Custom QuerySet and Manager without breaking DRY? - django

I'm trying to find a way to implement both a custom QuerySet and a custom Manager without breaking DRY. This is what I have so far:
class MyInquiryManager(models.Manager):
def for_user(self, user):
return self.get_query_set().filter(
Q(assigned_to_user=user) |
Q(assigned_to_group__in=user.groups.all())
)
class Inquiry(models.Model):
ts = models.DateTimeField(auto_now_add=True)
status = models.ForeignKey(InquiryStatus)
assigned_to_user = models.ForeignKey(User, blank=True, null=True)
assigned_to_group = models.ForeignKey(Group, blank=True, null=True)
objects = MyInquiryManager()
This works fine, until I do something like this:
inquiries = Inquiry.objects.filter(status=some_status)
my_inquiry_count = inquiries.for_user(request.user).count()
This promptly breaks everything because the QuerySet doesn't have the same methods as the Manager. I've tried creating a custom QuerySet class, and implementing it in MyInquiryManager, but I end up replicating all of my method definitions.
I also found this snippet which works, but I need to pass in the extra argument to for_user so it breaks down because it relies heavily on redefining get_query_set.
Is there a way to do this without redefining all of my methods in both the QuerySet and the Manager subclasses?

The Django 1.7 released a new and simple way to create combined queryset and model manager:
class InquiryQuerySet(models.QuerySet):
def for_user(self, user):
return self.filter(
Q(assigned_to_user=user) |
Q(assigned_to_group__in=user.groups.all())
)
class Inquiry(models.Model):
objects = InqueryQuerySet.as_manager()
See Creating Manager with QuerySet methods for more details.

Django has changed! Before using the code in this answer, which was written in 2009, be sure to check out the rest of the answers and the Django documentation to see if there is a more appropriate solution.
The way I've implemented this is by adding the actual get_active_for_account as a method of a custom QuerySet. Then, to make it work off the manager, you can simply trap the __getattr__ and return it accordingly
To make this pattern re-usable, I've extracted out the Manager bits to a separate model manager:
custom_queryset/models.py
from django.db import models
from django.db.models.query import QuerySet
class CustomQuerySetManager(models.Manager):
"""A re-usable Manager to access a custom QuerySet"""
def __getattr__(self, attr, *args):
try:
return getattr(self.__class__, attr, *args)
except AttributeError:
# don't delegate internal methods to the queryset
if attr.startswith('__') and attr.endswith('__'):
raise
return getattr(self.get_query_set(), attr, *args)
def get_query_set(self):
return self.model.QuerySet(self.model, using=self._db)
Once you've got that, on your models all you need to do is define a QuerySet as a custom inner class and set the manager to your custom manager:
your_app/models.py
from custom_queryset.models import CustomQuerySetManager
from django.db.models.query import QuerySet
class Inquiry(models.Model):
objects = CustomQuerySetManager()
class QuerySet(QuerySet):
def active_for_account(self, account, *args, **kwargs):
return self.filter(account=account, deleted=False, *args, **kwargs)
With this pattern, any of these will work:
>>> Inquiry.objects.active_for_account(user)
>>> Inquiry.objects.all().active_for_account(user)
>>> Inquiry.objects.filter(first_name='John').active_for_account(user)
UPD if you are using it with custom user(AbstractUser), you need to change
from
class CustomQuerySetManager(models.Manager):
to
from django.contrib.auth.models import UserManager
class CustomQuerySetManager(UserManager):
***

You can provide the methods on the manager and queryset using a mixin.
This also avoids the use of a __getattr__() approach.
from django.db.models.query import QuerySet
class PostMixin(object):
def by_author(self, user):
return self.filter(user=user)
def published(self):
return self.filter(published__lte=datetime.now())
class PostQuerySet(QuerySet, PostMixin):
pass
class PostManager(models.Manager, PostMixin):
def get_query_set(self):
return PostQuerySet(self.model, using=self._db)

