I have the following models:
class Dog(AbstractAnimals):
pass
class Meta:
app_label = 'animals'
class Cat(AbstractAnimals):
pass
class Meta:
app_label = 'animals'
Which are abstract of:
class AbstractAnimals(models.Model):
description = models.CharField()
This is where my question comes in. If I have a House model, it can have one or both of these
animals. I can think of two ways do to this, but I'm unsure which is better.
class House(models.Model):
user = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='animal', null=True)
1) Use 2 GenericForeignKey's on the House Model
2) Use 2 standard ForeignKey's.
3) another option?
Which is the better option, and or opinions? Thanks.
If House can have two animals and animal can be in only one house and you want your code to be prepared to easily add other animal types, then AbstractAnimal should have regular foreign key to House and House shouldn't have any foreign keys in this case.
This kind of relation between objects (or models or tables) is usually called "many to one": many animals are linked (or can be) to a single house.
Example usage of GenericForeignKeys is comment or tag model, where many unrelated content types can be commented or tagged (articles, blog entries, images, videos, etc.). In such case comment or tag model should have GenericForeignKey to "unknown" model. Some comments and tagging packages available on the Internet are using this pattern.
GenericForeignKey's in my opinion are best suitable when you want to write a reusable application or package in which models needs to be related to some unknown content types (other models) at the time of writing.
Related
I have a Django app where I want to use a set of abstract base classes to define certain models. I want to connect some of these abstract models through foreign keys.
I know that Django does not allow models.ForeignKey to abstract models. I've done some searching on StackOverflow and the general consensus solution seems to be - using GenericForeignKey. However, from my understanding, this results in an extra db-level SELECT call. I want to avoid this because quite a few of my abstract models have this kind of relationship. An example is given below:
class Person (models.Model):
name = models.CharField (max_length=256)
class Meta:
abstract = True
class Phone (models.Model):
phone_no = models.BigIntegerField ()
owner = models.ForeignKey (Person) # This is, of course, wrong. I'd like something like this.
class Meta:
abstract = True
Besides the posts about GenericForeignKey, I also came across a post about dynamic model generation. The link is given below. The question-poster themselves have provided the answer.
Defining an Abstract model with a ForeignKey to another Abstract model
I would like to ask:
if this still holds for the current versions of Django,
if there are any caveats I should be aware of, and
if there is perhaps a more contemporary solution?
Thank you.
I have solved the issue. As pointed by Willem and Mohit, I was thinking about the problem wrongly (I come from the Cpp world and am very new to the Python/Django programming mindset).
I ended up having the abstract classes defined without any relationships and then having concrete classes derived from these abstract ones actually define the relationships. This is also in keeping with the DRY principle.
I deleted my previous question, since it was terribly worded and my non-working examples were just confusing.
I have a series of models, such as Vehicle, Computer, Chair, and whatnot. My goal is to be able to attach an arbitrary number of images to each of them. That's my question: what's the best design pattern to achieve this?
What I've tried so far is to create an abstract model, AttachedImage. I then inherit from this model, and create more specific models, like VehicleImage, ComputerImage, or ChairImage. But this doesn't feel like the right way to pursue this.
As I mentioned in the comment on the deleted question, the correct way to do this would be to use generic relations.
Make your AttachedImage model concrete, but add content_type and object_id fields and the content_object GenericForeignKey. That content_object field can now point to an instance of any model.
To make the reverse relationships easier, you can add GenericRelation accessors to your Vehicle, Computer and Chair models.
Thanks to Daniel Roseman's, I found out about generic relationships. You can find a great overview and tutorial about them here. There are basically two ways to achieve this.
The first one is using generic relationships as explained in the tutorial I linked. This system is very versatile and powerful, but in my case, this would add an additional layer of complexity that quite frankly I don't need.
