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.
Here is the solution for the above scenario. There are three parts to setting up a GenericForeignKey:
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.
from django.contrib.contenttypes.models import ContentType contenttype_obj = ContentType.objects.get_for_model(project_object)
TimeLine.objects.filter(object_id=project_object.id, content_type=contenttype_obj)
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