I have two models in my django application, event/user, that have a many-to-many relationship.
Event can have many attendees(user).
User can attend many events.
Django automatically made a bridge table for for this relationship.
My question is how can I add new fields to this table using migrations as there is not a model for this Bridge table.
This can be done by creating a "through" table and adding the fields on it. Check out the docs at https://docs.djangoproject.com/es/1.10/topics/db/models/
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __str__(self): # __unicode__ on Python 2
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
This bridge table is called association table. Imo it is bad practice to edit this table that way. Connecting models by many to many field suggest that there is no model between. If your association table is supposed to represent additional data then you should create new model
class NewModel(models.Model):
attender = models.ForeignKey(User, null=False)
event = models.ForeignKey(Event, null=False)
# additional fileds
and remove many to many relationship from event
Related
I don't understand why should I use ManyToManyField if I will (or might) store extra information along the many-to-many relationships.
The doc here shows this example:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=50)
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(
Person,
through='Membership',
through_fields=('group', 'person'),
)
class Membership(models.Model):
group = models.ForeignKey(Group, on_delete=models.CASCADE)
person = models.ForeignKey(Person, on_delete=models.CASCADE)
inviter = models.ForeignKey(
Person,
on_delete=models.CASCADE,
related_name="membership_invites",
)
invite_reason = models.CharField(max_length=64)
Why even having the members field there, if the developer already defines the relationship model and also defines the ForeignKeys? :D Why not leaving out the members ManyToManyField ?? What is the benefit? Thank you.
I don't understand why should I use ManyToManyField if I will (or might) store extra information along the many-to-many relationships.
You do not need to do this. But it makes querying more effective. Indeed, you can now query with:
Group.objects.filter(members=some_person)
this is shorter and more descriptive than working with:
Group.objects.filter(membership__person=some_person)
It is thus a "coding shortcut" to go from the Group model to the Person model and vice versa.
This is a question about how to add a field to a many-to-many relationship in Django.
I have a model LandingPage and a model Product. (Code below). In my project, LandingPages can have many Products listed on them and those same Products can appear on multiple different LandingPages.
Product is connected to LandingPage via a ManyToManyField.
My Goal:
I am trying to figure out how to add a field so that I can set the order (1 through 10) for Products on their associated LandingPages. Reminder, Product instances can appear on multiple LandingPages, so each instance will need to have a different order attribute.
Ideally, I'd like to expose this functionality via the built-in Django admin. Right now it shows the relationships table, but not the order field as it does not yet exist. (Screenshots/mockups below).
My Code:
models.py
class LandingPage(models.Model):
"""Stores a single LandingPage and metadata.
"""
name = models.CharField(max_length=200, help_text="The name is only used internally. It is not visible to the public.")
slug = models.SlugField(default="", editable=False, max_length=150, null=False, verbose_name="Slug", help_text="This is not editable.")
# Additional fields that I do not believe are relevant
class Product(models.Model):
"""Stores a single Product and metadata.
"""
name = models.CharField(max_length=200, help_text="Used internally. Not visible to the public.")
active = models.BooleanField(default=False, verbose_name="Product is Live on Landing Pages", help_text="Determines whether the product should be visible on the assocaited landing page or not.")
landing_page = models.ManyToManyField(
LandingPage,
verbose_name="Landing Page",
help_text="The landing page or pages that this product is assocaited with.",
)
# Additional fields that I do not believe are relevant
admin.py
# Inline configuration used by LandingPageAdmin
class ProductInline(admin.TabularInline):
"""Creates Inline table format for displaying Product data."""
model = Product.landing_page.through
extra = 0
class LandingPageAdmin(admin.ModelAdmin):
"""Specifies LandingPage data in Admin."""
readonly_fields=('slug',)
inlines = [ProductInline]
save_as = True
# Inline configuration used by Product Admin
class LandingPageInline(admin.TabularInline):
"""Creates Inline table format for displaying LandingPage data."""
model = LandingPage.product_set.through
extra = 0
class ProductAdmin(admin.ModelAdmin):
"""Specifies Product data in Admin."""
inlines = [LandingPageInline]
save_as = True
Mockups (for clarity):
Current State
Desired State
(I added the desired functionality in red for clarity. The order integers should be editable so that the order can be re-arranged.)
My Question
How can I accomplish this goal of adding an editable order field to this pre-existing relationship?
Should I manually add an order field to the product-landingpage join table that was automatically created by Django? If I do that, is there a way to have the Django admin show that added field?
Or should I go about it a totally different way?
Thank you in advance!
I found the answer to this.
