Django: How to cascade custom managers defined in different abstract classes? - django

Question

I want to be able to make complex queries combining different custom manager functions from different Abstract classes.
My models are like these:
class GenderManager(models.Manager):
def male(self):
return self.filter(gender="M")
def female(self):
return self.filter(gender="F")
class SpeciesManager(models.Manager):
def lion(self):
return self.filter(species="L")
def tiger(self):
return self.filter(species="T")
class GenderModel(models.Model):
gender = models.CharField(max_length=1)
objects = GenderManager()
class Meta:
abstract = True
class SpeciesModel(models.Model):
species = models.CharField(max_length=1)
objects = SpeciesManager()
class Meta:
abstract = True
class Animal(GenderModel,SpeciesModel):
name = models.CharField(max_length=30)
age = models.DecimalField(max_digits=4, decimal_places=2)
The reason I want to split Gender and Species is that in my models sometimes I will need to inherit only from GenderModel and sometimes only from SpeciesModel.
In the cases where I want to inherit form both (like in Animal class), I would like to be able to make queries like this:
Animal.objects.male().tiger().filter(age__gte = 10.00)
But it doesn't work.
However, if I don't use custom manager functions it works:
Animal.objects.filter(gender="M").filter(species="T").filter(age__gte = 10.00)
How can I make it work with custom manager functions do make it DRY?
Thank you!

Answer 1

This can't be done due to how Managers actually work but there can be different ways to refactor your model and the logic, depending on the complexity and how independent the models actually need to be.
Since your manager is actually needed for the concrete class and not the abstract class, concanate both managers into a single manager, afterall you are filtering on the concrete class and not the Abstract.
class GenderModel(models.Model):
gender = models.CharField(max_length=1)
class Meta:
abstract = True
class SpeciesModel(models.Model):
species = models.CharField(max_length=1)
class Meta:
abstract = True
class AnimalManager(models.Manager):
def gender(self):
return self.filter(gender="M")
def female(self):
return self.filter(gender="F")
def lion(self):
return self.filter(species="L")
def tiger(self):
return self.filter(species="T")
def get_male_tigers(self):
return self.filter(species="T").filter(gender="M").all()
class Animal(GenderModel,SpeciesModel):
name = models.CharField(max_length=30)
age = models.DecimalField(max_digits=4, decimal_places=2)
objects = AnimalManager()
Then:
animals = Animal.objects.get_male_tigers()
Of course you can further refactor to your needs

Answer 2

objects, of course, can't be a reference to both GenderManager and SpeciesManager. Given how MRO works, in your example it is a SpeciesManager instance.
You can create a third manager:
class AnimalManager(GenderManager, SpeciesManager):
def male_and_tiger(self):
return self.male & self.tiger()
[all other combinations here]
class Animal(GenderModel,SpeciesModel):
[...]
objects = AnimalManager()
But really I think you are being possessed by the OOP inheritance demon :) A plain call to the orm is much better and more future proof (unless you want to add a new method to your SpeciesManager each time you add a new species)