Apparently, the number of exception classes related to has_many :through (HM:T) "scared Giles away":http://gilesbowkett.blogspot.com/2008/02/im-proud-not-to-understand-hasmany.html. I think most of them are there because I put the original ones there. Course, looking at them highlighted does make it seem a bit ridiculous.

HM:T associations are the only associations that actually look at other associations to figure out how to query the database. There aren't a simple set of conventions to go by that raise simple errors like "couldn't find the suspended_users table". If you actually look at the associations, they're not much more than simple ActiveRecordError subclasses with a custom message. It gives you nice, helpful messages like this:

class Post < ActiveRecord::Base
end
class User < ActiveRecord::Base
end
class Topic < ActiveRecord::Base
  has_many :posts
  belongs_to :user
end
class Forum < ActiveRecord::Base
  # commented out bits are things
  # i conveniently 'forgot' in this contrived example.
  #
  # has_many :topics 
  has_many :posts, :through => :topics
  has_many :creators, :through => :topics# , :source => :user
end

Forum.find(:first).posts
# => ActiveRecord::HasManyThroughAssociationNotFoundError: Could not find the association :topics in model Forum

Forum.find(:first).creators
# => ActiveRecord::HasManyThroughSourceAssociationNotFoundError: Could not find the source association(s) :creator or :creators in model Topic.  Try 'has_many :creators, :through => :topics, :source => '.  Is it one of :posts or :user?

Are the error messages helpful? I sure think so. But, do we really need all those exception classes? Sure, it'll keep the error messages DRY.

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Okay, maybe not. Is it sufficient to have a single ActiveRecordError class (or even a more specific ActiveRecord::AssociationsError)? I could definitely do something like this:

raise ActiveRecord::AssociationsError.new(:has_many, :through_association_not_found, model, reflection)

Whichever way's the best, I really don't think it's good enough validation to miss out on the feature. I can point to a few other more complex areas of the associations code (the crazy eager joins code is why I personally haven't used :include in any of my apps in over a year).