[1.8.x] Fixed #24757 -- Recreated MySQL index when needed during combined index removal

Thanks Thomas Recouvreux for the report and Tim Graham for the tests and
review.
Backport of ae635cc36 from master.

django/db/backends/base/schema.py

@@ -317,15 +317,7 @@ class BaseDatabaseSchemaEditor(object):
         news = set(tuple(fields) for fields in new_unique_together)
         # Deleted uniques
         for fields in olds.difference(news):
-            columns = [model._meta.get_field(field).column for field in fields]
+            self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)
-            constraint_names = self._constraint_names(model, columns, unique=True)
-            if len(constraint_names) != 1:
-                raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
-                    len(constraint_names),
-                    model._meta.db_table,
-                    ", ".join(columns),
-                ))
-            self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_names[0]))
         # Created uniques
         for fields in news.difference(olds):
             columns = [model._meta.get_field(field).column for field in fields]
@@ -341,19 +333,22 @@ class BaseDatabaseSchemaEditor(object):
         news = set(tuple(fields) for fields in new_index_together)
         # Deleted indexes
         for fields in olds.difference(news):
+            self._delete_composed_index(model, fields, {'index': True}, self.sql_delete_index)
+        # Created indexes
+        for field_names in news.difference(olds):
+            fields = [model._meta.get_field(field) for field in field_names]
+            self.execute(self._create_index_sql(model, fields, suffix="_idx"))
+
+    def _delete_composed_index(self, model, fields, constraint_kwargs, sql):
         columns = [model._meta.get_field(field).column for field in fields]
-            constraint_names = self._constraint_names(model, list(columns), index=True)
+        constraint_names = self._constraint_names(model, columns, **constraint_kwargs)
         if len(constraint_names) != 1:
             raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
                 len(constraint_names),
                 model._meta.db_table,
                 ", ".join(columns),
             ))
-            self.execute(self._delete_constraint_sql(self.sql_delete_index, model, constraint_names[0]))
+        self.execute(self._delete_constraint_sql(sql, model, constraint_names[0]))
-        # Created indexes
-        for field_names in news.difference(olds):
-            fields = [model._meta.get_field(field) for field in field_names]
-            self.execute(self._create_index_sql(model, fields, suffix="_idx"))
 
     def alter_db_table(self, model, old_db_table, new_db_table):
         """

django/db/backends/mysql/schema.py

@@ -62,6 +62,24 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                     field.db_index = False
         return super(DatabaseSchemaEditor, self)._model_indexes_sql(model)
 
+    def _delete_composed_index(self, model, fields, *args):
+        """
+        MySQL can remove an implicit FK index on a field when that field is
+        covered by another index like a unique_together. "covered" here means
+        that the more complex index starts like the simpler one.
+        http://bugs.mysql.com/bug.php?id=37910 / Django ticket #24757
+        We check here before removing the [unique|index]_together if we have to
+        recreate a FK index.
+        """
+        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)
+            if not constraint_names:
+                self.execute(
+                    self._create_index_sql(model, [model._meta.get_field(fields[0])], suffix="")
+                )
+        return super(DatabaseSchemaEditor, self)._delete_composed_index(model, fields, *args)
+
     def _alter_column_type_sql(self, table, old_field, new_field, new_type):
         # Keep null property of old field, if it has changed, it will be handled separately
         if old_field.null:

docs/releases/1.8.2.txt

@@ -27,3 +27,6 @@ Bugfixes
   :ticket:`24712`).
 
 * Fixed ``isnull`` lookup for ``HStoreField`` (:ticket:`24751`).
+
+* Fixed a MySQL crash when a migration removes a combined index (unique_together
+  or index_together) containing a foreign key (:ticket:`24757`).

tests/schema/tests.py

@@ -1084,6 +1084,24 @@ class SchemaTests(TransactionTestCase):
         self.assertRaises(IntegrityError, UniqueTest.objects.create, year=2012, slug="foo")
         UniqueTest.objects.all().delete()
 
+    def test_unique_together_with_fk(self):
+        """
+        Tests removing and adding unique_together constraints that include
+        a foreign key.
+        """
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+        # Ensure the fields are 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.
@@ -1127,6 +1145,24 @@ class SchemaTests(TransactionTestCase):
             ),
         )
 
+    def test_index_together_with_fk(self):
+        """
+        Tests removing and adding index_together constraints that include
+        a foreign key.
+        """
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+        # Ensure the fields are unique to begin with
+        self.assertEqual(Book._meta.index_together, ())
+        # Add the unique_together constraint
+        with connection.schema_editor() as editor:
+            editor.alter_index_together(Book, [], [['author', 'title']])
+        # Alter it back
+        with connection.schema_editor() as editor:
+            editor.alter_index_together(Book, [['author', 'title']], [])
+
     def test_create_index_together(self):
         """
         Tests creating models with index_together already defined