How to create a property of relations that can be prefetch_related by specific conditions

How to create a property of relations that can be prefetch_related by specific conditions

I want to set a prefetch_related relation that can be narrowed down to a certain model under specific conditions.

Introduction

For example, suppose you have models X and Y with an M: N relationship.

class X(models.Model):
    name = models.CharField(max_length=32, null=False, default="")
    ctime = models.DateTimeField(auto_now_add=True, null=False)
    is_valid = models.BooleanField(default=True, null=False)


class Y(models.Model):
    name = models.CharField(max_length=32, null=False, default="")
    ctime = models.DateTimeField(auto_now_add=True, null=False)
    is_valid = models.BooleanField(default=True, null=False)

    xs = models.ManyToManyField(X, related_name="ys")

I want to get the relation of X narrowed down by the following conditions for the object with Y.

--order by -ctime --Filter with ʻis_valid = True`

If you write it down directly, it will look like the following

y.xs.all().filter(is_valid=True).order_by("-ctime")

Requirements

--I want to get the relation narrowed down by the corresponding conditions from the instance of the model. ――I want this narrowed relationship to be able to prefetch_related as well.

For example, if you name the relation that satisfies the above conditions as valid_xs, you can use it as follows.

#from instance
y.valid_xs  # =>  [X,X,X,X]

# prefetch_related (N+1 query can be suppressed)
for y in Y.objects.all().prefetch_related("valid_xs"):
    print(y, y.valid_xs)

Implementation

prefetch_related ("valid_xs ") was difficult, so prefetch_related (Y.prefetch_valid_xs ()) is fine.

Define the following function.

def custom_relation_property(getter):
    name = getter.__name__
    cache_name = "_{}".format(name)

    def _getter(self):
        result = getattr(self, cache_name, None)
        if result is None:
            result = getter(self)
            setattr(self, cache_name, result)
        return result

    def _setter(self, value):
        setattr(self, cache_name, value)

    prop = property(_getter, _setter, doc=_getter.__doc__)
    return prop

Change the definition of the model as follows

class X(models.Model):
    name = models.CharField(max_length=32, null=False, default="")
    ctime = models.DateTimeField(auto_now_add=True, null=False)
    is_valid = models.BooleanField(default=True, null=False)

    @classmethod
    def valid_set(cls, qs=None):
        if qs is None:
            qs = cls.objects.all()
        return qs.filter(is_valid=True).order_by("-ctime")

class Y(models.Model):
    name = models.CharField(max_length=32, null=False, default="")
    ctime = models.DateTimeField(auto_now_add=True, null=False)
    is_valid = models.BooleanField(default=True, null=False)

    xs = models.ManyToManyField(X, related_name="ys")

    @custom_relation_property
    def valid_xs(self):
        return X.valid_set(self.xs.all())

    @classmethod
    def prefetch_valid_xs(cls):
        return Prefetch("xs", queryset=X.valid_set(), to_attr="valid_xs")

It can be used as follows.

#from instance
Y.objects.get(id=1).valid_xs  # => [X,X,X,X]

# prefetch_related (N+1 query can be suppressed)
for y in Y.objects.all().prefetch_related(Y.prefetch_valid_xs()):
    print(y, y.valid_xs)

reference

-Think a little more about django's prefetch_related (conditionally added relation eager loading)

It's a blog I wrote myself.