I often limit the lookup to fields of the model and forget about backward relations.
Consider the following relationship:
class Group(models.Model): name = models.CharField(max_length=100) class Student(models.Model): name = models.CharField(max_length=100) group = models.ForeignKey(Group)
A group can have many students.
We want to get the groups based on certain conditions on model
Student. An example is getting the groups which contain a student named ‘stud1’. If you can get it using model
Group and without using
Student, you can skip this post.
In such a scenario, we tend to use
Student model. It’s more intuitive because the Foreign Key relationship exists from Student to Group.
Let’s load the following data to test our queries:
group1 = Group.objects.create(name='Group 1') group1 = Group.objects.create(name='Group 2') Student.objects.create(name='stud1', group=group1) Student.objects.create(name='stud2', group=group1) Student.objects.create(name='stud3', group=group1) Student.objects.create(name='stud1', group=group2)
Get the groups which contain student named
We first try to get all the students which satisfy the criteria. Query for that would be:
And then we try to get the groups of those students. If we use the
Student model, we can’t get a queryset of
Group. So our approach would be to first get the ids of desired groups and then get a queryset of Group using those ids.
group_ids = Student.objects.filter(name='stud1').values_list('group', flat=True) groups = Group.objects.filter(id__in=group_ids)
Less obvious way:
What if we could use the
Group model directly?
This gives the exact same result as given by the Intuitive way.
So, even though there is no field called
Group and we didn’t specify any relationship to Student from Group, Django is smart enough to figure out the relationship for us.
Get the groups with name
Group1 which contain students named
group_ids = Student.objects.filter(name='stud1', group__name='Group1').values_list('group', flat=True) groups = Group.objects.filter(id__in=group_ids)
Less obvious way:
groups = Group.objects.filter(name='Group1', student__name='stud1')
Gives the exact same result as given by preceding two queries.
I always missed the backward relationship while following Django tutorial or whenever I read the docs. I tried finding it while writing this post to see if the docs missed mentioning it. I was wrong yet again, there is a section which mentions it.
Thank you for reading the Agiliq blog. This article was writen by akshar on Apr 28, 2014 in django .
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