        class Parent(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            c = models.PositiveIntegerField()
+            d = models.PositiveIntegerField()
+
+            class Meta:
+                unique_together = (
+                    ('a', 'b', 'c'),
+                )
+
+        class Child(models.Model):
+            a = models.PositiveIntegerField()
+            b = models.PositiveIntegerField()
+            d = models.PositiveIntegerField()
+            value = models.CharField(max_length=255)
+
+            parent = ForeignObject(
+                Parent,
+                on_delete=models.SET_NULL,
+                from_fields=('a', 'b', 'd'),
+                to_fields=('a', 'b', 'd'),
+                related_name='children',
+            )
+
+        field = Child._meta.get_field('parent')
+        errors = field.check(from_model=Child)
+        expected = [
+            Error(
+                "No subset of the fields 'a', 'b', 'd' on model 'Parent' is unique",
+                hint="Add a unique=True on any of the fields 'a', 'b', 'd' of 'Parent', or add "
+                     "them all or a subset to a unique_together constraint.",
+                obj=field,
+                id='fields.E310',
+            ),
+        ]
+        self.assertEqual(expected, errors)