Question

I have a pretty simple set of data requirements to do with pets and veterinarians.

• An owner can have many pets
• A pet can have one owner
• A pet can be treated by (belongs_to) many veterinarians
• A veterinarian can treat (have_many) pets

Owner and Veterinarian are both subclasses of User using STI.

Here’s some code:

class Owner < User
  has_many :pets
  has_many :veterinarians, :through => :pets
end

class Veterinarian < User
  attr_accessible :clinic_name

  has_many :pets
  has_many :owners, :through => :pets
end


class Pet < ActiveRecord::Base
  attr_accessible :name, :date_of_birth, :species, :breed, :gender, :neutered

  belongs_to :owner
  belongs_to :veterinarian
end

And here is the spec that is failing:

it "has various veterinarians" do
  o = Owner.make!(:email => '[e-mail]')
  v1 = Veterinarian.make!(:email => '[e-mail]')
  v2 = Veterinarian.make!(:email => '[e-mail]')
  p = Pet.make!(:name => 'fluffy')

  o.pets << p
  v1.pets << p
  v2.pets << p

  o.pets.should have(2).records
  o.veterinarians.should have(2).records
end

The make! stuff is to do with using machinist fixture replacement. It just factory creates the objects.

The failure occurs on the last line. It turns out that o.veterinarians only has 1 record. I understand that a pet is not a join table in the traditional sense, inasmuch as I don’t want to create a whole new pet each time I create a relationship between an owner and a veterinarian. Should I be using a schema more like Owner has_many Pets, Pet belongs_to :owner and Pet has_and_belongs_to_many Veterinarians?

Thanks!

Answer 1

You have to use a join table between the Pet and the Veterinarian. Let’s call it Treatment:

class Treatment < ...
...
  belongs_to :pet
  belongs_to :veterinarian
...
end

class Veterinarian < ...
...
  has_many :treatments
  has_many :pets, through: :treatments
...
end

class Pet < ...
...
  has_many :treatments
  has_many :veterinarians, through: :treatments
...
end