CODEWITHSUNDEEP
djangodjango-models
meepmeep
meepmeep

Reputation: 3118

django custom model field

If I have 2 interlinked models:

class Person(models.Model)
    name = models.CharField()

class Project(models.Model):
    person = models.ForeignKey(Person)
    title = models.CharField()

I frequently find myself trying to find the number of Projects associated with each Person:

person = Person.objects.get(id=1)
no_projects = Project.objects.filter(person=person).count()

Is there a way of adding this as a custom field to the Person model, such that I may just call person.no_projects?

Upvotes: 1

Views: 499

Answers (3)

Daniel Roseman
Daniel Roseman

Reputation: 599470

In fact, that functionality is (almost) built-in. Instead of querying Projects, you can just do person.project_set.count() to get that person's count of projects (and .all() to get the actual list).

Upvotes: 4

Colin Su
Colin Su

Reputation: 4609

You can use the property() to generate the dynamical field.

class Project(models.Model):
    person = models.ForeignKey(Person)
    title = models.CharField()

    def _get_no_projects(self):
        return Projects.objects.filter(person=self).count()

    no_projects = property(_get_no_projects)

But the dynamical fields can not be used in querying, because the they don't really store data into the database.

FYI: http://www.b-list.org/weblog/2006/aug/18/django-tips-using-properties-models-and-managers/

Upvotes: 1

Iain Shelvington
Iain Shelvington

Reputation: 32244

This can be done by adding a property to the Person class.

class Person(models.Model)
    name = models.CharField()

    @property
    def no_projects(self):
        return Project.objects.filter(person=self).count()

This can be called now like this

person = Person.objects.get(id=1)
person.no_projects

Upvotes: 4

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