scinote-web/app/models/user_my_module.rb

class UserMyModule < ApplicationRecord
  validates :user, presence: true, uniqueness: { scope: :my_module }
  validates :my_module, presence: true

  belongs_to :user, inverse_of: :user_my_modules, optional: true
  belongs_to :assigned_by,
             foreign_key: 'assigned_by_id',
             class_name: 'User',
             optional: true
  belongs_to :my_module, inverse_of: :user_my_modules,
                         optional: true
end