How to map 2 db columns to one property

Given a migration

class CreateTalks < ActiveRecord::Migration
  def self.up
    create_table :talks do |t|
      t.integer :duration_hours
      t.integer :duration_minutes
    end
  end

  def self.down
    drop_table :ta开发者_开发问答lks
  end
end

and model

class Talk < ActiveRecord::Base
end

and object

class Duration
   attr_accessor :hours, :minutes
end

how do I map the duration_hours and duration_minutes columns to a Duration property in Talk so that I can do

d = Talk.first.duration
hours = d.hours
minutes = d.minutes

I'm aware that in this case I could translate the hours and minutes into seconds and store these in a single column but I'm looking to understand how I could achieve this type of mapping with ActiveRecord.

Write custom accessor methods.

I'm not sure I understand what you're trying to accomplish... especially with storing hours and minutes in two different columns rather than storing a date/time and extracting the hours and minutes .... buuuut... my understanding of attr_accessor is that:

attr_accessor :duration_hours

is equal to:

def duration_hours
  @duration_hours
end

def duration_hours=(d)
  write_attribute(:duration_hours, d)
end

So what Eimantas is referring to customizing the above methods to store or display information as you see fit... you should already be able to do:

d = Talk.first
hours = d.duration_hours
minutes = d.duration_minutes

Right?

EDIT:

If you're trying to make a list of lectures or something and the amount of time each elapsed using your columns, the way I would do it is (for display purposes only):

def duration
  @duration = self.duration_hours.to_s + ":" + self.duration_minutes.to_s
end

Then call using talk.duration. Sloppy but it works.

Look into the composed_of utility in Rails; it allows you to map one or more database columns into instances of Ruby classes.

References:

• http://api.rubyonrails.org/classes/ActiveRecord/Aggregations/ClassMethods.html
• Section 18.6 of Agile Web Development with Rails Third Edition.