Fake a composite primary key? Rails

Question

I have a table with id|patient_id|client_id|active. A record is unique by patient_id, client_id meaning there should only be one enrollment per patient per client. Normally I would make that the primary key, but in rails I have id as my primary key.

What is the best way to enforce this? Validations?

Answer 1

Sounds like you have a model relationship of:

class Client < ActiveRecord::Base
  has_many :patients, :through => :enrollments
  has_many :enrollments
end

class ClientPatient < ActiveRecord::Base
  belongs_to :client
  belongs_to :patient
end

class Patient < ActiveRecord::Base
  has_many :clients, :through => :enrollments
  has_many :enrollments
end

To enforce your constraint I would do it in ActiveRecord, so that you get proper feedback when attempting to save a record that breaks the constraint. I would just modify your ClientPatient model like so:

class Enrollment < ActiveRecord::Base
  belongs_to :client
  belongs_to :patient
  validates_uniqueness_of :patient_id, :scope => :client_id
end

Be careful though because, while this is great for small-scale applications it is still prone to possible race conditions as described here: http://apidock.com/rails/v3.0.5/ActiveRecord/Validations/ClassMethods/validates_uniqueness_of under "Concurrency and Integrity"

As they describe there, you should also add a unique index to the table in the database. This will provide two immediate benefits:

• The validation check and any searches through this model based on these two id's will perform faster (since they're indexed)
• The uniqueness constraint will be enforced DB-side, and on the rare occurrence of a race condition you won't get bad data saved to the database... although users will get a 500 Server Error if you don't catch the error.

In a migration file add the following:

add_index :enrollments, [:patient_id, :client_id], :unique => true

Hopefully this was helpful :)

Edit (fixed some naming issues and a couple obvious bugs):

It's then very easy to find the data you're looking for:

Client.find_by_name("Bob Smith").patients
Patient.find_by_name("Henry Person").clients

Answer 2

Validations would work (Back them up with a unique index!), but there's no way to get a true composite primary key in vanilla Rails. If you want a real composite primary key, you're going to need a gem/plugin - composite_primary_keys is the one I found, but I'm sure there are others.

Hope this helps!