If you're a newbie with Django like I am, you can consider a much more straightforward, but less 'systematic' pattern, also detailed in that tutorial I linked: simply add a ForeignKey field for each one of the objects you want your main model to be related to. Following the example I used in my question:
class AttachedImage(models.Model):
uuid = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
author = models.ForeignKey(User, on_delete=models.CASCADE)
image = models.ImageField(upload_to=image_path) # TO FIX: añadir la ruta
is_default = models.BooleanField(default=False)
To this, you just add fields for the relationships you will need, such as:
parent_vehicle = models.ForeignKey(Vehicle, on_delete=models.CASCADE, null=True)
parent_computer = models.ForeignKey(Computer, on_delete=models.CASCADE, null=True)
parent_chair = models.ForeignKey(Chair, on_delete=models.CASCADE, null=True)
And just use them as you regularly would. The downside is ending up with all those extra fields you don't really need, but being NULLable, they won't take up much space. The other downside is that doing this with a big number of models is probably overkill, in which case you should really use the first solution.
If you don't want to use the GenericForeignKey, you can implement your own polymorphic models, with multiple table inheritance. Following are the pseudo-models for the same:
class Attachable(models.Model):
pass
class Image(models.Model):
attachable = models.ForeignKey(Attachable, related_name='images')
class Video(models.Model):
attachable = models.ForeignKey(Attachable, related_name='videos')
class Vehicle(Attachable):
vehicle attrs...
class Computer(Attachable):
computer attrs...
class Chair(Attachable):
chair attrs...
For later optimizations, Attachable can also have an attribute which describes its subtype.
We are developing a collection management project using Django, usable for different types of collections.
This problem quite naturally divides itself in two:
The common part, that will be shared by all collections.
The specializations, different for each collection type.
Example
To illustrate this a bit further, let's take a simplified example in pseudocode.
Common part
class ItemBase: # ideally abstract
name = CharField()
class Rental
item = ForeignKey("Item")
rented_to_person = CharField()
Specialization for a collection of cars
class ItemSpecialization
horse_power = Int()
The problem
The question is how we could organize the code in order to allow reuse of the common part without duplicating its content ?
We would imagine it would be best to have the common part as a non-installed application, and have each specialized configuration as a separate installed application. But this would cause a problem with the Rental concrete class, because it resides in the common-part application.
Any advices on how we could proceed ?
It really depends on what you want, you may use an abstract model class for common stuff, and inherit from that in specialized model classes.
Otherwise, if you really want one table for all common data, typically to be able to relate to it, then you'll need your specialized model to have a relation to the common model. It can be a foreign key, or you can use model inheritance, in which case the foreign key in question will be managed for you by django, but it'll be harder to use.
It sounds like you're looking for a OneToOneField field relationship. Based on your example:
class ItemBase:
name = models.CharField(max_length=50)
class Rental:
item = models.OneToOneField(ItemBase)
rented_to_person = models.CharField(max_length=50)
class ItemSpecialization
item = models.OneToOneField(ItemBase)
horse_power = models.IntegerField()
With this model hierarchy, you could fetch Rental or ItemSpecialzation objects and also gain access to ItemBase fields. It's basically OO inheritance for Django models. More details in the docs: https://docs.djangoproject.com/en/1.9/topics/db/examples/one_to_one/
as the question states, how do I do create a many-to-one relationship in django models?
Basically, I have two models: Article and Comic, I want to have one Comment model which will have a relationship with both Article and Comic, but not both.
So if a Comment object has a relationship with an Article object, then it wont have a relationship with a Comic object.
I am currently doing it the following way, which does not work:
class Article(models.Model):
#other fields
class Comic(models.Model):
#other fields
class Comment(models.Model):
article = models.ForeignKey(Article)
comic = models.ForeignKey(Comic)
I would really appreciate some help.
This is tricky. I think there are a couple ways you could model this.
Using your current way you could enforce your uniqueness constraint in the application.
class Comment(models.Model):
article = models.ForeignKey(Article)
comic = models.ForeignKey(Comic)
def save(self, *args, **kwargs):
# assert that there is either comic OR article but not both
super(Comment, self).save(*args, **kwargs)
with this way, what happens if you add another model that you want Comment to reference?? You will have to manually add the conditional for the new type in your save method and perform a migration.
