has_many children and has_many parents

Question

I'm trying to figure out a complex relation between a Model. I have a model called "Concept", which has two inheriting types called "Skill" and "Occupation". Basicly this means that each concept represents a category, but a concept can also be a skill or an occupation when going deep enough into the hierychal tree.

I'm solving this hierachy by using STI. So my schema for the Concepts table looks like this:

class CreateConcepts < ActiveRecord::Migration
  def self.up
    create_table :concepts do |t|
      t.string :uri, :null => false, :length => 255
      t.string :type, :null => true, :length => 255
      t.integer :isco_code, :null => true
      t.timestamps
    end
  end

  def self.down
    drop_table :concepts
  end
end

The type column determins whether the Concept is a real "Concept" or a "Skill"/"Occupation". The problem now however the following relations:

EDIT:

• A Concept can belong to a single parent Concept
• An Occupation can belong to a single parent Concept
• A Skill can belong to multiple parent Concepts
• A skill has no children
• An occupation has no children

so basicly you'd have something like this:

>                             concept1
>                  concept2                  concept3
>        concept4       concept5        concept6     concept7    skill1 
> occup1   skill2 occup2    skill5
> occup7    skill2  occup3   skill4
> occup4    skill1 occup8

I hope the picture is a bit clear what I'm trying to explain. Currently I have created the following migration to try to solve the parent-child relation but I'm not sure how to map this with the associations...

class CreateConceptLinks < ActiveRecord::Migration
  def self.up
    create_table :concept_links do |t|
      t.integer :parent_id, :null => false
      t.integer :child_id, :null => false
      t.timestamps
    end
  end

  def self.down
    drop_table :concept_links
  end
 end

What I want to end up with is the following posssibilities:

concepta.parents => a Concept object
conceptb.children => an array of Conept objects
Occupation.parents => a Concept object
Occupation.children => []
Skill.parents => an array of Concept objects
Skill.children => []

Hope this is even possible...

Answer 1

You can model hierarchical relations in rails. You've got most of the way there with your migrations. Adding the relations below should allow you to do the method calls you'd like:

def Concept < ActiveRecord::Base
  has_many :child_links, :class_name => 'ConceptLink', :foreign_key => 'parent_id'
  has_many :children, :through => :child_links

  has_many :parent_links, :class_name => 'ConceptLink', :foreign_key => 'child_id'
  has_many :parents, :through => :parent_links
end

def ConceptLink < ActiveRecord::Base
  belongs_to :child, :class_name => "Concept"
  belongs_to :parent, :class_name => "Concept"
end

I'd also take a look at this blog posting which does a very good of explaining parent-child mappings in rails.