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

Question

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 lt PhoneNumber gt  phoneNumbers    How can I express this in Django

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

While rolling stone s answer is good  straightforward and functional  I think there are two things it does not solve    If OP wanted to enforce a phone number cannot belong to both a Dude and a Business The inescapable feeling of sadness as a result of defining the relationship on the PhoneNumber model and not on the Dude Business models  When extra terrestrials come to Earth  and we want to add an Alien model  we need to modify the PhoneNumber  assuming the ETs have phone numbers  instead of simply adding a  phone numbers  field to the Alien model    Introduce the content types framework  which exposes some objects that allow us to create a  generic foreign key  on the PhoneNumber model  Then  we can define the reverse relationship on Dude and Business  from django contrib contenttypes fields import GenericForeignKey  GenericRelation from django contrib contenttypes models import ContentType from django db import models  class PhoneNumber models Model       number   models CharField        content type   models ForeignKey ContentType  on delete models CASCADE      object id   models PositiveIntegerField       owner   GenericForeignKey    class Dude models Model       numbers   GenericRelation PhoneNumber   class Business models Model       numbers   GenericRelation PhoneNumber    See the docs for details  and perhaps check out this article for a quick tutorial   Also  here is an article that argues against the use of Generic FKs

User · Answer

First of all we take a tour   01  one-to-many relationship   ASSUME   class Business models Model       name   models CharField max length 200                                  phone number   models OneToMany PhoneNumber   NB  Django do not support OneToMany relationship   class Dude models Model       name   models CharField max length 200                                  phone number   models OneToMany PhoneNumber   NB  Django do not support OneToMany relationship   class PhoneNumber models Model       number   models CharField max length 20                              NB  Django doesn t provides any OneToMany relationship  So we can t use upper method in Django  But we need to convert in relational model  So what can we do  In this situation we need to convert relational model into reverse relational model   Here   relational model   OneToMany  So  reverse relational model   ManyToOne  NB  Django support ManyToOne relationship  amp  in Django ManyToOne is represented by ForeignKey   02  many-to-one relationship   SOLVE   class Business models Model                                class Dude models Model                                class PhoneNumber models Model                                 business   models ForeignKey Business      dude   models ForeignKey Dude    NB  THINK SIMPLY

User · Answer

If the  many  model does not justify the creation of a model per-se  not the case here  but it might benefits other people   another alternative would be to rely on specific PostgreSQL data types  via the Django Contrib package  Postgres can deal with Array or JSON data types  and this may be a nice workaround to handle One-To-Many when the many-ies can only be tied to a single entity of the one    Postgres allows you to access single elements of the array  which means that queries can be really fast  and avoid application-level overheads  And of course  Django implements a cool API to leverage this feature   It obviously has the disadvantage of not being portable to others database backend  but I thougt it still worth mentionning   Hope it may help some people looking for ideas

[django] How to express a One-To-Many relationship in Django

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