I don't understand why we have to use ManyToManyField to declare a many to many association. To do so, I would create another table with two foreign keys, period!
Here is an example from the doc https://docs.djangoproject.com/en/dev/topics/db/models/#intermediary-manytomany:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
class Membership(models.Model):
person = models.ForeignKey(Person)
group = models.ForeignKey(Group)
extraField = models.DateField()
I would just write:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
class Group(models.Model):
name = models.CharField(max_length=128)
class Membership(models.Model):
person = models.ForeignKey(Person)
group = models.ForeignKey(Group)
extraField = models.DateField()
Is it correct? What is the difference?
There is nothing wrong with defining an intermediate model for the relationship. That way you can store extra information on the intermediate model (Membership), like when the person joined the group, or if the membership is still valid. However, most of the time you don't need the extra information and only want to store which person is member of which group. In those cases, you could just use a simple ManyToManyField. Django makes it very easy to work with those fields, so you can do group.members.add(user) and group.members.delete(user), compare that to:
Membership.objects.create(user=user, group=group)
Membership.objects.get(user=user, group=group).delete()
Disclaimer: pseudo-code, might not actually work
You can still use ManyToManyField with an intermediate model, this allows for most of the Django ManyToMany conveniences, but with some restrictions:
Unlike normal many-to-many fields, you can’t use add, create, or assignment (i.e., beatles.members = [...]) to create relationships.
Why? You can’t just create a relationship between a Person and a Group - you need to specify all the detail for the relationship required by the Membership model. The simple add, create and assignment calls don’t provide a way to specify this extra detail. As a result, they are disabled for many-to-many relationships that use an intermediate model. The only way to create this type of relationship is to create instances of the intermediate model.
The remove() method is disabled for similar reasons. However, the clear() method can be used to remove all many-to-many relationships for an instance.
Once you have established the many-to-many relationships by creating instances of your intermediate model, you can issue queries. Just as with normal many-to-many relationships, you can query using the attributes of the many-to-many-related model.
Source: docs.djangoproject.com
Related
Given these models:
class Event(models.Model):
name = models.CharField()
class Project(models.Model):
name = models.CharField()
class EventProject(models.Model):
event= models.ForeignKey(Event)
project= models.ForeignKey(Project)
Is there a way to get all the Events for each Project like this: with the property project.events = [<array of Event>]?
I found a similar question, however one of the tables has the members = models.ManyToManyField(Person, through='Membership') relationship so it's easier to do. I can't do as in the related question and add a relationship as I cannot change the model.
It is straight Forward
Event.objects.filter(eventproject__project=project)
Filtering through ForeignKey works both forward and backward in Django. You can use the model name in lower case for backward filtering.
https://docs.djangoproject.com/en/3.1/topics/db/queries/#lookups-that-span-relationships
Suppose I have following models:
class Author(Model):
name = CharField()
class Publication(Model):
name = CharField()
authors = ManyToManyField(Author)
class Meta:
abstract = True
class Book(Publication):
pass
class Article(Publication):
pass
class Journal(Publication):
pass
How to change code so that I can add through table to authors? If I write authors = ManyToManyField(Author, through='Relationship'), it will not work.
Django thankfully takes care of intermediate tables without any coding. You can even access them using .through on the M2M relationship manager, e.g. one_publication.authors.through.
You only need to specify the through table if you want to manage it yourself, e.g. because you want to add more fields than just the foreign keys of the two related entities. Is that the case here?
If yes, you have to create a Relationship model (consider giving it a more helpful name) that contains foreign keys to Publication and Author.
Update: If you want to add a default order to object lists from many-to-many relationships, an intermediate model would indeed be one way to achieve this:
class Relationship(models.Model):
author = models.ForeignKey(Author)
publication = models.ForeignKey(Publication)
# Any further fields that you need
class Meta:
ordering = ['author__last_name', 'author__first_name']
However, you can just as easily order your m2m relationships on querying them, without any intermediate model or through manager:
book = Book.objects.first()
for author in book.authors.order_by('last_name', 'first_name'):
# Will print in alphabetical order
print(f'Author: {author.first_name} {author.last_name}')
One caveat is that if you use prefetching, you need to specify the ordering in a Prefetch object to avoid executing the query twice, first for prefetching without ordering, than on access with ordering.
