查找具有多个“子”对象的 Django 模型记录？

Question

If I have two models in Django, Parent and Child , where Child has a foreign key relationship to Parent like this:

class Parent(models.Model):
    parent_name = models.CharField(max_length=128, blank=False, default='no name')

class Child(models.Model):
    child_name = models.CharField(max_length=128, blank=False, default='no name')
    parent = models.ForeignKey('app.Parent', on_delete=models.CASCADE, null=False)

How can I find all Parent records that have at least two Child records? Ideally, the solution would use a single .filter() statement on Parent .

Answer 1

You can annotate on the number of Child s and then filter on that number, like:

from django.db.models import Count

Parent.objects.annotate(
    
).filter()

This will generate a query like:

SELECT parent.*, COUNT(child.id) AS nchild
FROM parent
LEFT OUTER JOIN child ON parent.id = child.parent_id
GROUP BY parent.id

One can change the .filter(..) condition to all sorts of conditions on the number of childs nchilds , for example nchild=4 filters on Parent s with exactly four children, whereas ~Q(nchild=7) will exclude all Parent s with exactly seven children. We can thus make more complicated filters.