How to express a One-To-Many relationship in Django

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I'm defining my Django models right now and I realized that there wasn't a OneToManyField in the model field types. I'm sure there's a way to do this, so I'm not sure what I'm missing. I essentially have something like this:

class Dude(models.Model):
    numbers = models.OneToManyField('PhoneNumber')

class PhoneNumber(models.Model):
    number = models.CharField()

In this case, each Dude can have multiple PhoneNumbers, but the relationship should be unidirectional, in that I don't need to know from the PhoneNumber which Dude owns it, per se, as I might have many different objects that own PhoneNumber instances, such as a Business for example:

class Business(models.Model):
    numbers = models.OneToManyField('PhoneNumber')

What would I replace OneToManyField (which doesn't exist) with in the model to represent this kind of relationship? I'm coming from Hibernate/JPA where declaring a one-to-many relationship was as easy as:

@OneToMany
private List<PhoneNumber> phoneNumbers;

How can I express this in Django?

To handle One-To-Many relationships in Django you need to use ForeignKey.

The documentation on ForeignKey is very comprehensive and should answer all the questions you have:

https://docs.djangoproject.com/en/dev/ref/models/fields/#foreignkey

The current structure in your example allows each Dude to have one number, and each number to belong to multiple Dudes (same with Business).

If you want the reverse relationship, you would need to add two ForeignKey fields to your PhoneNumber model, one to Dude and one to Business. This would allow each number to belong to either one Dude or one Business, and have Dudes and Businesses able to own multiple Numbers. I think this might be what you are after.

class Business(models.Model):
    ...
class Dude(models.Model):
    ...
class PhoneNumber(models.Model):
    dude = models.ForeignKey(Dude)
    business = models.ForeignKey(Business)

What's the difference between django OneToOneField and , Many-to-one relationships¶. To define a many-to-one relationship, use ForeignKey : from django.db import models class Reporter(models.Model): first_name  To handle One-To-Many relationships in Django you need to use ForeignKey. The documentation on ForeignKey is very comprehensive and should answer all the questions you have: The current structure in your example allows each Dude to have one number, and each number to belong to multiple Dudes (same with Business).

In Django, a one-to-many relationship is called ForeignKey. It only works in one direction, however, so rather than having a number attribute of class Dude you will need

class Dude(models.Model):
    ...

class PhoneNumber(models.Model):
    dude = models.ForeignKey(Dude)

Many models can have a ForeignKey to one other model, so it would be valid to have a second attribute of PhoneNumber such that

class Business(models.Model):
    ...
class Dude(models.Model):
    ...
class PhoneNumber(models.Model):
    dude = models.ForeignKey(Dude)
    business = models.ForeignKey(Business)

You can access the PhoneNumbers for a Dude object d with d.phonenumber_set.objects.all(), and then do similarly for a Business object.

What is `related_name` used for in Django?, To be more clear - there's no OneToMany in Django, only ManyToOne - which is Foreignkey described above. You can describe OneToMany relation using  Many-to-one relationships; Browse. Prev: Many-to-many relationships; Next: One-to-one relationships; Table of contents; General Index; Python Module Index; You are here: Django 3.0 documentation. Using Django. Models and databases. Examples of model relationship API usage. Many-to-one relationships; Getting help FAQ Try the FAQ — it's got answers to many common questions.

To be more clear - there's no OneToMany in Django, only ManyToOne - which is Foreignkey described above. You can describe OneToMany relation using Foreignkey but that is very inexpressively.

A good article about it: https://amir.rachum.com/blog/2013/06/15/a-case-for-a-onetomany-relationship-in-django/

Models | Django documentation, How to declare Django model relationships one to many, many to many and one to one; what To define a one to many relationship in Django models you use the Although model relationship data types typically express their relationships​  In a one-to-many relationship, the parent is not required to have child records; therefore, the one-to-many relationship allows zero child records, a single child record or multiple child records. To define a many-to-one relationship, use ForeignKey .: The above queryset shows User u1 with multiple Articles. Hence One to Many.

You can use either foreign key on many side of OneToMany relation (i.e. ManyToOne relation) or use ManyToMany (on any side) with unique constraint.

