Add test for persisting inverse keys for HABTM substitution

[ersatzryan]

Setup

class Post
  include Mongoid::Document

  has_and_belongs_to_many :tags
end

class Tag
  include Mongoid::Document

  has_and_belongs_to_many :posts
end

tag = Tag.create
post = Post.create(tags: [tag])
post.tags = [Tag.last]

Expected Behavior

post.reload.tag_ids #=> [tag.id]
tags.reload.post_ids #=> [post.id]

Actual Behavior

post.reload.tag_ids #=> [tag.id]
tag.reload.post_ids #=> []

Commit has a failing test in the many_to_many_spec.rb, but we aren't sure how to fix with out breaking other functionality.

[akshayrao14]

Just wondering... Has this issue been fixed yet? I'm Mongoid 4.0.2 and this issue is giving us some pain.

[johnnyshields]

@ersatzryan can you try this on latest Mongoid master to see if still an issue?

[estolfo]

Fixed in this commit: 8ccd331