We have a problem in our schema design for our database that I would like to figure out the cleanest way to solve. The three models are defined in these simplified Django model classes:

class Animal(Model):
    name = CharField()
    infusion_rate = FloatField()
    infusate = ForeignKey(Infusate)  # but with specific tracer concentrations?

class Infusate(Model):
    name = CharField()
    tracers = ManyToManyForeignKey(Tracer)

class Tracer(Model):
    name = CharField()

Currently, we have a through model for the Infusate:Tracer M:M relationship that adds a concentration float field:

class InfusateTracer(Model):
    infusate = ForeignKey(Infusate)
    tracer = ForeignKey(Tracer)
    concentration = FloatField()

but we need to link the Animal model to a single infusate, but with specific concentrations for each of its multiple tracers. In other words, how do I relate an animal to a single infusate, but multiple tracers with specific concentrations belonging to that one infusate?

I was thinking that I could add a field to InfusateTracer and have Animal link to that instead. Let’s call it animal, so:

class InfusateTracer(Model):
    infusate = ForeignKey(Infusate)
    tracer = ForeignKey(Tracer)
    concentration = FloatField()
    animal = IntegerField()

but how do I enforce that the relationship between infusate and animal fields in the InfusateTracer model is M:1 (i.e. InfusateTracer.animal never co-occurs with multiple different InfusateTracer.infusate values (but multiple InfusateTracer.tracer values))? Is there some sort of unique constraint I can use to enforce that?

From a different angle, I could create an AnimalTracer linking/through model, but then I would have the same difficulty of retrieving a single unique infusate name associated with the animal.

Trying to figure out the most elegant solution that maintains data integrity is melting my brain.