[1.7.x] Fixed #23987 -- Made SQLite SchemaEditor always use effective_default().

Backport of 089047331d from master

django/db/backends/sqlite3/schema.py

@@ -69,8 +69,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         # Add in any created fields
         for field in create_fields:
             body[field.name] = field
-            # If there's a default, insert it into the copy map
+            # Choose a default and insert it into the copy map
-            if field.has_default():
+            if not isinstance(field, ManyToManyField):
                 mapping[field.column] = self.quote_value(
                     self.effective_default(field)
                 )

docs/releases/1.7.2.txt

@@ -135,3 +135,8 @@ Bugfixes
   growth in Django's own test suite (:ticket:`23969`).
 
 * Fixed timesince filter translations in Korean (:ticket:`23989`).
+
+* Fixed the SQLite ``SchemaEditor`` to properly add defaults in the absence of
+  a user specified ``default``. For example, a ``CharField`` with ``blank=True``
+  didn't set existing rows to an empty string which resulted in a crash when
+  adding the ``NOT NULL`` constraint (:ticket:`23987`).

tests/schema/tests.py

@@ -1180,3 +1180,27 @@ class SchemaTests(TransactionTestCase):
                 }
             )
             editor.alter_field(model, get_field(Author, field_class=ForeignKey), field)
+
+    def test_add_field_use_effective_default(self):
+        """
+        #23987 - effective_default() should be used as the field default when
+        adding a new field.
+        """
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Ensure there's no surname field
+        columns = self.column_classes(Author)
+        self.assertNotIn("surname", columns)
+        # Create a row
+        Author.objects.create(name='Anonymous1')
+        # Add new CharField to ensure default will be used from effective_default
+        new_field = CharField(max_length=15, blank=True)
+        new_field.set_attributes_from_name("surname")
+        with connection.schema_editor() as editor:
+            editor.add_field(Author, new_field)
+        # Ensure field was added with the right default
+        with connection.cursor() as cursor:
+            cursor.execute("SELECT surname FROM schema_author;")
+            item = cursor.fetchall()[0]
+            self.assertEqual(item[0], '')