The issue of association class name lookups when modules are involved has come up several times in my time at MongoDB, most recently in https://jira.mongodb.org/browse/MONGOID-5080. Our tutorial on the association page doesn't include the word "module" at all, hence appears to be silent on the matter.

Specifically, when models are defined in a module and they reference each other via associations, the class name must be specified on each reference where the target is in a module, even if the host is in the same module. For example, in the following snippet, both of the class_name options are required:

module Blog

  class Article

    include Mongoid::Document

    embeds_many :comments, class_name: "Blog::Comment"

  end

 

  class Comment

    include Mongoid::Document

    embedded_in :article, class_name: "Blog::Article"

  end

end

This ticket is to:

Verify this behavior for all 7 associations (4 referenced, 3 embedded)
Add tests if needed
Document it in tutorial with examples

Note that https://jira.mongodb.org/browse/MONGOID-5080 proposes to change Mongoid's behavior to where :class_name option would be required when referencing models in non-host namespaces, which would require different documentation and tests.