You can now use the from_queryset() method on you manager to change its base Queryset.
This allows you to define your Queryset methods and your manager methods only once
from the docs
For advanced usage you might want both a custom Manager and a custom QuerySet. You can do that by calling Manager.from_queryset() which returns a subclass of your base Manager with a copy of the custom QuerySet methods:
class InqueryQueryset(models.Queryset):
def custom_method(self):
""" available on all default querysets"""
class BaseMyInquiryManager(models.Manager):
def for_user(self, user):
return self.get_query_set().filter(
Q(assigned_to_user=user) |
Q(assigned_to_group__in=user.groups.all())
)
MyInquiryManager = BaseInquiryManager.from_queryset(InquiryQueryset)
class Inquiry(models.Model):
ts = models.DateTimeField(auto_now_add=True)
status = models.ForeignKey(InquiryStatus)
assigned_to_user = models.ForeignKey(User, blank=True, null=True)
assigned_to_group = models.ForeignKey(Group, blank=True, null=True)
objects = MyInquiryManager()

A slightly improved version of T. Stone’s approach:
def objects_extra(mixin_class):
class MixinManager(models.Manager, mixin_class):
class MixinQuerySet(QuerySet, mixin_class):
pass
def get_query_set(self):
return self.MixinQuerySet(self.model, using=self._db)
return MixinManager()
Class decorators make usage as simple as:
class SomeModel(models.Model):
...
#objects_extra
class objects:
def filter_by_something_complex(self, whatever parameters):
return self.extra(...)
...
Update: support for nonstandard Manager and QuerySet base classes, e. g. #objects_extra(django.contrib.gis.db.models.GeoManager, django.contrib.gis.db.models.query.GeoQuerySet):
def objects_extra(Manager=django.db.models.Manager, QuerySet=django.db.models.query.QuerySet):
def oe_inner(Mixin, Manager=django.db.models.Manager, QuerySet=django.db.models.query.QuerySet):
class MixinManager(Manager, Mixin):
class MixinQuerySet(QuerySet, Mixin):
pass
def get_query_set(self):
return self.MixinQuerySet(self.model, using=self._db)
return MixinManager()
if issubclass(Manager, django.db.models.Manager):
return lambda Mixin: oe_inner(Mixin, Manager, QuerySet)
else:
return oe_inner(Mixin=Manager)

based on django 3.1.3 source code, i found a simple solution
from django.db.models.manager import BaseManager
class MyQuerySet(models.query.QuerySet):
def my_custom_query(self):
return self.filter(...)
class MyManager(BaseManager.from_queryset(MyQuerySet)):
...
class MyModel(models.Model):
objects = MyManager()

There are use-cases where we need to call custom QuerySet methods from the manager instead of using the get_manager method of a QuerySet.
A mixin would suffice based on the solution posted in one of the accepted solution comments.
class CustomQuerySetManagerMixin:
"""
Allow Manager which uses custom queryset to access queryset methods directly.
"""
def __getattr__(self, name):
# don't delegate internal methods to queryset
# NOTE: without this, Manager._copy_to_model will end up calling
# __getstate__ on the *queryset* which causes the qs (as `all()`)
# to evaluate itself as if it was being pickled (`len(self)`)
if name.startswith('__'):
raise AttributeError
return getattr(self.get_queryset(), name)
For example,
class BookQuerySet(models.QuerySet):
def published(self):
return self.filter(published=True)
def fiction(self):
return self.filter(genre="fiction")
def non_fiction(self):
return self.filter(genre="non-fiction")
class BookManager(CustomQuerySetManagerMixin, models.Manager):
def get_queryset(self):
return BookQuerySet(self.model, using=self._db).published()
class Book(models.Model):
title = models.CharField(max_length=200)
genre = models.CharField(choices=[('fiction', _('Fiction')), ('non-fiction', _('Non-Fiction'))])
published = models.BooleanField(default=False)
author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="books")
objects = BookManager()
class Author(models.Model):
name = models.CharField(max_length=200)
With the above, we can access related objects (Book) like below without defining new methods in the manager for each queryset method.
fiction_books = author.books.fiction()

The following works for me.
def get_active_for_account(self,account,*args,**kwargs):
"""Returns a queryset that is
Not deleted
For the specified account
"""
return self.filter(account = account,deleted=False,*args,**kwargs)
This is on the default manager; so I used to do something like:
Model.objects.get_active_for_account(account).filter()
But there is no reason it should not work for a secondary manager.

