[Fixed]-How do I write a Django model with ManyToMany relationsship with self through a Model

by

24👍

✅

You can do something like this:

class Person(models.Model):
    name = models.CharField(max_length = 255)
    occupation = models.CharField(max_length = 255)
    friends = models.ManyToManyField('self', through = 'PersonFriends', 
          symmetrical = False)
    #     ^^^^^^^^^^^
    # This has to be false when using `through` models. Or else your 
    # model will not validate.

class PersonFriends(models.Model):
    source = models.ForeignKey(Person, related_name = 'source')
    #                                  ^^^^^^^^^^^^
    # You need different `related_name` for each when you have 
    # multiple foreign keys to the same table. 

    target = models.ForeignKey(Person, related_name = 'target')
    comment = models.CharField(max_length = 255)
👤Manoj Govindan

5👍

Everything is described in the official docs for ManyToManyField.through_fields (you can search for ‘recursive relationships’ phrase there to quickly find what you need):

for django 1.11 you have to specify through and (!) through_fields arguments:

class Person(models.Model):
    name = models.CharField(max_length=50)

    # note the additional arguments here
    friends = models.ManyToManyField(
        'self',

        # recursive relationships to self with intermediary
        # through model are always defined as non-symmetrical
        symmetrical=False,

        through='PersonFriend',

        # this argument is required to define a custom
        # through model for many to many relationship to self
        # position matters: 1 - source (from), 2 - target (to)
        through_fields=('person', 'friend'),        
    )


class PersonFriend(models.Model):
    # required relationship-defining foreign keys
    # (note that the order does not matter, it matters
    # in 'through_fields' argument in 'friends' field of the 'Person' model)
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    friend = models.ForeignKey(Person, on_delete=models.CASCADE)

    # additional fields
    comment = models.CharField()
👤Bob

3👍

class PersonFriends(models.Model):
    from_person = models.ForeignKey(Person, related_name='from_person')
    to_person = models.ForeignKey(Person, related_name='to_person')

this is from db table structure of a ManyToMany relation to self from my Model structure. Django defines it like that..

👤Mp0int

3👍

Without assuming that friendships are symmetrical. Because Buzz Lightyear might be Woody’s friend, but Woody isn’t friends with Buzz Lightyear till near the end of the film. You can simplify both models and still have reasonable lookup names. You would of course need to make sure that you define two PersonFriends if it’s a good friendship.

class Person(models.Model):
   name = models.CharField()
   occupation = models.CharField()

class PersonFriends(models.Model):
    from_person = models.ForeignKey(Person, related_name='friends_with')
    to_person = models.ForeignKey(Person, related_name='friends')
    comment = models.CharField()
    class Meta:
        unique_together = ('from_person', 'to_person')

This has the added bonus of a comment for each direction of the friendship. i.e. Tyrion thinks Sansa is a lovely and intelligent, but lost girl. Whereas Sansa might think that Tyrion is an ugly but clever and kind-hearted kinda guy.

👤Will S

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