Django ManyToMany field through with extra fields does not show on both relations

Question

I have a class Assembly

class Assembly(models.Model):

    room = models.ForeignKey("Room", related_name="assemblies")
    name = models.CharField(max_length=200)
    number = models.IntegerField()
    position = models.CharField(max_length=200, blank=True)
    components = models.ManyToManyField("material.Component", through="m2m_Assembly_Components")
    connections = models.ManyToManyField("Assembly", through="Connection")
    category = models.ForeignKey("Category", default=0)
    notes = models.TextField(blank=True)

that has a ManyToMany Realtionship between instances of itself (connections). I use an intermediary table Connection so that i can have additional fields for a connection between two instances of an Assembly.

class Connection(models.Model):

    source = models.ForeignKey("Assembly", related_name="source_assembly", null=True)
    destination = models.ForeignKey("Assembly", related_name="destination_assembly", null=True)
    length = models.IntegerField(null=True, blank=True)

If I have two Assemblies, lets say A and B, and I connect them by defining a new Connection with A as source and B as destination, I get B as A's connections (A.connections.all()), but I don't get A as B's connections.

If I don't use an intermediary table, just a models.ManyToManyField("Assembly") I get A as B's connections and B as A's connections.

What is my problem here?

Answer 1

I think you need to specify the through_fields argument to your ManyToManyField.

When Django autogenerates your through model, it knows which of the two ForeignKeys in it corresponds to the "local" end of the relationship, and which is the "remote" end. However, when you specify a custom intermediary model, this makes it more difficult. It is possible that Django just takes the first ForeignKey in the intermediary model pointing to the right target model, which in both cases happens to be source here (although I'm not sure this is really the case, and if it is, it might be a bug).

Try to see if using through_fields=('source', 'destination') helps.