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If you have developed web services using Django, chances are that you have come across the Generic Relations. It is a technique that allows you to link a model with multiple other models without knowing them before hand. It comes in quite handy when you have common traits which can be applied to several models. A typical example is a tagging system which is used as an example in the docs.
I have used it a couple times and found it very useful, however, there is one problem I have noticed with it - you cannot apply that trait (say tags) more than once in a model. To illustrate this issue and how I solved it, let’s look at my example.
First I had a model
Event which has images which I process using django-s3direct.
class Event(models.Model): ... images = JSONField(S3DirectField(), null=True, blank=True) ...
The first problem was that using the JSONField, a lot of work had to be done in the admin to be able to display the s3direct file upload widget correctly. As a result, I decided to create a separate model to wrap the s3direct field and use the widget provided by the package. Also, considering that I will be using images in other models too, I decided to use generic relations.
So now we have this:
class Image(models.Model): link = S3DirectField() content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE) object_id = models.PositiveIntegerField() content_object = GenericForeignKey('content_type', 'object_id') class Event(models.Model): ... images = GenericRelation(Image, related_name='events') ... class Place(models.Model): ... images = GenericRelation(Image, related_name='places') ...
Great! Now the Image model can be used in any model needing images and I can simply do this
event.images.all() to fetch all the images belonging
to a particular event.
GenericRelation is just a manager so we can use the
images field the same way we can use any manager:
Now the next problem - I suddenly now needed to have images uploaded by the event creators (official images) and images uploaded
by people who attended the event (let’s just say random images). So how do we separate this? surely I can’t add another
GenericRelation in the
Event model because it doesn’t even make sense. In fact,
images is not
actually a database field but just a manager and having the same manager twice won’t make any difference.
So how then can I use this so that I can differentiate between the official images and random images for events? The solution is quite simple but may not be very obvious at the beginning (at least it wasn’t to me )
The hack is to do this in the level of the
Image model. Precisely, I introduced a nullable field
purpose in the image model that
allows models consuming the relationship to specify the usage of particular
So now the code looks like this:
class Image(models.Model): link = S3DirectField() # A hack to allow models have "multiple" image fields purpose = models.CharField(null=True, blank=True) content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE) object_id = models.PositiveIntegerField() content_object = GenericForeignKey('content_type', 'object_id') class Event(models.Model): ... images = GenericRelation(Image, related_name='events') ... @property def official_images(self): return self.images.filter(purpose='official') @property def random_images(self): return self.images.filter(purpose='random')
So to create an official image, all I need do is
event.images.create(link='my link', content_object=event, purpose='official').
And I can get them by
Of course, the
purpose field is nullable so other models like
Place can keep using it the same way if multiple image
fields are not needed.
There may also be other ways to solve this issue and you can propose yours in the comments. In any case, I do hope this was of help to you.