Related

Django - combining custom querysets used as managers with abstract class inheritance

Basically, I have to achieve two goals in my project that are currently conflicting with each other in terms of implementation.
The goals are:
Enabling chainable filtering for model instances (my_model.objects.custom_filter1().custom_filter2() etc)
For those tables with is_deleted field, and I want to exclude records marked as deleted, so that I don't have to explicitly filter for deleted records (use my_model.objects.all() instead of my_model.objects.filter(is_deleted = false))
Currently I have the following code:
class MixinManager(models.Manager):
def get_queryset(self):
try:
return self.model.MixinQuerySet(self.model).filter(is_deleted=False)
except FieldError:
return self.model.MixinQuerySet(self.model)
class BaseMixin(models.Model):
objects = MixinManager()
class MixinQuerySet(QuerySet):
pass
class Meta:
abstract = True
class DeleteMixin(BaseMixin):
is_deleted = models.BooleanField(default=False)
class Meta:
abstract = True
class Sms(DeleteMixin):
# core fielrds
# objects = SmsQuerySet.as_manager()
class Meta:
managed = False
db_table = 'sms'
However, my first goal becomes seemingly infeasible. Prior to using abstract classes for achieving the second goal, I had the following code
to solve the second goal:
class SmsQuerySet(models.query.QuerySet):
def filter_1(self, user):
return self.filter(...)
def filter_2(self, user):
return self.filter(...)
def filter_3(self, user):
return self.filter(...)
class Sms(models.Model):
# core fields
objects = SmsQuerySet.as_manager()
This syntax solved my first goal.
THE QUESTION:
The problem is that I can't combine these two structures into one logic, so that I can write:
sms.objects.filter_1().filter_2().filter_3()
so that deleted records are exluded from sms.objects.
If I uncomment the # objects = SmsQuerySet.as_manager() line in the first code snippet, then the objects field of BaseMixin will be ignored and I will end up with deleted records in the final queryset. On the other hand, if I comment the line, than my custom querysets become unreachable.
I hope I succeeded to concisely describe what I am trying to do. Will be very grateful for any hint towards the solution!
It should work
from django.db import models
class SmsQuerySet(models.query.QuerySet):
def filter_1(self, user):
return self.filter(...)
def filter_2(self, user):
return self.filter(...)
def filter_3(self, user):
return self.filter(...)
class MixinManager(models.Manager):
def get_queryset(self):
try:
return SmsQuerySet(self.model).filter(is_deleted=False)
except FieldError:
return SmsQuerySet(self.model)
class Sms(models.Model):
# core fields
objects = MixinManager()

