Django many-to-many relationship with extra fields

Question

I am building a simple interface to a biological database using the django-admin to populate the db. I want tot to use a many-to-many relationship for a questionnaire to fish species (one questionnaire can have more than one species and one species can be present in more than one questionnaire). The two models in question:

class Species(models.Model):
    fish_spp_name = models.CharField(max_length=255, unique=True)


class Questionaire(models.Model):
    # ...
    fish_caught = models.ManyToManyField(Species)

now, I want to my data to contain a number of each species caught, per questionnaire. So, for example, I can associate 3 different species with questionnaire id=1, but how do I include that, say 2 of the first species, 1 of the second and 4 of the third were caught?

Answer 1

Define another models Caught to hold the information per catch. Give it a related_name to make it easier to refer to in your code. You might also want to unique_together appropriate fields.

class Species(models.Model):
    name = models.CharField(max_length=255, unique=True)

    def __unicode__(self):
        return '%s/%d' % self.name

class Questionaire(models.Model):
    pass

class Caught(models.Model):
    species = models.ForeignKey(Species)
    number = models.IntegerField()
    questionaire = models.ForeignKey(
        Questionaire, related_name='catches')

    def __unicode__(self):
        return '%s/%d' % (self.species.name, self.number)

Use it like this:

(InteractiveConsole)
>>> from app.models import *
>>> s1 = Species(name='Salmon')
>>> s1.save()
>>> s2 = Species(name='Mackerel')
>>> s2.save()
>>> q = Questionaire()
>>> q.save()
>>> c1 = Caught(species=s1, number=7, questionaire=q)
>>> c2 = Caught(species=s2, number=5, questionaire=q)
>>> c1.save()
>>> c2.save()
>>> q.catches.all()
[<Caught: Salmon/7>, <Caught: Mackerel/5>]
>>>