Datamapper: Sorting results through association

Question

I'm working on a Rails 3.2 app that uses Datamapper as its ORM. I'm looking for a way to sort a result set by an attribute of the associated model. Specifically I have the following models:

class Vehicle
  include DataMapper::Resource

  belongs_to :user
end

class User
  include DataMapper::Resource

  has n, :vehicles
end

Now I want to be able to query the vehicles and sort them by the name of the driver. I tried the following but neither seems to work with Datamapper:

> Vehicle.all( :order => 'users.name' )
ArgumentError: +options[:order]+ entry "users.name" does not map to a property in Vehicle

> Vehicle.all( :order => { :users => 'name' } )
ArgumentError: +options[:order]+ entry [:users, "name"] of an unsupported object Array

Right now I'm using Ruby to sort the result set post-query but obviously that's not helping performance any, also it stops me from further chaining on other scopes.

Answer 1

I spent some more time digging around and finally turned up an old blog which has a solution to this problem. It involves manually building the ordering query in DataMapper.

From: http://rhnh.net/2010/12/01/ordering-by-a-field-in-a-join-model-with-datamapper

def self.ordered_by_vehicle_name direction = :asc
  order = DataMapper::Query::Direction.new(vehicle.name, direction)
  query = all.query
  query.instance_variable_set("@order", [order])
  query.instance_variable_set("@links", [relationships['vehicle'].inverse])
  all(query)
end

This will let you order by association and still chain on other scopes, e.g.:

User.ordered_by_vehicle_name(:desc).all( :name => 'foo' )

It's a bit hacky but it does what I wanted it to do at least ;)

Answer 2

Note: I'm not familiar with DataMapper and my answer might not be within the standards and recommendations of using DataMapper, but it should hopefully give you the result you're looking for.

---

I've been looking through various Google searches and the DataMapper documentation and I haven't found a way to "order by assocation attribute". The only solution I have thought of is "raw" SQL.

The query would look like this.

SELECT vehicles.* FROM vehicles
LEFT JOIN users ON vehicles.user_id = users.id
ORDER BY users.name

Unfortunately, from my understanding, when you directly query the database you won't get the Vehicle object, but the data from the database.

From the documentation: http://datamapper.org/docs/find.html. It's near the bottom titled "Talking directly to your data-store"

Note that this will not return Zoo objects, rather the raw data straight from the database

Answer 3

Vehicle.joins(:user).order('users.name').all

or in Rails 2.3,

Vehicle.all(:joins => "inner join users on vehicles.user_id = user.id", :order => 'users.name')