# 2 queries plus one for every book
books = Book.objects.prefetch_related('authors')
for book in books:
for author in book.authors.order_by('last_name', 'first_name'):
print(f'Author: {author.first_name} {author.last_name}')
# 2 queries regardless of number of books
books = Book.objects.prefetch_related(Prefetch('authors',
queryset=Author.objects.order_by('last_name', 'first_name')))
for book in books:
for author in book.authors.all():
print(f'Author: {author.first_name} {author.last_name}')
Here's a version of my models.py file. I've removed irrelevant fields and the model names are made up, for security reasons:
class FilmStudio(models.Model):
name = models.CharField(max_length=200, unique=True)
class ProductionCompany(models.Model):
name = models.CharField(max_length=200)
film_studio = models.ForeignKey(FilmStudio)
class Meta:
# Ensure that a given combination of ProductionCompany name and FilmStudio object is unique
unique_together = ('name', 'film_studio')
class Film(models.Model):
title = models.CharField(max_length=200)
production_company = models.ForeignKey(ProductionCompany)
class Actor(models.Model):
name = models.CharField(max_length=200)
films = models.ManyToManyField(Film, blank=True)
Although it is not explicitly defined, there is a many-to-many relationship between an Actor and a FilmStudio. This is evidenced by the following call to the Python API:
FilmStudio.objects.filter(productioncompany__film__actor__name='Samuel L. Jackson').distinct()
This returns all of the FilmStudio objects which Samuel L. Jackson is related to, and each one only once. What I'd like is to define extra fields on the relationship between an Actor and a FilmStudio (it doesn't work too well in this example, I know, but it makes sense for my scenario).
Following what is described in Extra fields on many-to-many relationships, I could use an intermediate model to define extra fields on the relationship between a Film and an Actor, for instance.
But this doesn't seem to help me with my problem. I don't want to define the Actor to FilmStudio relationship explicitly, since it's an existing relationship based on other relationships.
Is it possible to define fields on the relationship that I'm describing?
As far as I know, you are not able to do that.
The reason for that is that it is nowhere to store the extra fields of that relationship. If I understand you correctly, these "extra fields" are not implicit in the actor-film or productionstudio-film relationships, so even though you say they are implicit, the extra fields themselves are explicit.
You could try to emulate it by creating an explicit direct relationship whenever it is needed. Then you could simulate the extra fields using the model as an abstraction, but I am not sure if this is what you want. If you opt for this kind of solution you can use default values (in your abstraction) for filling in relationships that don't have a instance yet.
Does that explanation make sense to you?
EDIT:
(I have not double checked that the code works, so be vary)
OK, so you have the original models:
class FilmStudio(models.Model):
name = models.CharField(max_length=200, unique=True)
class ProductionCompany(models.Model):
name = models.CharField(max_length=200)
film_studio = models.ForeignKey(FilmStudio)
class Meta:
# Ensure that a given combination of ProductionCompany name and FilmStudio object is unique
unique_together = ('name', 'film_studio')
class Film(models.Model):
title = models.CharField(max_length=200)
production_company = models.ForeignKey(ProductionCompany)
class Actor(models.Model):
name = models.CharField(max_length=200)
films = models.ManyToManyField(Film, blank=True)
# The "solution" would be:
class ActorProductionComapny(models.Model):
production_company = models.ForeignKey(ProductionCompany, related_name='actors')
actor = models.ForeignKey(Actor, related_name='companies')
# your extra fields here
someproperty = models.CharField(max_length=200)
class Meta:
# let's say one per actor
unique_together = ('production_company', 'actor')
This is going to get messy really quickly
We use a F object like this:
FilmStudio.objects.filter(productioncompany__film__actor__name='Samuel L. Jackson',
productioncompany__film__actor=F('actors__actor'),
actors__someproperty="Plays poker with CEO").distinct()
The tricky part is going to be handling default values (i.e. when there is no value) This would have to be implemented using a custom Manager, but then I am out of my depth.
I will try to explain as well as I can, but it's going to be tricky.
If you want to make a filter on the relationship you may have to do something like this:
def filter_prod(pq_query, someproperty, actor_name):
if someproperty == "Default":
# Great, this means we can ignore the parallel relationship:
return pq_query.filter(productioncompany__film__actor__name=actor_name)
else:
# Here comes the hard part
FilmStudio.objects.filter(productioncompany__film__actor__name=actor_name,
productioncompany__film__actor=F('actors__actor'),
actors__someproperty=someproperty).distinct()
The thing I am trying to illustrate here is that there are two kinds of actor-productioncompany relationships, those with custom field values (non-default), and those without.
Now, you can make a custom getter that looks something like this:
class ProductionCompany(models.Model):
name = models.CharField(max_length=200)
film_studio = models.ForeignKey(FilmStudio)
def get_actors(self):
# This one is not lazy, so be aware
actors = list(self.actors)
# Get a list of actor IDs
actor_ids = [a.actor_id for a in actors]
for actor in Actor.objects.filter(films__production_company_id=self.id):
if actor.id not in actor_ids:
actors.append(ActorProductionComapny(actor=actor, production_company=self)
actor_ids.append(actor.id)
return actors
class Meta:
# Ensure that a given combination of ProductionCompany name and FilmStudio object is unique
unique_together = ('name', 'film_studio')
This should not save the relationship to the database until you call .save() on an instance. You can also add a custom save method that ignores/aports .save() calls where all the values are default. Just remember to check if it is a new instance or not, because you don't want it to cancel a "set back to default" call. You could also make it delete on a "set back to default", but check if you are allowed to do that within .save().
