class Mongoid::Relations::Eager::BelongsTo

def group_by_key
  @metadata.foreign_key
end

def key
  @metadata.primary_key
end

def preload
  raise Errors::EagerLoad.new(@metadata.name) if @metadata.polymorphic?
  @docs.each do |d|
    set_relation(d, nil)
  end
  each_loaded_document do |doc|
    id = doc.send(key)
    set_on_parent(id, doc)
  end
end