如何使用MongoDB Ruby驱动程序执行“group”(group by)?

有关 MongoDB Group using Ruby driver

如果我想在SQL中执行如下操作:

select page_id, count(page_id) from a_table group by page_id

我以为MongoDB的医生说

http://api.mongodb.org/ruby/current/Mongo/Collection.html#group-instance_method

group(key, condition, initial, reduce, finalize = nil)
# returns an array

Analytic.collection.group(  "fucntion (x) return {page_id : x.page_id}", 
                            nil, 
                            {:count => 0},  
                            "function(x, y) { y.count++ }"  )

[{"count"=>47.0}] 

http://kylebanker.com/blog/2009/11/mongodb-count-group/

db.pageviews.group(
{
 key: {'user.agent': true}, 
 initial: {sum: 0}, 
 reduce: function(doc, prev) { prev.sum += 1}
});

Analytic.collection.group(  ['page_id'], nil, 
  {:count => 0},  "function(x, y) { y.count++ }"  )