I’m using: rails 6.1.4 and ruby 3.1.1p18

It’s been a minute since I’ve played with a many-to-many association in rails and I’m a bit lost on this one. Any help/tips will be greatly appreciated.

I’m happy to edit post to add more info if requested.

I’ve made a directory inside my rails app living_muay_thai/ and inside it I’m trying to create a many to many relationship between two models. I have all the code in place. Using the console (development), I try to create a record in the joins table I get an error:

3.1.1 :009 > sb.errors.full_messages
 => ["Living muay thai student must exist", "Living muay thai badge must exist"]

My code:

generate models/migrations:

rails generate model living_muay_thai/Student
rails generate model living_muay_thai/Badge

At this point I can’t remember how I created the joins table. I think I tried a few different generate lines but all ended up with errors on the index name too long, so I ended up editing it by hand to make that error happy.

Migrations:

Students:

class CreateLivingMuayThaiStudents < ActiveRecord::Migration[6.1]
  def change
    create_table :living_muay_thai_students do |t|
      t.string :fname 
      t.string :lname 
      t.timestamps
    end
  end
end

--------------------------
Badges:

class CreateLivingMuayThaiBadges < ActiveRecord::Migration[6.1]
  def up
    create_table :living_muay_thai_badges do |t|
      t.string  :color 
      t.string  :category
      t.integer :number

      t.timestamps
    end
  end

  def down
    drop_table :living_muay_thai_badges
  end
end

------------------
StudentBadges (joins table)

class CreateLivingMuayThaiStudentBadges < ActiveRecord::Migration[6.1]
  def change
    create_table :living_muay_thai_student_badges do |t|
      t.integer :student_id
      t.integer :badge_id

      t.timestamps
    end
    add_index :living_muay_thai_student_badges, :student_id, name: 'students_index'
    add_index :living_muay_thai_student_badges, :badge_id, name: 'badges_index'
  end
end

For the student badges migration I did it this way (above) because doing the way a couple tutorials mentioned I was getting an error… Index name too long... So, this way worked.

Models:

Students:

# == Schema Information
#
# Table name: living_muay_thai_students
#
#  id         :bigint           not null, primary key
#  fname      :string
#  lname      :string
#  created_at :datetime         not null
#  updated_at :datetime         not null
#
class LivingMuayThai::Student < ApplicationRecord
    has_many :living_muay_thai_student_badges
    has_many :living_muay_thai_badges, through: :living_muay_thai_student_badges

    has_many :living_muay_thai_levels
end

----------------------
Badges:

# == Schema Information
#
# Table name: living_muay_thai_badges
#
#  id         :bigint           not null, primary key
#  category   :string
#  color      :string
#  number     :integer
#  created_at :datetime         not null
#  updated_at :datetime         not null
#
class LivingMuayThai::Badge < ApplicationRecord
    has_many :living_muay_thai_student_badges
    has_many :living_muay_thai_students, through: :living_muay_thai_student_badges
end

---------------------
StudentBadge

# == Schema Information
#
# Table name: living_muay_thai_student_badges
#
#  id         :bigint           not null, primary key
#  badge_id   :integer
#  student_id :integer
#  created_at :datetime         not null
#  updated_at :datetime         not null
#
# Indexes
#
#  badges_index    (badge_id)
#  students_index  (student_id)
#
class LivingMuayThai::StudentBadge < ApplicationRecord
  belongs_to :living_muay_thai_student
  belongs_to :living_muay_thai_badge
end

I haven’t fleshed out the contollers yet. I wanted to test this in the console before I did that. I’m not sure it that has any causal relation to my problem. (??)

Console:

# create a student:
3.1.1 :001 > s1=LivingMuayThai::Student.create(:fname => 'John', :lname=>'Smith')
# works

# create a badge:
3.1.1 :002 > b1=LivingMuayThai::Badge.create(:color => 'White', :category => 'Beginner', :number => 1)
# works

# create record in joins table:
3.1.1 :003 > sb=LivingMuayThai::StudentBadge.create(:student_id => s1.id, :badge_id => b1.id)
 => #<LivingMuayThai::StudentBadge:0x00007f713867a970 id: nil, student_id: 11, badge_id: 140, created_at: nil, updated_at: nil> 
3.1.1 :004 > sb.valid?
 => false 
3.1.1 :005 > sb.errors.full_messages
 => ["Living muay thai student must exist", "Living muay thai badge must exist"]

Thanks again for looking at this. Much appreciated.

>Solution :

The problem is how you define your associations and what columns and tables name you have. Now they are not corresponds to the Rails convention

Define belongs_to like this with explicit column names of join table

class LivingMuayThai::StudentBadge < ApplicationRecord
  belongs_to :living_muay_thai_student, class_name: 'LivingMuayThai::Student', foreign_key: :student_id
  belongs_to :living_muay_thai_badge, class_name: 'LivingMuayThai::Badge', foreign_key: :badge_id
end