Automatically generate custom migrations in Django

Question

Django provides a really nice feature called makemigrations where it will create migration files based on the changes in models. We are developing a module where we will want to generate custom migrations.

I haven't found much info about creating custom migrations in the Django docs. There is documentation on the various Operation classes that can yield migrations but there's nothing about creating custom Operation classes that can yield custom migrations.

The autodetector module for generating new migrations also doesn't seem to leave much room for adding custom Operation classes: https://github.com/django/django/blob/master/django/db/migrations/autodetector.py#L160

It seems this is completely static. Are there other ways to generate custom migrations, perhaps by using existing classes with a custom management command?

Answer 1

You can create a custom class to hook into the makemigrations class and add your custom migrations stuff then execute using the "runscript" command. Below is a sample module where the file is named custom_migrations.py and located in a "scripts" folder off one of your apps:

from django.core.management.commands.makemigrations import Command
"""
To invoke this script use:
   manage.py runscript custom_migrations --script-args [app_label [app_label ...]] name=my_special_migration verbosity=1
"""    

class MyMigrationMaker(Command):
    '''
    Override the write method to add more stuff before finishing
    '''


    def write_migration_files(self, changes):
        print("Do some stuff to \"changes\" object here...")
        super().write_migration_files(changes)



def run(*args):
    nargs = []
    kwargs = {}
    # Preload some options with defaults and then can be overridden in the args parsing
    kwargs['empty'] = True
    kwargs['verbosity'] = 1
    kwargs['interactive'] = True
    kwargs['dry_run'] = False
    kwargs['merge'] = False
    kwargs['name'] = 'custom_migration'
    kwargs['check_changes'] = False
    for arg in args:
        kwarg = arg.split('=', 1)
        if len(kwarg) > 1:
            val = kwarg[1]
            if val == "True":
                arg_val = True
            elif val == "False":
                arg_val = False
            elif val.isdigits():
                arg_val = int(val)
            else:
                arg_val = val
            the_kwargs[kwarg[0]] = arg_val
        else:
            nargs.append(arg)
    MyMigrationMaker().handle(*nargs, **kwargs)