I have the user model like :
has_many :users, class_name: 'User'
belongs_to :master_user, class_name: 'User', optional: true, inverse_of: :users
I Would like to find :
User.first.master_user it's ok
MasterUser.users but get an error : "NameError: uninitialized constant MasterUser"
You are getting that error because you have not defined a MasterUser model. I am guessing you only have a User model as described in your question. If you want to find the users belonging to a "master_user" then you need to find a "master_user" first, then request its users. It would look something like this:
user_with_a_master = User.where.not(master_user_id: nil).first
master = user_with_a_master.master_user
master_users = master.users
