Question

I recently posted this question: How to model a many self-referential relationship with many parents?

I got an adequate answer, but I was told that I could model this better using Mongo/Mongoid. I always wanted to attempt using a different database paradigm so maybe this will be a good start.

Is it true that this would be easier to model using Mongo? If so, could you help lay it out for me?

I basically have two options right? 1 is to create an array through a document that references Skill id’s through it? E.g, Skill.prerequisites = [Skill1, Skill2, Skill3]. Something that works with that structure right?

Otherwise it may be that I need to run something logical on that association so I’d have to create a separate model. This is where I get confused. Given I may need to create a separate model, does that exist as a document or an embedded document? What are my limitations w/ each strategy?

Answer 1

OK, so let’s go with your skill approach. Here’s how I’d model it in Mongoid:

class Skill
  include Mongoid::Document

  field :prereqruisite_skill_ids, type: Array, default: []

  def prereqruisite_skills
    Skill.any_of(:_id => prereqruisite_skill_ids).all
  end

  def add_prereqruisite!(skill)
    self.prereqruisite_skill_ids << skill
    safely.save!
  end
end

With Mongo, this is the only way I would do it.

With ActiveRecord, you could:

class Skill < AR::Base
  has_many :skill_prereqruisites
  has_many :prerequisites, through: skill_prereqruisites, class_name: "Skill"
end

class SkillPrereqruisite < AR::Base
  belongs_to :skill_one, foreign_key: "skill_one_id"
  belongs_to :skill_two, foreign_key: "skill_two_id"
end