Question

I cant find out where is an error with this relation:

class Education < ActiveRecord::Base
  attr_accessible :description, :graduation, :student_id, :faculty_id, :department_id

  has_one :faculty
  belongs_to :student
end
#  id          :integer          not null, primary key
#  student_id  :integer
#  faculty_id  :integer
#  description :text
#  graduation  :string(255)

class Faculty < ActiveRecord::Base
  attr_accessible :department_id, :name

  belongs_to :education
  belongs_to :department
end

# == Schema Information
#
# Table name: faculties
#
#  id            :integer          not null, primary key
#  name          :string(255)
#  department_id :integer

Why I have to add education_id to faculties table?
Each Education has only one faculty and faculty belongs to many educations.

>> Education.last.faculty
  Education Load (0.3ms)  SELECT "educations".* FROM "educations" ORDER BY "educations"."id" DESC LIMIT 1
  Faculty Load (0.2ms)  SELECT "faculties".* FROM "faculties" WHERE "faculties"."education_id" = 1 LIMIT 1
SQLite3::SQLException: no such column: faculties.education_id: SELECT  "faculties".* FROM "faculties"  WHERE "faculties"."education_id" = 1 LIMIT 1
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: faculties.education_id: SELECT  "faculties".* FROM "faculties"  WHERE "faculties"."education_id" = 1 LIMIT 1
from /Users/rege/.rvm/gems/ruby-1.9.3-p194@ckdatabase/gems/sqlite3-1.3.6/lib/sqlite3/database.rb:91:in `initialize'

Answer 1

Whichever side you put a ‘belongs_to’ in will invariably have an id of the associated objects – it’s kind of by definition. If you belong_to something you don’t also belong_to another instance.

If you want each Education to have only one Faculty, there are two alternatives:

• use a one-to-one: when you also want the Faculty to only have one Education
• use a belongs_to with a one-to-many: when you want each Faculty to have multiple Educations

So you might try this instead:

class Education < ActiveRecord::Base
  attr_accessible :description, :graduation, :student_id, :faculty_id, :department_id

  belongs_to :faculty
  belongs_to :student
end

class Faculty < ActiveRecord::Base
  attr_accessible :department_id, :name

  has_many :education
  belongs_to :department
end