Django
Relazioni molti-a-molti

Con un modello passante

class Skill(models.Model):
    name = models.CharField(max_length=50)
    description = models.TextField()

class Developer(models.Model):
    name = models.CharField(max_length=50)
    skills = models.ManyToManyField(Skill, through='DeveloperSkill')

class DeveloperSkill(models.Model):
    """Developer skills with respective ability and experience."""

    class Meta:
        order_with_respect_to = 'developer'
        """Sort skills per developer so that he can choose which
        skills to display on top for instance.
        """
        unique_together = [
            ('developer', 'skill'),
        ]
        """It's recommended that a together unique index be created on
        `(developer,skill)`. This is especially useful if your database is
        being access/modified from outside django. You will find that such an
        index is created by django when an explicit through model is not
        being used.
        """


    ABILITY_CHOICES = [
        (1, "Beginner"),
        (2, "Accustomed"),
        (3, "Intermediate"),
        (4, "Strong knowledge"),
        (5, "Expert"),
    ]

    developer = models.ForeignKey(Developer, models.CASCADE)
    skill = models.ForeignKey(Skill, models.CASCADE)
    """The many-to-many relation between both models is made by the
    above two foreign keys.

    Other fields (below) store information about the relation itself.
    """

    ability = models.PositiveSmallIntegerField(choices=ABILITY_CHOICES)
    experience = models.PositiveSmallIntegerField(help_text="Years of experience.")

Si consiglia di creare un indice univoco su (developer,skill) . Questo è particolarmente utile se il tuo database è accessibile / modificato da un altro django. Troverete che tale indice è creato da Django quando non viene utilizzato un modello esplicito tramite.

Relazione semplice da molti a molti.

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

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

Qui definiamo una relazione in cui un club ha molte Person e membri e una persona può essere membro di diversi Club .

Sebbene definiamo solo due modelli, django crea effettivamente tre tabelle nel database per noi. Questi sono myapp_person , myapp_club e myapp_club_members. Django crea automaticamente un indice univoco sulle myapp_club_members(club_id,person_id) .

Usando ManyToMany Fields

Usiamo questo modello dal primo esempio:

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

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

Aggiungi Tom e Bill al Nightclub:

tom = Person.objects.create(name="Tom", description="A nice guy")
bill = Person.objects.create(name="Bill", description="Good dancer")

nightclub = Club.objects.create(name="The Saturday Night Club")
nightclub.members.add(tom, bill)

Chi è nel club?

for person in nightclub.members.all():
    print(person.name)

Ti darò

Tom
Bill