For even more complex queries (mix of default and non-default) you have Q-objects (further down on the page from F objects)
In short, you need to create an extra model to store this extra relational data between Actor and FilmStudio.
class Actor(models.Model):
name = models.CharField(max_length=200)
films = models.ManyToManyField(Film, blank=True)
film_studios = models.ManyToMany(FilmStudio, through='ActorFilmStudio')
class ActorFilmStudio(models.Model):
actor = models.ForeignKey(Actor)
film_studio = models.ForeignKey(FilmStudio)
# define extra data fields here
data1 = models.TextField()
data2 = models.IntegerField()
One way to think about this: the data you're trying to store belongs to an Actor-FilmStudio relation, and is not related in anyway to Film or ProductionCompany.
Your existing ability to retrieve the a set of Actors for a given FilmStudio (or vice-versa) does not necessarily imply you can store relational data belonging to these two models using the models defined in your example.
Keep in mind that each of the models you defined in your example are backed by a table in your database. In the case of Actor.films field, Django creates an extra table to store the many-to-many relationship data.
Since you're looking to store relational data between Actor and FilmStudio, you need to consider where the data will be stored in your database. Can you store the data in the Film model? or the ProductionCompany model?
I have two classes in my models.py file:
class Person:
person_name = models.CharField(max_length = 50)
class Course:
course_name = models.CharField(max_length = 50)
course_person = models.ManyToManyField(Person)
In my modified example, one person is takes many courses and one course is taken by many people, hence ManyToMany.
When I let Django auto-generate my table, I get an extra ID field.
I want the autogenerated person_course manytomany table to consist of the two composite keys person_id and course_id only. Note: Both of them are auto-generated, auto-incremented fields.
I have also tried defining my ManyToMany class and attempted to link the fields using the keyword through=, but that did not help.
I have asked Google but without much help. Many some geniuses among you can provide some hint :)
Django currently does not support composite primary key by default
What you can do instead is keep the auto generated id as the (surrogate) primary key and then define a unique_together relationship in the through table.
class Meta:
unique_together = (course, person)
This way, you can guarantee unique entries in the through table, and when you reference the id it is the equivalent of referencing the unique (course, person) which is what we want anyways.
There are some third party apps that implement this feature if you want. However, unless an absolute necessity (like a legacy system support), I would just keep it simple and implement unique_together.
from django.db import models
class Person(models.Model):
SHIRT_SIZES = (
('S', 'Small'),
('M', 'Medium'),
('L', 'Large'),
)
name = models.CharField(max_length=60)
shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)
I needed to assign one or more categories to a list of submissions, I initially used a table with two foreign keys to accomplish this until I realized Django has a many-to-many field, however following the documentation I haven't been able to duplicate what I did with original table.
My question is : Is there a benefit to using many-to-many field instead of manually creating a relationship table? If better, are there any example on submitting and retrieving many-to-many fields with Django?
From the Django docs on Many-to-Many relationships:
When you're only dealing with simple many-to-many relationships such
as mixing and matching pizzas and toppings, a standard ManyToManyField
is all you need. However, sometimes you may need to associate data
with the relationship between two models.
In short: If you have a simple relationship a Many-To_Many field is better (creates and manages the extra table for you). If you need multiple extra details then create your own model with foreign keys. So it really depends on the situation.
Update :- Examples as requested:
From the docs:
class Person(models.Model):
name = models.CharField(max_length=128)
def __unicode__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __unicode__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person)
group = models.ForeignKey(Group)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
You can see through this example that membership details (date_joined and invite_reason) are kept in addition to the many-to-many relationship.
However on a simplified example from the docs:
class Topping(models.Model):
ingredient = models.CharField(max_length=128)
class Pizza(models.Model):
name = models.CharField(max_length=128)
toppings = models.ManyToManyField(Topping)
There seems no need for any extra data and hence no extra model.
Update 2 :-
An example of how to remove the relationship.
In the first example i gave you have this extra model Membership you just delete the relationship and its details like a normal model.
for membership in Membership.objects.filter(person__pk=1)
membership.delete()
Viola! easy as pie.
For the second example you need to use .remove() (or .clear() to remove all):
apple = Toppings.objects.get(pk=4)
super_pizza = Pizza.objects.get(pk=12)
super_pizza.toppings.remove(apple)
super_pizza.save()
And that one is done too!