Django
Relaciones de muchos a muchos

Con un modelo pasante.

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.")

Se recomienda que se cree un índice único en conjunto (developer,skill) . Esto es especialmente útil si se está accediendo / modificando su base de datos desde fuera de django. Encontrará que dicho índice es creado por django cuando no se está utilizando un modelo explícito a través de.

Simple muchos a muchos relación.

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)

Aquí definimos una relación donde un club tiene muchas Person y miembros y una Persona puede ser miembro de varios Club diferentes.

Aunque solo definimos dos modelos, django crea tres tablas en la base de datos para nosotros. Estas son myapp_person , myapp_club y myapp_club_members. Django crea automáticamente un índice único en las myapp_club_members(club_id,person_id) .

Uso de muchos campos de muchos

Usamos este modelo del primer ejemplo:

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)

Agrega a Tom y Bill al club nocturno:

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)

Quien esta en el club

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

Te regalaré

Tom
Bill