Creating a django manager with a parameter

I have the following situation
I have a manager class that filters a queryset according to a field. The problem is that the field name is different according to the class but the value to which it filters comes from the same place (so i thought i don't need several managers). This is what i did so far.
class MyManager(models.Manager):
def __init__(self, field_name):
super(MyManager, self).__init__()
self.field_name = field_name
def get_queryset(self):
# getting some value
kwargs = { self.field_name: some_value }
return super(MyManager, self).get_queryset().filter(**kwargs)
class A:
# some datamembers
#property
def children(self):
return MyUtils.prepare(self.b_set.all())
class B:
objects = MyManager(field_name='my_field_name')
a = models.ForeignKey(A, null=False, blank=False)
When i run tests i that retrieve from the DB a B object, and try to read the children property i get the following error:
self = <django.db.models.fields.related_descriptors.RelatedManager object at 0x7f384d199290>, instance = <A: A object>
def __init__(self, instance):
> super(RelatedManager, self).__init__()
E TypeError: __init__() takes exactly 2 arguments (1 given)
I know its because of the constructor parameter because when i remove it (or give it a default value) all of the tests work.
How can i overcome this? Is this the right way of achieving this?
Tech stuff:
Django 1.9.5
test framework py.test 2.9.1
Thanks
Another option would be to generate the Manager class dynamically, such as:
def manager_factory(custom_field):
class MyManager(models.Manager):
my_field = custom_field
def get_queryset(self):
# getting some value
kwargs = {self.my_field: 'some-value'}
return super(MyManager, self).get_queryset().filter(**kwargs)
return MyManager()
class MyModel(models.Model):
objects = manager_factory('custom_field')
This way you can decouple the Manager from the Model class.
As you can see, that error is happening because Django instantiates a new Manager whenever you make a related objects call; that instantiation wouldn't get the extra parameter.
Rather than getting the value this way, you could try making it an attribute of the model and then referencing it via self.model.
class MyManager(models.Manager):
def get_queryset(self):
# getting some value
kwargs = { self.model.manager_field_name: some_value }
class B:
manager_field_name = 'my_field_name'
objects = MyManager()

How to access custom QuerySet methods from the Manager of a ForeignKey

I'm using Django Managers to make a higher API to interact with my database and keeping my code cleaner and more readable. But in case I have a Foreignkey relationship, I can't use the manager of the ForeignKey model. The queries are more complex as below, but I just simplified the example so, It can be easier to read and get the idea of the question:
models.py:
class Community(models.Model):
objects = CommunityManager()
...
class Inscription(models.Model):
objects = InscriptionManager()
...
community = models.ForeignKey("Community", related_name="inscriptions")
created_at = models.DateTimeField()
managers.py:
from datetime import date
from django.db import models
class InscriptionQuerySet(models.query.QuerySet):
def by_day(self, day=date.today()):
return self.filter(created_at__day=day)
... # more queries
class InscriptionManager(models.Manager):
def get_query_set(self):
return InscriptionQuerySet(self.model, using=self._db)
def today(self):
return self.get_query_set().by_day()
... # more queries
class CommunityQuerySet(models.query.QuerySet):
def by_type(self, type):
return self.filter(type=type)
... # more queries
class CommunityManager(models.Manager):
def get_query_set(self):
return OrganistaionQuerySet(self.model, using=self._db)
def by_type(self, type):
return self.get_query_set().by_type(type)
... # more queries
Usage:
Inscription.objects.by_day() # return correctly all the inscriptions made today
Community.objects.by_type('type1') # return correctly all Communities that match
Problem: but here lies the problem
community_b = Community.objects.get(id=12)
community_b.inscriptions.by_day()
>>> AttributeError: 'ForeignRelatedObjectsDescriptor' object has no attribute 'by_day'
How can I fixe this. How to customize the manager to take in consideration the models relation.
I don't see the need of both Manager and QuerySet in your approach. You could just do away with a QuerySet. This is how I generally implement custom managers, and I tried accessing custom manager method on reverse relationship in one of my projects, and it works fine:
class InscriptionQuerySet(models.QuerySet):
def by_day(self, day=date.today()):
return self.filter(created_at__day=day)
def today(self):
return self.by_day()
class CommunityQuerySet(models.QuerySet):
def by_type(self, type):
return self.filter(type=type)
And then in your models, change your objects like this:
class Community(models.Model):
objects = CommunityQuerySet.as_manager()
...
class Inscription(models.Model):
objects = InscriptionQuerySet.as_manager()
I think you would be able to access custom queryset method with this setup.

