ActiveRecord dynamic attribute-based finders

Published 03 Aug 2008

I’m more used to Java ORM frameworks as for example JPA and Hibernate but started to play around with ActiveRecord lately. Probably all has been said already and I will spare you the introduction and point you to some documentation instead. It took me some time to realize that there is much more than what is shown mostly in online articles, blog postings and books. 

Let’s start with the typical example of a book model containing articles defined with a simple has_many relation.

class Book < ActiveRecord::Base  
  has_many :chapters
end

class Chapter < ActiveRecord::Base  
  belongs_to :book
end

A typical query for all books would then look like this:

Book.find(:all)

and a query for all published books would be something similar to:

Book.find(:all, :conditions => [ 'status = ?', false])

Now to simplify this we can instead use the dynamic finder:

Book.find_all_by_status(false)

Until now this is pretty straightforward, but now comes the interesting part. Imagine now that you have got a book and you want to retrieve a list of chapters:

@book = Book.find(params[:id])
@chapters = @book.chapters.find_all_by_title(title)

Notice how one can use the same dynamic finders directly on the child association? Much easier than making a query using the book_id foreign key in the chapters table!

In fact, ActiveRecord automatically makes the class-level methods of the Chapter model available directly for the associations. This also works in case you have added your own finder methods in the model.

class Chapter < ActiveRecord::Base  
  belongs_to :book  

  def self.find_finished  
    Chapter.find_all_by_status(false)  
  end
end

Note, the “self” since we are adding a class-level method to Chapter. ActiveRecord will automatically make this now available in the association:

@chapters = @book.chapters.find_finished

Hope you got the idea. It simply is beautiful. Next time we should probably look into what’s behind the dynamic finder method.