Many-to-one relationships | Django documentation, In relational databases, a one-to-many relationship occurs when a parent record in one table can potentially reference several child records in another table. The many-to-one relationship that Django provides is ForeignKey. A foreign key is always declared on the “one” side of the relationship, e.g., if you have a Band model and each Band has several Musician models related to it, then you would put a ForeignKey in the Musician model like so: class Band (models. Model): # class Musician (models.

django is smart enough. Actually we don't need to define oneToMany field. It will be automatically generated by django for you :-). We only need to define foreignKey in related table. In other words, we only need to define ManyToOne relation by using foreignKey.

class Car(models.Model):
    // wheels = models.oneToMany() to get wheels of this car [**it is not required to define**].


class Wheel(models.Model):
    car = models.ForeignKey(Car, on_delete=models.CASCADE)  

if we want to get the list of wheels of particular car. we will use python's auto generated object wheel_set. For car c you will use c.wheel_set.all()

How to express a One-To-Many relationship in Django, To handle One-To-Many relationships in Django you need to use ForeignKey . The documentation on ForeignKey is very comprehensive and  To define a one to many relationship in Django models you use the ForeignKey data type on the model that has the many records (e.g. on the Item model). Listing 7-22 illustrates a sample of a one to many Django relationship. Listing 7-22. One to many Django model relationship

Relationships in Django models, First, we will need a Person class to describe and store a person involved in a movie When two models have a one-to-many relationship, we use a ForeignKey  Many-to-many relationships; Getting help FAQ Try the FAQ — it's got answers to many common questions. Index, Module Index, or Table of Contents Handy when looking for specific information. django-users mailing list Search for information in the archives of the django-users mailing list, or post a question. #django IRC channel

2. How to model one to many relationships?, There arises many times in the development of modern web applications where the business requirements inherently describe relationships that are recursive. One well known example of such a business rule is in the description of  Here are some other examples of one-to-many relationships: People-Addresses (Each person can live at one address, but each address can house one or more people.) Owners-Pets (Each pet has one owner, but each owner can have one or more pets.) Farmer-Equipment (Each piece of farming equipment is owned by one farmer,

[SOLVED] How to express a One-To-Many relationship in Django , Model relationships are very important to express ideas in your database model. the Duration: 25:56 Posted: Jan 10, 2019

Comments
  • Could you give me an example given the problem above? I'm probably just missing it completely, but I've been reading the Django documentation for some time now and am still unclear as to how to create this kind of relationship.
  • might want to make both of the ForeignKeys not required (blank=True, null=True), or add some sort of custom validation to make sure that there is at least one or the other. what about the case of a business having a generic number? or an unemployed dude?
  • @j_syk good point about custom validation. but it seems kind of hacky to include both a foreignkey to dude and a foreignkey to business, and then do custom (external to the model definition) validation. seems like there has to be a cleaner way, but I can't figure it out either.
  • In this situation it is appropriate to use the ContentTypes Framework which allows for generic relationships to different objects. However, using content types can get very complex very quickly, and if this is your only need, this simpler (if hacky) approach might be desirable.
  • One relevant thing to mention is the "related_name" argument to ForeignKey. So in the PhoneNumber class you'd have dude = models.ForeignKey(Dude, related_name='numbers') and then you can use some_dude_object.numbers.all() to get at all the related numbers (if you don't specify a "related_name" it'll default to "number_set").
  • I was under the assumption that ForeignKey meant "one-to-one." Using your above example, I should have a Dude that has many PhoneNumbers right?
  • I edited my answer to reflect this. Yes. ForeignKey is only one-to-one if you specify ForeignKey(Dude, unique=True), so with the above code you will get a Dude with multiple PhoneNumbers.
  • @rolling stone- thank you, I was adding that after realizing my mistake as you commented. Unique=True doesn't work exactly like OneToOneField, I meant to explain that ForeignKey only uses a one-to-one relationship if you specify Unique=True.
  • +1 for doing it from the PhoneNumber. Now it's starting to make sense. ForeignKey is essentially many-to-one, so you need to do it backwards to get a one-to-many :)
  • Can somebody explain the significance of the name of the field phonenumber_set? I don't see it defined anywhere. Is it the name of the model, in all lower case, appended with "_set" ?
  • You can use ManyToManyField.