methods of the manager on subqueries QuerySet objects

[Wojciech Banaś <fizista@…>]

Extends the operation methods of the manager on subqueries QuerySet objects.

The "standard manager" you can only like this:

SomeModel.object.some_method_manager().all().filter(...). ...

But this does not work:

SomeModel.object.all().some_method_manager()

And this it works with the use of this class / patches:

SomeModel.object.all().some_method_manager().filter(...).some_method_manager2(). ... 
SomeModel.object.some_method_manager().some_method_manager2().some_method_manager3(). ... 

Extension of the methods manager on all child objects "QuerySeth".

Example:

• class model

      class Animals(models.Model):
          type = models.CharField('Type of animal') # dog, cat, elephant
          color = models.CharField('Colour Name')
          
          objects = AnimalsManager()
  

• class manager

    class AnimalsManager(ManagerQuerySetDecorator):
    
        def get_dogs(self):
            return self.get_query_set().filter(type='dog')
        get_dogs.queryset_method = True
        
        def get_cats(self):
            return self.get_query_set().filter(type='cat')
        get_cats.queryset_method = True
        
        def get_white_animals(self):
            return self.get_query_set().filter(color='white')
        get_white_animals.queryset_method = True
        
        def get_black_animals(self):
            return self.get_query_set().filter(color='black')
        get_black_animals.queryset_method = True
        
        def get_brown_animals(self):
            return self.get_query_set().filter(color='black')

• add models data

    Animals(type='dog', color='black').save()
    Animals(type='dog', color='bown').save()
    Animals(type='dog', color='white').save()
    Animals(type='cat', color='black').save()
    Animals(type='cat', color='bown').save()
    Animals(type='cat', color='white').save()

• examples of actions a new manager

    animals = Animals.objects.get_black_animals().get_dogs()
    # return list black dogs

    animals = Animals.objects.get_white_animals().get_cats()
    # return list white cats
    
    # When ffff equals False, or not defined (as it is in 
    # the original model manager), we can not perform such an operation:
    animals = Animals.objects.get_cats().get_brown_animals()
    # return:
    # AttributeError: 'QuerySet' object has no attribute 'get_brown_animals'
    
    # but:
    animals = Animals.objects.get_brown_animals().get_cats()
    # return brown cats, because get_cat() present in all parent object instances.

In Annex file a class action extends the manager.

If there is interest in this patch, then add in the future full of tests, and prepare a ready-made patch for django code.

[Wojciech Banaś <fizista@…>]

New manager class

[Wojciech Banaś <fizista@…>]

Improved and simplified version.

[Wojciech Banaś <fizista@…>]

Based on the ticket 17271, I rewrote the code and got a very nice and simple patch.

It is located in the attachment manager.2.py.

[Wojciech Banaś <fizista@…>]

Even small changes

[Carl Meyer]

As a longer-term project, I think finding a way to unify Manager and QuerySet could be a real improvement to the ORM API, if we can find a way to do it that is both sane and backwards compatible. The technique here, dynamically creating a new QuerySet subclass on every access and adding manager methods to it, is (probably mostly) backwards compatible, but I am not at all convinced that it is sane, or an acceptable performance hit.

[Wojciech Banaś <fizista@…>]

Dynamic class is created only once

[Andrey Popp]

Replying to carljm:

As a longer-term project, I think finding a way to unify Manager and QuerySet could be a real improvement to the ORM API, if we can find a way to do it that is both sane and backwards compatible. The technique here, dynamically creating a new QuerySet subclass on every access and adding manager methods to it, is (probably mostly) backwards compatible, but I am not at all convinced that it is sane, or an acceptable performance hit.

Speaking of that, do you have any clue, why not make ManagerDescriptor returns QuerySet instead of Manager? Looking at sources I can't see any useful functionality which is up to Manager — most code are just produces and proxies calls to QuerySet.

[Aymeric Augustin]

I'm skeptical about the clarity and maintainability of code that create classes dynamically too.

[Luke Plant]

For reference, an alternative solution is here: ​https://github.com/zacharyvoase/django-qmixin

[Wojciech Banaś <fizista@…>]

Another solution, tested, working, downwards compatible, django devel path

[Wojciech Banaś <fizista@…>]

Indeed, the dynamic class caused a lot of problems. I've enclosed a different solution based on various other solutions. It is downward compatible. This gives two possible ways of implementation.

• inside model class

objects_second = PersonManagerSecond(ManagerQuerySet, ['get_fun_people', 'get_not_fun_people'])

or inside manager class

class PersonManager(models.Manager):
    def __init__(self):
        super(PersonManager, self).__init__()
        self.implement_queryset_cls(ManagerQuerySet)
        self.implement_queryset_methods('get_fun_people', 'get_not_fun_people')

For more details refer to attachment: manager_queryset.diff

This implementation passed all django tests.

[Wojciech Banaś <fizista@…>]

Another solution, tested, working, downwards compatible, decorator

[Jacob]

Marking Someday: we want to do this, but it's entirely unclear exactly how. If you've got a specific proposal, please take it to django-developers for discussion!

[Anssi Kääriäinen]

I am closing this as duplicate of #20625 - that ticket has more momentum than this one.