Fixes #18896. Add tests verifying that you can get IntegrityErrors using get_or_create through relations like M2M, and it also adds a note into the documentation warning about it

docs/ref/models/querysets.txt

@@ -1409,6 +1409,41 @@ has a side effect on your data. For more, see `Safe methods`_ in the HTTP spec.
 
 .. _Safe methods: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1
 
+.. warning::
+
+  You can use ``get_or_create()`` through :class:`~django.db.models.ManyToManyField`
+  attributes and reverse relations. In that case you will restrict the queries
+  inside the context of that relation. That could lead you to some integrity
+  problems if you don't use it consistently.
+
+  Being the following models::
+
+      class Chapter(models.Model):
+          title = models.CharField(max_length=255, unique=True)
+
+      class Book(models.Model):
+          title = models.CharField(max_length=256)
+          chapters = models.ManyToManyField(Chapter)
+
+  You can use ``get_or_create()`` through Book's chapters field, but it only
+  fetches inside the context of that book::
+
+      >>> book = Book.objects.create(title="Ulysses")
+      >>> book.chapters.get_or_create(title="Telemachus")
+      (<Chapter: Telemachus>, True)
+      >>> book.chapters.get_or_create(title="Telemachus")
+      (<Chapter: Telemachus>, False)
+      >>> Chapter.objects.create(title="Chapter 1")
+      <Chapter: Chapter 1>
+      >>> book.chapters.get_or_create(title="Chapter 1")
+      # Raises IntegrityError
+
+  This is happening because it's trying to get or create "Chapter 1" through the
+  book "Ulysses", but it can't do any of them: the relation can't fetch that
+  chapter because it isn't related to that book, but it can't create it either
+  because ``title`` field should be unique.
+
+
 bulk_create
 ~~~~~~~~~~~
 

tests/get_or_create/models.py

@@ -28,3 +28,12 @@ class ManualPrimaryKeyTest(models.Model):
 
 class Profile(models.Model):
     person = models.ForeignKey(Person, primary_key=True)
+
+
+class Tag(models.Model):
+    text = models.CharField(max_length=256, unique=True)
+
+
+class Thing(models.Model):
+    name = models.CharField(max_length=256)
+    tags = models.ManyToManyField(Tag)

tests/get_or_create/tests.py

@@ -6,7 +6,7 @@ import traceback
 from django.db import IntegrityError
 from django.test import TestCase, TransactionTestCase
 
-from .models import Person, ManualPrimaryKeyTest, Profile
+from .models import Person, ManualPrimaryKeyTest, Profile, Tag, Thing
 
 
 class GetOrCreateTests(TestCase):
@@ -77,3 +77,28 @@ class GetOrCreateTransactionTests(TransactionTestCase):
             pass
         else:
             self.skipTest("This backend does not support integrity checks.")
+
+
+class GetOrCreateThroughManyToMany(TestCase):
+
+    def test_get_get_or_create(self):
+        tag = Tag.objects.create(text='foo')
+        a_thing = Thing.objects.create(name='a')
+        a_thing.tags.add(tag)
+        obj, created = a_thing.tags.get_or_create(text='foo')
+
+        self.assertFalse(created)
+        self.assertEqual(obj.pk, tag.pk)
+
+    def test_create_get_or_create(self):
+        a_thing = Thing.objects.create(name='a')
+        obj, created = a_thing.tags.get_or_create(text='foo')
+
+        self.assertTrue(created)
+        self.assertEqual(obj.text, 'foo')
+        self.assertIn(obj, a_thing.tags.all())
+
+    def test_something(self):
+        Tag.objects.create(text='foo')
+        a_thing = Thing.objects.create(name='a')
+        self.assertRaises(IntegrityError, a_thing.tags.get_or_create, text='foo')