Course x Discipline

has_and_belongs_to_many.rb

# course.rb
# rails g model course title:string 
class Course < ActiveRecord::Base
  has_and_belongs_to_many :disciplines
  
  scope :programming, -> {
    where("title LIKE ?", '%Programming%')
  }
end

# discipline.rb
# rails g model discipline title:string
class Discipline < ActiveRecord::Base
  has_and_belongs_to_many :courses
end

# join table
# rails g migration create_courses_disciplines:index course_id:integer discipline_id:integer:index
class CreateCoursesDisciplines < ActiveRecord::Migration
  def change
    create_table :courses_disciplines, id: false do |t|
      t.integer :course_id, index: true
      t.integer :discipline_id, index: true
    end
  end
end

['Arte', 'Musica', 'Literatura'].each {|d| Discipline.find_or_create_by(title: d) }

10.times do |i|
 c = Course.find_or_create_by(title: "Course #{i.next}")
 c.disciplines = Discipline.all
end

c = Course.first
c.disciplines.count # 3
# can't do this
# c.active_disciplines.count # 0

d = Discipline.first
d.courses.count # 10
d.courses.programming.count # 0

Course.create(title: 'Programming in Ruby', disciplines: [d])

d.courses.count # 11
d.courses.programming.count # 1

has_many_through.rb

# course.rb
# rails g model course title:string 
class Course < ActiveRecord::Base
  has_many :course_disciplines
  has_many :disciplines, through: :course_disciplines

  has_many :active_disciplines, -> {
    where("course_disciplines.active = ? ", true)
  }, through: :course_disciplines, source: :discipline
end

# discipline.rb
# rails g model discipline title:string
class Discipline < ActiveRecord::Base
  has_many :course_disciplines
  has_many :courses, through: :course_disciplines

  has_many :programming_courses, -> {
    where("courses.title LIKE ?", '%Programming%')
  }, through: :course_disciplines, source: :course
end

# course_discipline.rb
# rails g model CourseDiscipline course:references discipline:references active:boolean
class CourseDiscipline < ActiveRecord::Base
  belongs_to :course
  belongs_to :discipline
end


['Arte', 'Musica', 'Literatura'].each {|d| Discipline.find_or_create_by(title: d) }

10.times do |i|
 c = Course.find_or_create_by(title: "Course #{i.next}")
 c.disciplines = Discipline.all
end


c = Course.first
c.disciplines.count # 3
c.active_disciplines.count # 0

d = Discipline.first
d.courses.count # 10
d.programming_courses.count # 0

Course.create(title: 'Programming in Ruby', disciplines: [d])

d.courses.count # 11
d.programming_courses.count # 1

Unique Constraints

# rails g migration add_indices_to_courses_disciplines
class AddIndicesToCoursesDisciplines < ActiveRecord::Migration
  def change
    # using has_and_belongs_to_many
    table_name =  :courses_disciplines
    # using has_many :through
    table_name =  :course_disciplines
    add_index table_name, [:course_id, :discipline_id], unique: true
  end
end

Course.first.disciplines = []
Course.first.disciplines << Discipline.last
Course.first.disciplines << Discipline.last 
# ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_courses_disciplines_on_course_id_and_discipline_id"

Discipline.last.courses = []
Discipline.last.courses << Course.last
Discipline.last.courses << Course.last
# ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_course_disciplines_on_course_id_and_discipline_id"