Note about many-to-many relationships¶

There are two many-to-many relationships between Member and Product:

A given member can offer multiple products, and a given product can be offered by multiple members. We can call this the providers of a product.
A given member can want multiple products, and a given product can be wanted by multiple members. We can call this the customers of a product.

Using Django's interface for many-to-many relationships, we might express this as follows:

providers = models.ManyToManyField(
    'lets.Member', through='lets.Offer', related_name='offered_products')
customers = models.ManyToManyField(
    'lets.Member', through='lets.Demand', related_name='wanted_products')

Which you can read as follows:

Offer is the "intermediate model" used "to govern the m2m relation Product.providers / Member.offered_products.
Demand is the intermediate model used to govern the m2m relation Product.customers / Member.wanted_products.

A ManyToManyField is originally a shortcut for telling Django to create an automatic, "invisible", additional model, with two ForeignKey fields. But in most real-life situations you anyway want to define what Django calls "extra fields on many-to-many relationships", and thus you must explicitly name that "intermediate model" of your ManyToManyField. That's why we prefer to define an explicit intermediate model for each m2m relation instead of using ManyToManyField. Less magic, easier to extend.