_set in Django for a queryset

Question

What you are seeing is a reverse related object lookup.

In your example:

class Blog(models.Model):
    pass

class Entry(..):
    blog = Blog(..)

Now, given object e of type Entry, you would do e.blog to access the related object Blog - which is a forward relation. The _set is a reverse lookup class variable django puts in for you.

So, given object b - you would do:

entries = b.entry_set.all()

The reason the reverse is a queryset is, ForeignKey is 1-to-many relationship. Hence, the reverse is a queryset.

The _set object is made available when related_name is not specified.

Briefly speaking:

Suppose you have a model Car and a model Wheel. Wheel has a foreign key relationship with Car as follows:

class Car(models.Model):
    pass

class Wheel(models.Model):
    car = models.ForeignKey(Car, on_delete=models.CASCADE) # on_delete parameter is mandatory in Django 2.0

Let's say w is an instance of Wheel, and c is an instance of Car:

>>> w.car # returns the related Car object to w
>>> c.wheel_set.all() # returns all Wheel objects related to c

Detailed explanation

Using the models defined above, a Wheel object w can get its associated Car object by accessing the car attribute: w.car.

If a model has a ForeignKey, instances of that model will have access to the related foreign object via a simple attribute of the model.

According to the official Django documentation:

Django also creates API accessors for the “other” side of the relationship – the link from the related model to the model that defines the relationship.

In this case, a Car object c has access to a list of all related Wheel objects via the wheel_set attribute: c.wheel_set.all().

If a model has a ForeignKey, instances of the foreign-key model will have access to a Manager that returns all instances of the first model. By default, this Manager is named FOO_set, where FOO is the source model name, lowercased. This Manager returns QuerySets, which can be filtered and manipulated.