Rails mutliple has_one relationships as a has_many

Question

I have a scenario where I need to have multiple has_one relationships and be able to access them all at once through a relationship.

Currently I have an STI table, we'll call it Animals. There are subclasses: Lion, Meerkat, Boar

And they all need to relate to multiple Group objects.
So a Lion can be in multiple groups, but a Group may only have one Lion related.

I want to limit and refer to specifically one Lion, Meerkat and Boar. I've tried a simple bridge table, has_many :animals, through: :associated_animals. But there doesn't seem to be an easy way to pull out group.lion. I've also tried doing multiple has_one :lion/:meerkat/:boar relationships, but there was no obvious way to do group.animals.

Is there an STI way to get this functionality through default rails associations or do I have to do a combination of both the has_one :lion and has_many :animals?

Valdis Valdis · Accepted Answer · 2015-03-09T18:38:32

Have you tried it like this: app/models/animal.rb

class Animal < ActiveRecord::Base                                                                                                                                                                                                          
  belongs_to :group                                                                                                                                                                                                                        
end                                                                                                                                                                                                                                        

class Lion < Animal; end                                                                                                                                                                                                                   
class Meerkat < Animal; end

Of course you can split out the Lion and Meerkat classes to separate ruby files

app/models/group.rb

 class Group < ActiveRecord::Base                                                                                                                                                                                                           
   has_many :animals                                                                                                                                                                                                                    
   has_one :lion                                                                                                                                                                                                                        
   has_one :meerkat                                                                                                                                                                                                                     
 end

after you run your migrations your schema should look something like this: db/schema.rb

ActiveRecord::Schema.define(version: 20150309234835) do                                                                                                                                                                                    

  create_table "animals", force: :cascade do |t|                                                                                                                                                                                           
    t.string   "name"                                                                                                                                                                                                                                                                                                                                                                                                                                          
    t.string   "type"                                                                                                                                                                                                                      
    t.datetime "created_at", null: false                                                                                                                                                                                                   
    t.datetime "updated_at", null: false                                                                                                                                                                                                   
    t.integer  "group_id"                                                                                                                                                                                                                  
  end                                                                                                                                                                                                                                      

  create_table "groups", force: :cascade do |t|                                                                                                                                                                                            
    t.string   "name"                                                                                                                                                                                                                      
    t.datetime "created_at", null: false                                                                                                                                                                                                   
    t.datetime "updated_at", null: false                                                                                                                                                                                                   
  end                                                                                                                                                                                                                                      

end

Now you should be able to write queries using group:

g = Group.find(1)
g.animals
g.lion
Lion.first.group
Animal.last.group

Rails mutliple has_one relationships as a has_many

1 Answers