mongo_association_finders.rb

module FromParam
  extend ActiveSupport::Concern

  module ClassMethods
    def [](key)
      find_by_slug(key) || find_by_id(key)
    end

    def from_param(param)
      self[param]
    end

    def from_param!(param)
      from_param(param) or raise MongoMapper::DocumentNotFound
    end
  end
end

MongoMapper::Document.plugin(FromParam)

class Post
  include MongoMapper::Document
  key  :slug
  many :comments
end

class Comment
  include MongoMapper::Document
  key :slug
  belongs_to :post
end

post = Post.create(:slug => 'post')
com1 = Comment.create(:slug => 'comment1', :post => post)
com2 = Comment.create(:slug => 'comment1', :post => post)

# This raises an ArgumentError as expected b/c Comment does respond_to :from_param 
Post.first.comments.from_param

# However, because from_param doesn't return a Plucky::Query, it continues up the
# method missing chain.
Post.first.comments.from_param('comment1')