Django how to find if the object is referenced by ForeignKey from another class in model.py-CodePudding

I have two classes shown below, I wanted to add a function to File to check if the file is referenced by any data inside the Project class (similar how "was published recently" is done here: https://docs.djangoproject.com/en/4.0/_images/admin12t.png ).

class File(models.Model):
    def __str__(self):
        return self.name
    file = models.FileField(upload_to='uploads/')

class Project(models.Model):
    def __str__(self):
        return self.name
    files = models.ManyToManyField(File)

CodePudding user response：

class File(models.Model):
    file = models.FileField(upload_to='uploads/')

    def __str__(self):
        return self.name

    def is_used_in_a_project(self):
        return self.project_set.exists()

Django automatically exposes a <model>_set attribute on the other side of a ManyToManyField relation. This is a queryset containing all of the instances that are linked to it via the m2m relation on the other model.

See https://docs.djangoproject.com/en/4.0/topics/db/examples/many_to_many/

You can alter the name of this "reverse relation" attribute by setting the related_name of the ManyToManyField.

https://docs.djangoproject.com/en/4.0/ref/models/fields/#manytomany-arguments

e.g. you could define:

class Project(models.Model):
    files = models.ManyToManyField(File, related_name="linked_projects")

class File(models.Model):
    def is_used_in_a_project(self):
        return self.linked_projects.exists()