Non persistent ActiveRecord model attributes

Question

I want to add to an existing model some attributes that need not be persisted, or even mapped to a database column. Is there a solution to specify such thing ?

Answer 1

In my case I wanted to use a left join to populate custom attribute. It works if I don't add anything but I also want to be able to set the attribute on a new object and of course it doesn't exist. If I add attr_accessor then it always returns nil after a select. Here's the approach I've ended up with that works for setting on new object and retrieving from left join.

after_initialize do
  self.foo = nil unless @attributes.key?("foo")
end

def foo
  @attributes["foo"]
end

def foo=(value)
  @attributes["foo"] = value
end