Django provides GenericForeignKey field that would allow you to reference any model from Comment. https://docs.djangoproject.com/en/1.8/ref/contrib/contenttypes/#generic-relations
This would allow you to create a generic reference from Comment to either Article or Comic, and since it is only one field, would by default be mutually exclusive. I find querying and using GenericeForeignKeys awkward; but they are still an option, that might work fine for your use case.
Another powerful option, (my favorite) could be to create a polymorphic model, which would also be mutually exclusive.
Each Comment could reference a generic Piece of Content, using model inheritance. (I did not test the following, so it will probably not work as copied/pasted)
class Content(models.Model):
objects = InheritanceManager()
# shared content fields could be stored in this model
class Article(Content):
# article specific fields
class Comic(Content):
# comic specific fields
class Comment(models.Model):
content = models.OneToOneField(Content)
This is a powerful way to model the relationship of Comment to any Content. This DOES add additional query overhead, and should warrant an audit for your use case.
InheritanceManager is a utility provided by django-model-utils package, and is pretty lightweight. I have used in in production environment and it is performant, as long as you understand the additional query overheard involved with modeling your data using it. https://django-model-utils.readthedocs.org/en/latest/managers.html#inheritancemanager
The query overhead is explained in the documentation.
If you think you will add additional Content subclasses in the future this could be a scalable way to model your relationship, and provides more flexibility in filtering then GenericForeignKeys.
Well, you can add another field to you Comment model. Like
class Comment(models.Model):
article = models.ForeignKey(Article, null = True)
comic = models.ForeignKey(Comic, null = True)
assigned = models.BooleanField(initial=False)
Once a comment object is created, put either article or comic to point at another object and make assigned = True.
I want to model an article with revisions in Django:
I have following in my article's models.py:
class Article(models.Model):
title = models.CharField(blank=False, max_length=80)
slug = models.SlugField(max_length=80)
def __unicode__(self):
return self.title
class ArticleRevision(models.Model):
article = models.ForeignKey(Article)
revision_nr = models.PositiveSmallIntegerField(blank=True, null=True)
body = models.TextField(blank=False)
On the artlcle model I want to have 2 direct references to a revision - one would point to a published revision and another to a revision that is being actively edited. However from what I understand, OneToOne and ForeignKey references generate a backreference on the other side of the model reference, so my question is, how do i create a one-way one-to-one reference in Django?
Is there some special incantation for that or do I have to fake it by including state into revision and custom implementations of the fields that ask for a revision in specific state?
Edit: I guess, I've done somewhat poor job of explaining my intent. Let's try it on a higher abstraction level:
My original intent was to implement a sort of revisioned article model, where each article may have multiple revisions, where one of those revisions may be "published" and one actively edited.
This means that the article will have one-to-many relationship to revisions (represented by ForeignKey(Article) reference in ArticleRevision class) and two one way references from Article to revision: published_revision and edited_revision.
My question is mainly, how can I model this with Django's ORM.
The back-references that Django produces are programatic, and do not affect the underlying Database schema. In other words, if you have a one-to-one or foreign key field on your Article pointing to your Revision, a column will be added to the Article table in the database, but not to the Revision table.
Thus, removing the reverse relationship from the revision to the article is unnecessary. If you really feel strongly about it, and want to document in your code that the backlink is never used, a fairly common Django idiom is to give the fields a related_name attribute like _unused_1. So your Article model might look like the following:
class Article(models.Model):
title = models.CharField(blank=False, max_length=80)
slug = models.SlugField(max_length=80)
revision_1 = models.OneToOneField(ArticleRevision, related_name='_unused_1')
revision_2 = models.OneToOneField(ArticleRevision, related_name='_unused_2')
def __unicode__(self):
return self.title
That said, it's rare that a one-to-one relationship is actually useful in an application (unless you're optimizing for some reason) and I'd suggest carefully reviewing your DB schema to make sure this is really what you want. It may make sense to keep a single ForeignKey field on your ArticleRevision pointing back to an Article (since an ArticleRevision will, presumably, always need to be associated with an Article) and adding another column to Revision indicating whether it's published.
What is wrong with the link going both ways? I would think that the OneToOneField would be the perfect choice here. Is there a specific reason why this will be a detriment to your application? If you don't need the backreference why can't you just ignore it?