Automatically guess the inverse associations for STI

[yui-knk]

ActiveRecord associations automatically guess the inverse associations.
But this feature does not work correctly on assoctions for STI.
For example, before this commit

class Post < ActiveRecord::Base
  belongs_to :author
end

class SpecialPost < Post; end

class Author < ActiveRecord::Base
  has_many :posts
  has_many :special_posts
end

author.posts.first.author works correctly, but
author.special_posts.first.author does not work correctly.

[rails-bot]

r? @rafaelfranca

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[arthurnn]

at a first pass this looks correct.
@rafaelfranca @sgrif would you mind also taking a look?

[mxie]

It'd be awesome to get this merged sometime soon. This is the exact behavior I would've expected when interacting with a child record. Instead, for a setup like this:

class Challenge < ApplicationRecord
  has_many :questions
end

class Question < ApplicationRecord
  belongs_to :challenge
end

class MultipleChoiceQuestion < Question
  # where `type` == "MultipleChoiceQuestion"
end

the following

challenge = Challenge.new
MultipleChoiceQuestion.create(challenge: challange)

would give me: ActiveRecord::AssociationNotFoundError: Association named 'challenge' was not found on Assessments::RankingQuestion; perhaps you misspelled it?

It seems to know that it's an association, but not really...

[sgrif]

@yui-knk This looks fine to me. Can you rebase?

[yui-knk]

@sgrif Rebased!