Understanding 'GenericForeignKey' in Django

In some cases we might want to store generic model object, rather a particular specific model as 'ForeignKey'. Here is the scenario of such kind. Suppose there are models like User, Project, Ticket and TimeLine as below.

class Ticket(models.Model):
    name = models.CharField(max_length=200, verbose_name=_("name"))
    slug = models.SlugField(max_length=250, null=False, blank=True, verbose_name=_("slug"))

class Project(models.Model):
    name = models.CharField(max_length=200, verbose_name=_("name"))
    slug = models.SlugField(max_length=250, null=False, blank=True, verbose_name=_("slug"))

class User(models.Model):
    name = models.CharField(max_length=200, verbose_name=_("name"))
    slug = models.SlugField(max_length=250, null=False, blank=True, verbose_name=_("slug"))

class Timeline(models.Model):
    involved_object = *****
    event_type = models.CharField(max_length=250, default="created")

If we want to store the user activities with these models like "created User, created Project, created Task" in a timeline, we have to create all the three models(User, Project, Task) as 'ForeignKey' fields. This is not a good programming practice. To overcome this, Django's content types framework provides a special field type (GenericForeignKey) which allows the relationship to be with any model. 

Using 'GenericForeignKey':

Here is the solution for the above scenario. There are three parts to setting up a GenericForeignKey:

• Give your model a ForeignKey to ContentType. The usual name for this field is “content_type”.
• Give your model a field that can store primary key values from the models you’ll be relating to. For most models, this means a PositiveIntegerField. The usual name for this field is “object_id”.
• Give your model a GenericForeignKey, and pass it the names of the two fields described above. If these fields are named “content_type” and “object_id”, you can omit this – those are the default field names GenericForeignKey will look for.

So by following the above three steps, the TimeLine will look as following.

class Timeline(models.Model):
    content_type = models.ForeignKey(ContentType, related_name="content_type_timelines")
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('content_type', 'object_id')
    event_type = models.CharField(max_length=250, default="created")

For example, if we want to store the event "created Project" after the project is created, the following snippet will do things for us.

t1 = TimeLine(content_object=project_object)
t1.save()

But due to the way 'GenericForeignKey' is implemented, you cannot use such fields directly with filters (filter() and exclude(), for example) via the database API. Because a GenericForeignKey isn’t a normal field object, the following examples will not work:

TimeLine.objects.filter(content_object=project_object)
# This will also fail
TimeLine.objects.get(content_object=project_object)

To retrieve the TimeLine object with the project object, we have to follow these steps.

• Get the 'ContentType' object with the following code. 

  from django.contrib.contenttypes.models import ContentType
      contenttype_obj = ContentType.objects.get_for_model(project_object)

• "object_id" is stored with project_object.id

• TimeLine.objects.filter(object_id=project_object.id, content_type=contenttype_obj)