user.rb

class User < ActiveRecord::Base
  
  string :name
  
  def foo
    name = "Bar"  
  end
  
  def working_foo
    self.name = "Bar"  
  end
  
end

user = User.new
user.name = "FooBar"
puts user.name # => FooBar

# This makes no change
user.foo
puts user.name # => FooBar

# This makes the change in the current instance of user
user.working_foo
puts user.name # => Bar