class A(models.Model):
    things = models.ManyToManyField("B", through=ThroughModel)

class B(models.Model):
    text = models.TextField()

class ThroughModel(models.Model):
    a = models.ForeignKey(A)
    b = models.ForeignKey(B)
    extra = models.BooleanField()

#this will return a list of ThroughModel objects
ThroughModel.objects.filter(b=instance_of_b, extra=True)

#this will return a list of A objects based on an extra field on the through table
A.objects.filter(things__ThroughModel__extra=True)

#keep in mind that limiting by one of the foreign keys on the through model is easier
A.objects.filter(things=instance_of_b)