Can I add a manager to a manytomany relationship? - django

Question

I have two models that has a manytomany relationship with a 'through' table in some way?
class Bike(models.Model):
nickname = models.CharField(max_length=40)
users = models.ManyToManyField(User, through='bike.BikeUser')
The BikeUser class
class BikeUser(models.Model):
bike = models.ForeignKey(Bike)
user = models.ForeignKey(User)
comment = models.CharField(max_length=140)
I would like to add functionality to the Bike class for working with users, is there a best practice way of doing this. I would like to avoid adding too many methods to the Bike class and rather have some kind of manager to work through
Something like:
bike.bikeusers_set.commonMethod()
or
bike.bikeusers.commonMethod()
What would be the best way to accomplish this?

Answer 1

Once you have the BikeUser model, you can add a custom manager to the model.
Something like:
class BikeUserManager(models.Manager):
def commonMethod():
pass
class BikeUser(models.Model):
bike = models.ForeignKey(Bike)
user = models.ForeignKey(User)
comment = models.CharField(max_length=140)
objects = BikeUserManager()
But you can only use it from the BikeUser Model:
BikeUser.objects.commonMethod()
What you want is to use this manager as a related manager:
http://docs.djangoproject.com/en/dev/topics/db/managers/#controlling-automatic-manager-types
Add the use_for_related_fields=True to the manager class.
class MyManager(models.Manager):
use_for_related_fields = True

Answer 2

use_for_related_fields is deprecated from django 2.0. Use for related fields is possible via base manager.
old:
class CustomManager(models.Model):
use_for_related_fields = True
class Model(models.Model):
custom_manager = CustomManager()
new:
class Model(models.Model):
custom_manager = CustomManager()
class Meta:
base_manager_name = 'custom_manager'
source of example
Remember about restrictions for get_queryset() method.

Answer 3

Code
def m2m_with_manager(related_manager, model_manager_name: str):
"""Replaces the default model manager tied to related manager with defined"""
model_manager = getattr(related_manager.model, model_manager_name)
return model_manager.filter(**related_manager.core_filters)
Example
for class Author and Books, where one Author can have multiple books
class Book:
author = FK(Author, related_name='books')
best = models.Manager(...)
Usage
wanted_qs = m2m_with_manager(author.books, model_manager_name='best')