Add --empty option to makemigrations

from django.apps import apps

from django.core.management.base import BaseCommand, CommandError

from django.db import connections, DEFAULT_DB_ALIAS, migrations

from django.db.migrations.migration import Migration

from django.db.migrations.loader import MigrationLoader

from django.db.migrations.autodetector import MigrationAutodetector

from django.db.migrations.questioner import MigrationQuestioner, InteractiveMigrationQuestioner

            help="Just show what migrations would be made; don't actually write them."),

        make_option('--merge', action='store_true', dest='merge', default=False,

            help="Enable fixing of migration conflicts."),

        make_option('--empty', action='store_true', dest='empty', default=False,

            help="Create an empty migration."),

    )

    help = "Creates new migration(s) for apps."

        self.interactive = options.get('interactive')

        self.dry_run = options.get('dry_run', False)

        self.merge = options.get('merge', False)

        self.empty = options.get('empty', False)

        # Make sure the app they asked for exists

        app_labels = set(app_labels)

        if self.merge and conflicts:

            return self.handle_merge(loader, conflicts)

        # Detect changes

        # Set up autodetector

        autodetector = MigrationAutodetector(

            loader.graph.project_state(),

            ProjectState.from_apps(apps),

            InteractiveMigrationQuestioner(specified_apps=app_labels),

        )

        # If they want to make an empty migration, make one for each app

        if self.empty:

            if not app_labels:

                raise CommandError("You must supply at least one app label when using --empty.")

            # Make a fake changes() result we can pass to arrange_for_graph

            changes = dict(

                (app, [Migration("custom", app)])

                for app in app_labels

            )

            changes = autodetector.arrange_for_graph(changes, loader.graph)

            self.write_migration_files(changes)

            return

        # Detect changes

        changes = autodetector.changes(graph=loader.graph, trim_to_apps=app_labels or None)

        # No changes? Tell them.

                self.stdout.write("No changes detected")

            return

        self.write_migration_files(changes)

    def write_migration_files(self, changes):

        """

        Takes a changes dict and writes them out as migration files.

        """

        directory_created = {}

        for app_label, app_migrations in changes.items():

            if self.verbosity >= 1: