Django queryset filter for backwards related fields

Question

How can you filter a model based on a model that relates to it? Example below...this works, but I think this hits the DB twice and is pretty inelegant. Is there a way to do it directly with querysets? Maybe somehow with select_related() , but haven't been able to figure that one out. I want to return a QuerySet of Project .

from django.db import models

class Person(models.Model):
    pass

class Project(models.Model):
    pass

class Action(models.Model):
    person = models.ForeignKey(Person)
    project = models.ForeignKey(Project)

# Better way to do this?
def projects_by_person(person):
    actions = Action.objects.filter(person=person)
    project_ids = actions.values_list('project')
    return Project.objects.filter(id__in=project_ids)

Answer 1

Try this. I haven't tested it let me know if you have any issues

#Untested Code
Project.objects.filter(action__person = person)

Answer 2

Is it true that you have a many-to-many relation between Person and Project ? If so, you can simplify your setup like this:

class Person(models.Model):
    projects = models.ManyToManyField('Project')
    name = models.CharField(max_length=100)     # just an example

class Project(models.Model):
    # ... some fields here ...

You can then eg issue the following query to get all the projects from people who are called John:

Project.objects.filter(person_set__name="John")

The usage of select_related() can speed up the lookup a bit when you have lots of queries which follow relations between different database tables but you don't need it to accomplish what you want.

Answer 3

You can find my case at below:

There are teachers and courses. Teachers can have multiple course but course has done by only one teacher.

class Course(models.Model):
    teacher = models.ForeignKey(Teacher, on_delete=models.CASCADE)
    -------

class Teacher(models.Model):
    name = models.CharField(max_length=50)
    ------

Inside the individual Teacher page I want to list all courses given by this individual teacher. You can find the related view function below.

def teacher(request, teacher_id):
teacher = get_object_or_404(Teacher, pk=teacher_id)
courses = Course.objects.filter(teacher = teacher)

context = {
    'teacher': teacher,
    'courses': courses
}