Portable Content Plugin & Page Type Apps

GLAMkit page types and plugins are portable, which means they can be copied into a project without affecting database table names and (relative) imports.

It’s important to make the content plugin and page type apps that ship with GLAMkit as generic as possible, so that developers can re-use them as-is for the most common cases, while making it possible to customise when needed.

When we do need to customise, we should be able to do so simply by copying the entire app (admin classes, forms, migrations, models, templates, views, etc.) from GLAMkit into a project, and having it work exactly as it did before.

If we can ensure that the database table names don’t change and imports don’t break when installing an app from within a project package, this becomes straightforward.

How to make a portable app

Following these guidelines will make it easy to copy an app into a project when customisation is required, without complicated database migrations.

  • Dynamically derive the default AppConfig class and AppConfig.name attribute from the module name:

    # __init__.py
    default_app_config = '%s.apps.AppConfig' % __name__
    # apps.py
    from django.apps import AppConfig
    class AppConfig(AppConfig):
        name = '.'.join(__name__.split('.')[:-1])  # Name of package where `apps` module is located

    These values won’t break when the app package is copied to a new location.

  • Hardcode the AppConfig.label attribute, deriving from the canonical AppConfig.name. For example, by replacing . with _:

    # apps.py
    class AppConfig(AppConfig):
        name = ...
        label = 'icekit_page_types_layout_page'

    By default, Django only uses the last component of the app name, which might result in labels that are likely to conflict with other apps.

  • Always use relative imports within the app package, not only model imports.

To copy a portable app package into a project, just copy the whole package to a new location and update your INSTALLED_APPS setting.

How to make an existing app portable

When making an existing app more portable, things are a little more complicated when it is already being used by other projects. But we can still do it.

In the app which is being made portable, make the following changes:

  • Update the AppConfig.label attribute or rename the app package.

  • Update the Meta.app_label attribute on existing models, if they already have a value set.

  • Replace any absolute imports within the app with relative imports.

  • If the app package was renamed, expose its modules at their old location with a deprecation warning.

  • Add a db_table: {old app label} option to all CreateModel operations in existing migrations, if not already defined, giving the previous app label as the value.

  • Run manage.py makemigrations {app_label}. Django will detect that the db_table option has changed and will create a migration to rename the table for us.

  • Add the following RunSQL operation to the newly created migration:

    migrations.RunSQL("UPDATE django_content_type SET app_label='{new app label}' WHERE app_label='{old app label}';"),

Applying the app rename in existing projects

In every project that uses an app with an updated app label, make the following changes:

  • Replace absolute model imports with get_app() and get_model() calls on the app registry.

  • Update absolute imports, if required, to avoid any deprecation warnings.

  • Update the app_label argument to get_*() calls on the app registry.

  • Update the app label in the dependencies and run_before attributes in existing migrations.

  • Update foreign key and many to many fields in all apps.

  • Execute the following SQL directly on the database for your renamed apps:

    UPDATE django_migrations SET app='{new app label}' WHERE app='{old app label}';