Django Manager Chaining

See this snippet on Djangosnippets: http://djangosnippets.org/snippets/734/

Instead of putting your custom methods in a manager, you subclass the queryset itself. It's very easy and works perfectly. The only issue I've had is with model inheritance, you always have to define the manager in model subclasses (just: "objects = QuerySetManager()" in the subclass), even though they will inherit the queryset. This will make more sense once you are using QuerySetManager.

Here is the specific solution to my problem using the custom QuerySetManager by Simon that Scott linked to.

from django.db import modelsfrom django.contrib import adminfrom django.db.models.query import QuerySetfrom django.core.exceptions import FieldErrorclass MixinManager(models.Manager):        def get_query_set(self):        try:            return self.model.MixinQuerySet(self.model).filter(deleted=False)        except FieldError:            return self.model.MixinQuerySet(self.model)class BaseMixin(models.Model):    admin = models.Manager()    objects = MixinManager()    class MixinQuerySet(QuerySet):        def globals(self):            try:                return self.filter(is_global=True)            except FieldError:                return self.all()    class Meta:        abstract = Trueclass DeleteMixin(BaseMixin):    deleted = models.BooleanField(default=False)    class Meta:        abstract = True    def delete(self):        self.deleted = True        self.save()class GlobalMixin(BaseMixin):    is_global = models.BooleanField(default=True)    class Meta:        abstract = True

Any mixin in the future that wants to add extra functionality to the query set simply needs to extend BaseMixin (or have it somewhere in its heirarchy). Any time I try to filter the query set down, I wrapped it in a try-catch in case that field doesn't actually exist (ie, it doesn't extend that mixin). The global filter is invoked using globals(), while the delete filter is automatically invoked (if something is deleted, I never want it to show). Using this system allows for the following types of commands:

TemporaryModel.objects.all() # If extending DeleteMixin, no deleted instances are returnedTemporaryModel.objects.all().globals() # Filter out the private instances (non-global)TemporaryModel.objects.filter(...) # Ditto about excluding deleteds

One thing to note is that the delete filter won't affect admin interfaces, because the default Manager is declared first (making it the default). I don't remember when they changed the admin to use Model._default_manager instead of Model.objects, but any deleted instances will still appear in the admin (in case you need to un-delete them).

I spent a while trying to come up with a way to build a nice factory to do this, but I'm running into a lot of problems with that.

The best I can suggest to you is to chain your inheritance. It's not very generic, so I'm not sure how useful it is, but all you would have to do is:

class GlobalMixin(DeleteMixin):    is_global = models.BooleanField(default=True)    objects = GlobalManager()    class Meta:        abstract = Trueclass GlobalManager(DeleteManager):    def globals(self):        return self.get_query_set().filter(is_global=1)

If you want something more generic, the best I can come up with is to define a base Mixin and Manager that redefines get_query_set() (I'm assuming you only want to do this once; things get pretty complicated otherwise) and then pass a list of fields you'd want added via Mixins.

It would look something like this (not tested at all):

class DeleteMixin(models.Model):    deleted = models.BooleanField(default=False)    class Meta:        abstract = Truedef create_mixin(base_mixin, **kwargs):    class wrapper(base_mixin):        class Meta:            abstract = True    for k in kwargs.keys():        setattr(wrapper, k, kwargs[k])    return wrapperclass DeleteManager(models.Manager):    def get_query_set(self):        return super(DeleteManager, self).get_query_set().filter(deleted=False)def create_manager(base_manager, **kwargs):    class wrapper(base_manager):        pass    for k in kwargs.keys():        setattr(wrapper, k, kwargs[k])    return wrapper

Ok, so this is ugly, but what does it get you? Essentially, it's the same solution, but much more dynamic, and a little more DRY, though more complex to read.

First you create your manager dynamically:

def globals(inst):    return inst.get_query_set().filter(is_global=1)GlobalDeleteManager = create_manager(DeleteManager, globals=globals)

This creates a new manager which is a subclass of DeleteManager and has a method called globals.

Next, you create your mixin model:

GlobalDeleteMixin = create_mixin(DeleteMixin,                                 is_global=models.BooleanField(default=False),                                 objects = GlobalDeleteManager())

Like I said, it's ugly. But it means you don't have to redefine globals(). If you want a different type of manager to have globals(), you just call create_manager again with a different base. And you can add as many new methods as you like. Same for the manager, you just keep adding new functions that will return different querysets.

So, is this really practical? Maybe not. This answer is more an exercise in (ab)using Python's flexibility. I haven't tried using this, though I do use some of the underlying principals of dynamically extending classes to make things easier to access.

Let me know if anything is unclear and I'll update the answer.