Overriding QuerySet.delete() in Django

I have a Django model that holds settings core to the function of an app. You should never delete this model. I'm trying to enforce this application-wide. I've disabled the delete function in the admin, and also disabled the delete method on the model, but QuerySet has it's own delete method. Example:
MyModel.objects.all()[0].delete() # Overridden, does nothing
MyModel.objects.all().delete() # POOF!
Ironically, the Django docs say has this to say about why delete() is a method on QuerySet and not Manager:
This is a safety mechanism to prevent you from accidentally requesting Entry.objects.delete(), and deleting all the entries.
How having to include .all() is a "safety mechanism" is questionable to say the least. Instead, this effectively creates a backdoor that can't be closed by conventional means (overriding the manager).
Anyone have a clue how to override this method on something as core as QuerySet without monkey-patching the source?
You can override a Manager's default QuerySet by overriding the Manager.get_query_set() method.
Example:
class MyQuerySet(models.query.QuerySet):
def delete(self):
pass # you can throw an exception
class NoDeleteManager(models.Manager):
def get_query_set(self):
return MyQuerySet(self.model, using=self._db)
class MyModel(models.Model)
field1 = ..
field2 = ..
objects = NoDeleteManager()
Now, MyModel.objects.all().delete() will do nothing.
For more informations: Modifying initial Manager QuerySets
mixin approach
https://gist.github.com/dnozay/373571d8a276e6b2af1a
use a similar recipe as #manji posted,
class DeactivateQuerySet(models.query.QuerySet):
'''
QuerySet whose delete() does not delete items, but instead marks the
rows as not active, and updates the timestamps
'''
def delete(self):
self.deactivate()
def deactivate(self):
deleted = now()
self.update(active=False, deleted=deleted)
def active(self):
return self.filter(active=True)
class DeactivateManager(models.Manager):
'''
Manager that returns a DeactivateQuerySet,
to prevent object deletion.
'''
def get_query_set(self):
return DeactivateQuerySet(self.model, using=self._db)
def active(self):
return self.get_query_set().active()
and create a mixin:
class DeactivateMixin(models.Model):
'''
abstract class for models whose rows should not be deleted but
items should be 'deactivated' instead.
note: needs to be the first abstract class for the default objects
manager to be replaced on the subclass.
'''
active = models.BooleanField(default=True, editable=False, db_index=True)
deleted = models.DateTimeField(default=None, editable=False, null=True)
objects = DeactivateManager()
class Meta:
abstract = True
other interesting stuff
http://datahackermd.com/2013/django-soft-deletion/
https://github.com/hearsaycorp/django-livefield

Django: Custom model manager problem

I am trying a simple custom manager, but I can't concatenate custom queries:
class MyManager(models.Manager):
def some_filter(self):
qs = self.get_query_set()
return qs.filter(score__gt = 10).order_by("-score")
class Game(models.Model):
score = models.IntegerField(blank=True, default=0)
objects = MyManager()
games = Game.objects.filter(any_filter).some_filter()[:5]
But I get QuerySet' object has no attribute 'some_filter'
Edit: It appears that the question is how to concatenate custom filter functions together: seomthing like games = Game.objects.some_filter1().some_filter2()[:5] just won't work for me.
You can apply some_filter() to MyManager object before QuerySet method filter()
games = Game.objects.some_filter().filter(any_filter)[:5]
Otherwise you should add some_filter method to QuerySet
I found a way to do it. A new QuerySet class need to be defined too
class GameQS(QuerySet):
def some_filter1(self):
return self.filter(score__gt = 10).order_by("-score")
def some_filter2(self):
return self.filter(score__gt = 50).order_by("-score")
class GameManager(models.Manager):
def get_query_set(self):
return GameQS(self.model, using=self._db)
def some_filer1(self):
return self.get_query_set().some_filter1()
def some_filter2(self):
return self.get_query_set().some_filter2()
If you are new in using model managers, first see GabiMe's answer
From django 1.7 onward, use queryset as manager class method to keep code DRY
MyQueryset.as_manager()
If you are using older verssions refer this question for more ways to optimize the code.