Return embedded documents in query - ruby-on-rails

Question

Is it possible to perform a query and return the embedded documents?
Currently, I have:
class Post
include MongoMapper::Document
many :comments
end
class Comment
include MongoMapper::EmbeddedDocument
belongs_to :post
key :author
key :date
key :body
end
Here is a query that is almost there:
Post.all("comments.date" => {"$gt" => 3.days.ago})
This will return all the post objects but not the comments. I guess I could do something like:
Post.all("comments.date" => {"$gt" => 3.days.ago}).map(&:comments)
But this would return all comments from the posts. I'd like to get all of the comments that met this condition. Perhaps Comment should not be embedded.

Answer 1

I assume you're looking for all comments newer than three days ago? Since your Comments are just embedded documents, they do not exist without the Post object so there's no way to "query" them separately (this is actually a future feature of MongoDB). However, you could easily add a convenience method to help you out:
class Comment
include MongoMapper::EmbeddedDocument
def self.latest
Post.all(:conditions => {"comments.date" => {"$gt" => 3.days.ago}}).map{|p| p.comments}.flatten
end
end
This method would get you all of the comments that have been updated in the last three days but they would not entirely be in order. A better solution might be to use Map/Reduce to pull the latest comments:
class Comment
include MongoMapper::EmbeddedDocument
def self.latest
map = <<-JS
function(){
this.comments.forEach(function(comment) {
emit(comment.created_at, comment)
});
}
JS
r = "function(k,vals){return 1;}"
q = {'comments.created_at' => {'$gt' => 3.days.ago}}
Post.collection.map_reduce(m,r,:query => q).sort([['$natural',-1]])
end
end
Caveat: the above is completely untested code and just exists as an example, but theoretically should return all comments from the last three days sorted in descending order.