AR: add attribute to model

rubyonrails-talk
1

Hi!

Well, I thought this was easy, but for some reason it won't work... Here is what I want to do:

I am working on an API to a large database. The basic mapping is done, but I want to implement some convenience methods that take information for one object and does some calculations with it. The result should be accessible as attribute.

So in a simple example:

We have an object that has a start and a stop attribute, i.e. columns in the table. Now I want to know the distance, but directly via an readable attribute.

Class Something

attr_accessor :distance

def initialize   @distance = self.stop-self.start end

end

However, if I use my_object.distance, it returns nil...

What am I doing wrong?

P.S. I also tried def initialize(distance) - but figured that only makes sense if I specifically create a new object, not if the attribute is created from within the class. Didn't work either, anyway.

Cheers,

Marc

2

Hi!

Well, I thought this was easy, but for some reason it won't work... Here is what I want to do:

I am working on an API to a large database. The basic mapping is done, but I want to implement some convenience methods that take information for one object and does some calculations with it. The result should be accessible as attribute.

So in a simple example:

We have an object that has a start and a stop attribute, i.e. columns in the table. Now I want to know the distance, but directly via an readable attribute.

Class Something

attr_accessor :distance

def initialize @distance = self.stop-self.start end

If that's an ActiveRecord objectg, not calling super (and changing the signature of initialize) is a bad thing. you could avoid that altogether with

def distance   @distance ||= stop - start end

Fred

3

Frederick Cheung wrote: