Create a has and belongs to many relationship in Rails 4.2

habtm.md

This will create a parents table, a kids table and a relationship between the two tables.

1. Create the Parents table

   bundle exec rails generate model parents fname:string lname:string
   

2. Create the Kids table

   bundle exec rails generate model kids fname:string lname:string
   

3. Create the Join table between parents and kids. NOTE the naming order is important: Rails expects it to be named in alphabetical order. kids_parents is valid. parents_kids is not valid

   bundle exec rails generate migration create_kids_parents
   

4. Update the migration so that the proper columns are created

   class CreateKidsParents < ActiveRecord::Migration
       def change
           create_table :kids_parents, id: false do |t|
               t.integer :kid_id
               t.integer :parent_id
           end
       end
   end
   

5. Run the migrations

   bundle exec rake db:migrate
   

6. Update the models

   class Kid < ActiveRecord::Base
       has_and_belongs_to_many :parents
   end
   
   class Parent < ActiveRecord::Base
       has_and_belongs_to_many :kids
   end
   

7. Add some test data and try it out

   bundle exec rails c
   homer = Parent.create(fname: "Homer", lname: "Simpson")
   marge = Parent.create(fname: "Marge", lname: "Simpson")
   bart = Kid.create(fname: "Bart", lname: "Simpson")
   lisa = Kid.create(fname: "Lisa", lname: "Simpson")
   homer.kids << [bart, lisa]
   marge.kids << [bart, lisa]
   
   homer.kids
   #this should return an array with bart and lisa
   
   bart.parents 
   #this should return an array of marge and homer