Django Model Injection

modelinject.py

# -*- coding: utf-8 -*-

"""
Model class injectors, to make Django 1.9+'s app registry easier to work with.

Django 1.9 made it an error to import an app's models submodule when importing
the app module itself. Instead, you should use `apps.get_model(app_label,
model_name)` to get the model class, and signals should be wired up in the
`ready()` hook of a custom `AppConfig` subclass.

The `@inject_models` decorator makes it easy to transition functions that use
models to a dependency-injection format, meaning you can have your cake and eat
it. Code that used to do the following:

    from myapp.models import MyModel

    def lookup_model(name):
        return MyModel.objects.filter(name=name)

Can be restructured to the following:

    from modelinject import inject_models, imodel

    @inject_models
    def lookup_model(name, MyModel=imodel('myapp.MyModel')):
        return MyModel.objects.filter(name=name)

At call-time, `@inject_models` will replace the `imodel()` with the result of
calling `django.apps.apps.get_model('myapp', 'MyModel')`. The downside is that
you're incurring a call to `get_model()` every time `lookup_model` is called,
but this should be a good stop-gap when upgrading a codebase from Django 1.8 or
prior to 1.9 or later.
"""

import collections
import functools
import inspect

from django.apps import apps


_ModelLabel = collections.namedtuple('_ModelLabel', ['app_label', 'model_name'])


def inject_models(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        call_args = inspect.getcallargs(func, *args, **kwargs)
        injected_args = {}
        for arg_name, arg_value in call_args.iteritems():
            if isinstance(arg_value, _ModelLabel):
                injected_args[arg_name] = apps.get_model(*arg_value)
            else:
                injected_args[arg_name] = arg_value
        return func(**injected_args)
    return wrapper


def imodel(model_id):
    app_label, model_name = model_id.split('.')
    return _ModelLabel(app_label, model_name)