[1.8.x] Fixed #24972 -- Fixed removing unique_together indexes on MySQL.

Backport of 65296b3be3 from master

django/db/backends/mysql/schema.py

@@ -73,11 +73,9 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         """
         first_field = model._meta.get_field(fields[0])
         if first_field.get_internal_type() == 'ForeignKey':
-            constraint_names = self._constraint_names(model, fields[0], index=True)
+            constraint_names = self._constraint_names(model, [first_field.column], index=True)
             if not constraint_names:
-                self.execute(
-                    self._create_index_sql(model, [model._meta.get_field(fields[0])], suffix="")
-                )
+                self.execute(self._create_index_sql(model, [first_field], suffix=""))
         return super(DatabaseSchemaEditor, self)._delete_composed_index(model, fields, *args)
 
     def _set_field_new_type_null_status(self, field, new_type):

docs/releases/1.8.3.txt

@@ -69,3 +69,5 @@ Bugfixes
 
 * Fixed ``prefetch_related()`` on databases other than PostgreSQL for models
   using UUID primary keys (:ticket:`24912`).
+
+* Fixed removing ``unique_together`` constraints on MySQL (:ticket:`24972`).

tests/schema/models.py

@@ -80,6 +80,15 @@ class BookWithSlug(models.Model):
         db_table = "schema_book"
 
 
+class BookWithoutAuthor(models.Model):
+    title = models.CharField(max_length=100, db_index=True)
+    pub_date = models.DateTimeField()
+
+    class Meta:
+        apps = new_apps
+        db_table = "schema_book"
+
+
 class IntegerPK(models.Model):
     i = models.IntegerField(primary_key=True)
     j = models.IntegerField(unique=True)

tests/schema/tests.py

@@ -20,8 +20,9 @@ from django.test import TransactionTestCase, skipIfDBFeature
 from .fields import CustomManyToManyField, InheritedManyToManyField
 from .models import (
     Author, AuthorWithDefaultHeight, AuthorWithEvenLongerName, Book, BookWeak,
-    BookWithLongName, BookWithO2O, BookWithSlug, IntegerPK, Note, NoteRename,
-    Tag, TagIndexed, TagM2MTest, TagUniqueRename, Thing, UniqueTest, new_apps,
+    BookWithLongName, BookWithO2O, BookWithoutAuthor, BookWithSlug, IntegerPK,
+    Note, NoteRename, Tag, TagIndexed, TagM2MTest, TagUniqueRename, Thing,
+    UniqueTest, new_apps,
 )
 
 
@@ -1185,6 +1186,28 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_unique_together(Book, [['author', 'title']], [])
 
+    def test_unique_together_with_fk_with_existing_index(self):
+        """
+        Tests removing and adding unique_together constraints that include
+        a foreign key, where the foreign key is added after the model is
+        created.
+        """
+        # Create the tables
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(BookWithoutAuthor)
+            new_field = ForeignKey(Author)
+            new_field.set_attributes_from_name('author')
+            editor.add_field(BookWithoutAuthor, new_field)
+        # Ensure the fields aren't unique to begin with
+        self.assertEqual(Book._meta.unique_together, ())
+        # Add the unique_together constraint
+        with connection.schema_editor() as editor:
+            editor.alter_unique_together(Book, [], [['author', 'title']])
+        # Alter it back
+        with connection.schema_editor() as editor:
+            editor.alter_unique_together(Book, [['author', 'title']], [])
+
     def test_index_together(self):
         """
         Tests removing and adding index_together constraints on a model.