Access SQL computed columns through ActiveRecord - mysql

Question

I have a Person model, which includes a property representing the data of birth (birth_date).
I also have a method called age(), which works out the current age of the person.
I now have need to run queries based on the person's age, so I have replicated the logic of age() as a computed column in MySQL.
I cannot workout how I would make this additional column part of the default select statement of the model.
I would like to be able to access the age as if it were a native property of the Person model, to perform queries against it and access the value in my views.
Is this possible, or am barking up the wrong tree?
I thought I might be able to define additional fields through default_scope or scope, but these methods seem to only recognise existing fields. I also tried default_scope in tandem with attr_assessor.
Possible workarounds I've considered but would prefer not to do:
Create an actual property called age and populate through the use of callbacks. The date is always changing, so this obviously would be be reliable.
Replicate the logic in ActiveRecord as age() and in a scope as a where cause. This would achieve what I need, but doesn't feel very DRY.
I am already caching the results of the age() method. it is the ability to use the field in where clauses that I am most interested in.
There must be a way to define dynamic fields through SQL that I can access through the model by default.
Any help would be appreciated.
Rich
UPDATE
An example of my failed attempt to utilise scopes:
default_scope :select => "*, 2 as age"
attr_accessor :age
age is blank, I assume because scopes only deal with limiting, not extending.

Answer 1

kim3er your solution to your problem is simple. Follow these steps:
Loose the attr_accessor :age from your model. You simply don't need it.
Leave the default scope at the Person model: default_scope :select => "*, 2 as age"
Lastly open up a console and try
p = Person.first
p.age
=> 2
When you define a select using as, Rails will automagically add those methods on the instances for you! So the answer to your question:
There must be a way to define dynamic fields through SQL that I can access through the model by default.
is:
Rails

Answer 2

I'm not an expert by any stretch, but it seems you want:
class Person < ActiveRecord::Base
scope :exact_age, lambda { |a| where('age = ?', a) }
scope :age_gt, lambda { |a| where('age > ?', a) }
but that said, I've just started looking at Arel, seems pretty cool