how to create a 3 table join migration in ruby on rails - ruby-on-rails

Question

I have 3 tables, students messages and coaches.
now i want to create a join migration table with message_id coach_id and student_id
please help to create the migration query
any help is highly appreciated
getting this error when i tried the below code
== 20150924072052 AssociationTable: migrating
=================================
-- create_table(:associations)
rake aborted!
StandardError: An error has occurred, all later migrations canceled:
Mysql2::Error: Key column 'student_id' doesn't exist in table: ALTER TABLE `associations` ADD CONSTRAINT `fk_rails_122f0db022` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`)`

Answer 1

It will be something like this:
$ rails g migration AssociationTable
... which will create a file such as the following:
#db/migrate/association_table_[timestamp].rb
class AssociationTable < ActiveRecord::Migration
def change
create_table :associations do |t|
t.references :student
t.references :message
t.references :coache
t.timestamps null: false
end
end
end
This will create a table with the following columns:
id
student_id
message_id
coach_id
created_at
updated_at
This will be used in a has_many :through relationship, which requires you to have a join model:
#app/models/association.rb
class Association < ActiveRecord::Base
belongs_to :student
belongs_to :message
belongs_to :coach
end
--
To update you regarding the choice between has_many :through and has_and_belongs_to_many, here is what you need to know:
The main difference between the two is the fact that has_many :through utilizes a join model. Join models are basically a model through which ActiveRecord will populate dependent associative data. In short, it "joins" two models together.
Although the join model is the big difference between HABTM and HMT, there is another technical reason why you'd choose between them - HMT permits you to have extra attributes in the join model.
This is important for something like this:
#app/models/doctor.rb
class Doctor < ActiveRecord::Base
has_many :appointments
has_many :patients, through: :appointments
end
#app/models/appointment.rb
class Appointment < ActiveRecord::Base
#columns id | doctor_id | patient_id | time | created_at | updated_at
belongs_to :doctor
belongs_to :patient
end
#app/models/patient.rb
class Patient < ActiveRecord::Base
has_many :appointments
has_many :doctors, through: :appointments
end
The join model (appointment) will therefore be able to have specific data that you'll be able to use with the other models:
#doctor = Doctor.find 1
#appointments = #doctor.appointments.where(time: ____)
#patients = #appointments.patients
Lots of queries. I hope you get the idea.
--
has_and_belongs_to_many is a lot simpler, although I'm not sure if it works for 3 tables (don't see why it shouldn't).
This removes the need for a join model, and in the process prevents you from being able to use extra attributes in the association (notice how the join table has no id or timestamp attributes?). The naming convention for HABTM tables is albabetical_plurals - in your case recipient_messages
Again, I don't know if this will work for a 3-way join, but here's how you'd do it:
#app/models/student.rb
class Student < ActiveRecord::Base
has_and_belongs_to_many :messages, join_table: "recipients_messages", foreign_key: "recipient_id", association_foreign_key: "message_id"
end
#app/models/message.rb
class Message < ActiveRecord::Base
has_and_belongs_to_many :recipients, join_table: "recipients_messages", foreign_key: "message_id", association_foreign_key: "recipient_id"
end
Thinking about your request specifically, I'd say that you'd be better with has_many :through.
Reason being that if you're sending messages, you need a way to know to whom the message was sent, its content and whether it's been read.
I would use messages as the join model:
#app/models/message.rb
class Message < ActiveRecord::Base
#columns id | student_id | coach_id | message | read | created_at | updated_at
belongs_to :student
belongs_to :coach
end
#app/models/coach.rb
class Coach < ActiveRecord::Base
has_many :messages
end
#app/models/student.rb
class Student < ActiveRecord::Base
has_many :messages
end