setter_private.rb

require 'bundler/setup'
require 'active_record'
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Schema.define do
  create_table "users" do |t|
    t.string "name"
  end
end

class User < ActiveRecord::Base
  private

  def name=(value)
    super
  end
end

user = User.new(name: "kir")
# unknown attribute 'name' for User. (ActiveModel::UnknownAttributeError)