class User
  include Mongoid::Documentfield :name, type: String
  has_many :stuff, dependent: :restrict_with_error

  def foo(my_keyword_arg: 'foo')
    puts "#{my_keyword_arg}!"
  end
end

class Stuff
  include Mongoid::Documentfield :foo, type: String
  belongs_to :user
  
  def foo_my_user
    user.foo(my_keyword_arg: 'bar')
  end
end

user = User.create!(name: 'my user')
stuff = Stuff.create!(foo: 'bar', user: user)
stuff.user.foo # => outputs 'foo!' as expected
stuff.user.foo(my_keyword_arg: 'baz') # => raises ArgumentError
stuff.foo_my_user # => raises ArgumentError