hmta.rb

##Class

class User
  has_many :memberships
  has_many :groups, through: :memberships
end

class Membership
  belongs_to :user
  belongs_to :group
end

class Group
  has_many :memberships
end

##Migration

class CreateMemberships < ActiveRecord::Migration
  def change
    create_table :memberships, do |t|
      t.references :user_id, index: true
      t.references :group_id, index: true

      t.timestamps
    end
  end
end