Issues using through option

[jwoertink]

This is loosely along the same lines as #98.

When using the through option, as I defined in the other issue, I would expect to use the plural name of the association like has_many :owners, through: :car_owners. The issue is that this line doesn't singularize that term which would mean it's looking for a foreign_key car_owners_id (unless the foreign key is specified).

Now, if I'm reading this correctly, since the through option is specified here, it will run this code which calls id. In my case, my "through" table has no id field.

Another example for this issue:

schema "facts" do
  has_many :fact_checkers, FactChecker
  has_many :checkers, Checker, through: :fact_checker # this has to be single to work or foreign_key would need to be specified
end

# only has a fact_id and checker_id
schema "fact_checkers" do
   belongs_to :fact, Fact
   belongs_to :checker, Checker
end

schema "checkers" do
   has_many :fact_checkers, FactChecker
   has_many :facts, through: :fact_checker # single again here too
end