HABTM relation validation

gistfile1.rb

class Variant < ActiveRecord::Base
  has_and_belongs_to_many :posts
end

class Post < ActiveRecord::Base
  has_and_belongs_to_many :variants
end

# specs
require 'spec_helper'

describe Post, :wip do

  describe '#validate_variant' do
    it 'adds errors if the post doen\'t have at least a variant' do
      post = build(:post)
      post.valid?
      expect(post.errors[:variant_ids]).to be_present
    end

    it 'returns true when the post have at least one variant', :now do
      variant = create(:variant)
      post = variant.posts.build(attributes_for(:post))
      post.valid?
      expect(post.errors[:variant_ids]).to be_empty
    end
  end

end