Migrating from a 1-to-many association to a many-to-many association

category.rb

class Category < ActiveRecord::Base
  # Old association
  belongs_to :topic
  # New association
  has_and_belongs_to_many :topics
end 

category_spec.rb

RSpec.describe Category do
  ...
  it 'accepts max 3 topics' do
    topics = (1..4).map { |n| FactoryGirl.create :topic, name: "Topic #{n}" }
    category = FactoryGirl.build :category, topics: topics
    
    expect(category).not_to be_valid
  end
  ...
end

category_spec_2.rb

RSpec.describe Category do
  ...
  it 'accepts max 3 topics' do
    topics = (1..4).map { |n| FactoryGirl.create :topic, name: "Topic #{n}" }
    category = FactoryGirl.build :category, topics: topics
    
    expect(category).not_to be_valid
    expect(category.errors.messages[:topics]).to include("can be a maximum of 3")
  end
  ...
end

category_with_validations.rb

class Category < ActiveRecord::Base
  # Old association
  belongs_to :topic
  # New association
  has_and_belongs_to_many :topics
  
  validates :topics, length: { maximum: 3 }
end 

category_with_validations_2.rb

class Category < ActiveRecord::Base
  # Old association
  belongs_to :topic
  # New association
  has_and_belongs_to_many :topics
  
  validates :topics, length: { maximum: 3, message: "can be a maximum of 3" }
end 

controller.rb

class CategoriesController < ApplicationController
  ...
  
  def update
    @category = Category.find params[:id]
    @category.update_attributes categories_params
  end
  
  private
    def categories_params
      params.require(:category).permit(:name, { topic_ids: [] })
    end
end

form.erb

<%= form_for(@category) do |f| %>
  ...
  <%= f.collection_select :topic_ids, Topic.all, :id, :name, { }, { include_blank: false, multiple: true } %>
  ...
<% end %>

length.rb

value_length = value.respond_to?(:length) ? value.length : value.to_s.length

migration.rb

class CreateTopicsListsAssociation < ActiveRecord::Migration
  def change
    create_table :categories_topics, id: false do |t|
      t.belongs_to :category, index: true
      t.belongs_to :topic, index: true
    end
  end
end

task.rake

desc 'Migrate to many-to-many categories topics associations'
task 'topics:migrate_associations' => :environment do
  Category.where.not(topic_id: nil).each do |category|
    topic = category.topic
    category.topics = [topic]
    category.topic = nil
    category.save!
  end
end

topic.rb

class Topic < ActiveRecord::Base
  # Old association
  # has_many :categories
  # New association
  has_and_belongs_to_many :categories
end