I have tripped over this myself, and I think I have a tenuous grasp on what’s going on that may help. I think the original example above would work like this:

class Soda ; :distributor_sodas
end

class Distributor :distributor_sodas
end

class DistributorSoda < ActiveRecord::Base
belongs_to :soda
belongs_to :distributor
end

First, the class name: DistributorSoda is always singular. The part I find confusing is that you (might) expect the underlying join table, and therefore the name used for the has_many declaration to be “distributors_sodas”, as it would in a HABTM relationship, but instead its “distributor_sodas”. In other words, its treated as one word pluralized instead of two words split by the underscore, with each pluralized. This makes sense when you consider that many join tables that are full fledged models go by a better name, like “store” in your example, or “membership” to give another popular example. I think this also explains why specifying :class_name and :foreign_key will also work; its not helping Rails figure out the proper pluralizatio so much as it is bypassing the whole pluralization issue by naming the join table/class/foreign keys explicitly.