Associations and Math between Models

Well that's kind of what I need help with, I'm not sure where it needs
to be defined in a many-to-many relationship. I've already created the
join table that contains both ids.

The point is that your example makes no sense; you've defined this
as a "to-many" relationship -- which Model_1 are you looking for?

Hassan Schroeder wrote in post #1074840:

But you're not showing us a view, you're showing us a model.

Do you really want `some_method` to return one value or an array
of values?

If you only want one value, why don't you pass the Model_1 instance
(or the attribute) you're interested in to the Model_2 instance?

def some_method(model_1_attribute)
   attr_a * model_1_attribute
end