why retrieve milestone again in GlobalMilestone.rb

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/global_milestone.rb#L20

   milestones = child_milestones.select(:id, :title).group_by(&:title).map do |title, grouped|
      milestones_relation = Milestone.where(id: grouped.map(&:id))
      new(title, milestones_relation)
    end

since child_milestones have already been retrieved from the database, why don't we just use them directly instead of Milestone.where to retrieve them again?

    milestones = child_milestones.group_by(&:title).map do |title, grouped|
      milestones_relation = grouped
      new(title, milestones_relation)
    end

Another place needs to be changed is

  def milestoneish_ids
    milestones.select(:id)
  end

to def milestoneish_ids milestones.map(&:id) end

global_milestone.rb.diff