Foreign Key to Foreign Key Relationships and Single Table Inheritance in Ruby on Rails

ActiveRecord is great and efficient for simple normalized databases, but once you start tread off the beaten path ActiveRecord starts to lose a lot of its mojo. A few months ago I had a rather abstract database that utilized a number of models based on Single Table Inheritance (STI). Prior to refactoring our our database diagram looked something like an upside down untranslated Tokyo subway map. Fortunately single table inheritance saved our butt big time and we were able to knock off a number of confusing entities from the database and refactor in some common functionality. There was but one problem though. Before our moving to single table inheritance we had a few classes that looked something like this:

class Organization < ActiveRecord::Base
end
 
class Client < ActiveRecord::Base
  belongs_to :organization
end
 
class StaffMember < ActiveRecord::Base
  belongs_to :organization
  has_many :clients, :through => :organization
end

After moving to single table inheritance it was something like this:

class Organization < ActiveRecord::Base
end
 
class Client < Organization
  belongs_to :organization, :foreign_key => 'parent_id'
end
 
class StaffMember < ActiveRecord::Base
  belongs_to :organization
  has_many :clients, :through => :organization
end

It looked fine. I had only changed one thing in the client model, but now the has_many association in the StaffMember model just doesn't work anymore. I kept getting an error akin to:

ActiveRecord::StatementInvalid: Mysql::Error: #42000Not unique table/alias: 'organization': SELECT organization.* FROM organization organization.parent_id = organization.id WHERE ((organization.organization_id = 1)) AND ( (`organization`.`type` = 'Client' ) )

Well that just plain sucked... all of that refactoring seemed to have gone to waste. I scoured the net looking for a solution to my conundrum and I couldn’t find a single reasonable answer to it. I could have used the "finder_sql" option but then it only returns read only pseudo models. There just had to be a better way. I begrudgingly started to dig through ActiveRecord hoping to find a quick fix. Alas, I came back empty handed. I had almost lost hope when I realized I could just like the two foreign keys together between the Client and StaffMember model to get the same result as using the Rails has_many :through option. So I dug around a little bit more in ActiveRecord and added an “alternate_key” option to the HasManyAssociation class to define a different foreign_key instead of always needing to link to a primary_key. So now I could link directly to the Client model from the StaffMember model without needing to use the :through option. An example can be seen below:

class Organization < ActiveRecord::Base
end
 
class Client < Organization
  belongs_to :organization
end
 
class StaffMember < ActiveRecord::Base
  belongs_to :organization
  has_many :clients, :alternate_key => 'organization_id', :foreign_key => 'organization_id'
end

Now @staff_member.clients should return a list of clients that are part of that StaffMember’s organization.

Download it

Questions, Comments, Contiributions? Contact Me.