permission.rb

class Permission < ApplicationRecord
  has_many :roles through: :roles_permissions
end

role.rb

class Role < ApplicationRecord
  has_many :users through: :users_roles
  has_many :permissions through: :roles_permissions
end

roles_permission.rb

class RolesPermission < ApplicationRecord
  belongs_to :role
  belongs_to :permission
end

user.rb

class User < ApplicationRecord
  has_many :roles through: :users_roles
end

users_role.rb

class UsersRole < ApplicationRecord
  belongs_to :user
  belongs_to :role
end