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I have a user model, farmer model, doctor model, and education model.

A farmer has a user and many educations.

A doctor has a user and many educations.

How do I setup the database for the education model?

Should it have a farmer_id AND a doctor_id?

But a education cannot belong to a farmer AND and doctor at the same time. It's one or the other.

So my education database entry would either have a farmer_id OR a doctor_id filled in, but not both.

Is there a way to guarantee that only one of the ids could be filled in at a time?

Or is there a better way to associate these models?

Your help would be appreciated!

Oh, and don't worry about the names of the models (farmer, doctor, etc.). It's just an example.

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2 Answers 2

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I see two possible solutions for this scenario.

The first one is to make use of polymorphic associations for education. That could look like this:

class Farmer < ActiveRecord::Base
  belongs_to :user
  has_many :educations, :as => :profession
end

class Doctor < ActiveRecord::Base
  belongs_to :user
  has_many :educations, :as => :profession
end

class Education < ActiveRecord::Base
  belongs_to :profession, :polymorphic => true
end

So instead of education having a doctor_id or a farmer_id it has one profession_id and one profession_type.

The second solution would be to make use of Single Table Inheritance. And in your scenrio, that could be accomplished by letting a Doctor be a User instead of belonging to a User. And of course the same thing for a Farmer. That could look like this:

class User < ActiveRecord::Base
  has_many :educations
end

class Farmer < User
end

class Doctor < User
end

class Education < ActiveRecord::Base
  belongs_to :user
end

And in this scenario you would add a type column to the User model to store what type of class it is and then only having a user_id in the Education model

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9 Comments

Every time i've used STI in rails i've bitterly regretted it. I don't know if it's just me being dumb or if it just sucks. I would go with the polymorphic relationship solution.
Polymorphic would probably be my choice as well.
So the Profession model has a profession_id and profession_type? Both would be integer fields in the database? I could have the profession_type represented by an enum in the Profession model?
The Education model has a profession_id and profession_type, yes. The profession_id is an integer but the profession_type is a string (or varchar in database)
Good to hear it worked out. Another way you could have done it in the console would be f1=Doctor.new;f1.save;f2=f1.educations.build;f2.save The build method lets you associate at the same time as you instantiate a model.
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I think its appropriate to have the relations this way based on roles.

Class User
  has_one :role
  has_many :educations
end

Class Role
  #What ever roles you have.
  #Farmer or Doctor
  belongs_to :user
end


class Education
  belongs_to :user
end

This way you will store the user_id in the education object, which solves your problem.

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2 Comments

Your comment "Whatever roles you have. Farmer or Doctor." Does that mean I should have, for example: class Doctor belongs_to :user end or what? I don't get it. Or am I supposed to have a enum that represents role type? And that enum would be stored in the Role table as an integer?
Not necessarily, you ll have only 2 records in the Role table. Role will basically have, these fields. Role.create(:name => "Farmer"), Role.create(:name => "Doctor"). And then you ll have to store the id of that role object in the respective user object.User.create(:role_id => what_ever_id_of_the_role, :all_your_fields)

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