The solution is create an intermediary model and connect it using "through". Example below:
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __str__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
Official docs are here: https://docs.djangoproject.com/en/3.1/topics/db/models/#intermediary-manytomany
Others in my situation may find it useful to read this question/answer as it does a good job of explaining various solutions: Define an order for ManyToManyField with Django
Hello i am trying to understand Django Data Models and it´s possibilities. After using one-to-one and m2m i am now trying to understand the m2m extra fields. So i followed the Example of the Django Doc and populated the models with some Data.
models.py
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __str__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
So far everything is fine, but as i looked into the model tables, via pgAdmin 4 ( i am using PostgreSQL), i could not find any members column in the Group model. So i made a little visual of my model and some screenshots of pgAdmin, for better understanding.
As you can see Group has only a id and name column.
Question
my question: Is this m2m members relation of Group, only some kind of "virtual" created relation via the Membership.person.fk and Membership.group.fk? Meaning there is no field which will be populated? I try to imagine that it works like a recursive, instance query?
Suppose I have the following two models task and person.
Now each person can have multiple tasks (one to many relationship).
Now in the admin app for a person I can only add one foreign key per record thus I'll have to create multiple records for a person having a different task but the same email and name fields.
Is there anything I can do in personAdmin that would allow me to add multiple foreign keys for the same person and in the backend it would create multiple records ?
class task(models.Model):
description = models.CharField(max_length=100)
class person(models.Model):
task = models.ForeignKey(Author, on_delete=models.CASCADE)
email =models.CharField(max_length=100)
name = models.CharField(max_length=100)
class personAdmin(admin.ModelAdmin):
pass
You could add a third table with both the first two tables as FOREIGN KEY.
So this is what you have currently
class ToDo_Task(models.Model):
description = models.CharField(max_length=100)
class User(models.Model):
task = models.ForeignKey(Author, on_delete=models.CASCADE)
email =models.CharField(max_length=100)
name = models.CharField(max_length=100)
Adding a third table
class person_task(models.Model):
task = models.ForeignKey(ToDo_Task, on_delete = models.CASCADE)
person = models.ForeignKey(User, on_delete = models.CASCADE)
...
You may add even more fields in the new table.
I'm having trouble understanding the use of ManyToMany models fields with a through model. I can easily achieve the same without the ManyToMany field. Considering the following from Django's docs:
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, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
What I don't understand, is how is using the ManyToMany field better than simply dropping it and using the related manager. For instance, the two models will change to the following:
class Group(models.Model):
name = models.CharField(max_length=128)
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE, related_name='members')
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
What am I missing here?
You're right, if you define the membership table explicitly then you don't need to use a ManyToManyField.
The only real advantage to having it is if you'd find the related manager convenient. That is, this:
group.members.all() # Persons in the group
looks nicer than this:
Person.objects.filter(membership_set__group=group) # Persons in the group
In practice, I think the main reason for having both is that often people start with a plain ManyToManyField; realize they need some additional data and add the table explicitly; and then continue to use the existing manager because it's convenient.
So I just wanted to add to anyone who is looking at this and may want another example to save them research. For one, I think it's important to note that in OP's questions, he should of removed the Group model not the People model and removed the matching field from the Membership model. That way, the model goes back to it's original meaning.
When looking at a many-to-many relationship, the through field can almost be contrived as the "why" to the many-to-many relationship. If we give the nomenclature a different name, it might change what the reader sees:
class Person(models.Model):
name = models.CharField(max_length=128)
class Club(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='RegistrationReceipt')
class RegistrationReceipt(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
club = models.ForeignKey(Club, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
paid_dues = models.BooleanField(default = True)
fee_payment_date = models.DateTimeField()
Now, you can imagine yourself adding all sorts of logic whenever a member joins this club. When they joined? Why did they join? Did they pay? When is their payment date? etc. You can obviously tackle this relationship in different ways, but you can see more clearly the use of "through" in a Many-to-Many relationship.
Also, for those that know SQL. The through attribute/field is the way you customize the intermediary table, the one that Django creates itself, that one is what the through field is changing.
I have some problem with the answer from Kevin Christopher Henry.
I don't think that the equivalent of the group.members.all() without a through="members" is Person.objects.....
Instead I think it is group.person_set.all() if the M2M field is on Person side. Or group.persons.all() if the M2M field is inside Group.
But I think without through=.. you have no control over the created table. It contains and will contain just 2 fields: both ID's of the related rows.
But with through=.. you can create the model yourself and add (now or later) the additional fields, which often can have a good reason. Example of such field: valid_from = DateField(), or so.