How to Return map of parent_id to children_ids in ActiveRecord/SQL?

Question

Say I have this:

class Post < ActiveRecord::Base
  has_many :relationships, :as => :parent
  has_many :images, :through => :relationships
end

class Relationship
  belongs_to :parent, :polymorphic => true
  belongs_to :child, :polymorphic => true
end

class Image < ActiveRecord::Base
  has_many :relationships, :as => :child
  has_many :posts, :through => :relationships
end

I want to then, as leanly as I can, get back a hash from this call:

Relationship.all(:select => "parent_id, child_id")

...that looks like this:

{:parent_id_1 => [:child_id_1, :child_id_7] ...}

{1 => [1, 7]} #...

Instead, that above call gives me this:

[{"parent_id"=>"1", "child_id"=>"1"}{"parent_id"=>"1", "child_id"=>"7"}...]

So then I have to merge them, and I'm getting a lot of stuff back I don't need. Is there a way to get back something like {1 => [1, 7]} using pure SQL?

Update

I just did this for now, still looking for a cleaner solution:

[{"parent_id"=>"1", "child_id"=>"1"}{"parent_id"=>"1", "child_id"=>"7"}...].inject({}) do |result, node|
  result[node["parent_id"]] ||= []
  result[node["parent_id"]] << node["child_id"]
  result
end