Exact Reduce function

Question

I have a collection of the type :

{
  "_id" : ObjectId("51f1fcc08188d3117c6da351"),
  "cust_id" : "abc123",
  "ord_date" : ISODate("2012-10-03T18:30:00Z"),
  "status" : "A",
  "price" : 25,
  "items" : [{
  "sku" : "ggg",
  "qty" : 7,
  "price" : 2.5
}, {
  "sku" : "ppp",
  "qty" : 5,
  "price" : 2.5
}]
}

I want to fetch only the "items" object whose "items.qty">5 and and"items.sku"=="ggg".

I applied Map reduce:

cmd { "mapreduce" : "orders" , "map" : "function map(){var items_out={items:[]};for(i in this.items){items_out.items.push(this.items[i].sku);};emit(this._id,[items_out]);}" , "reduce" : "function reduce(key,values){return {'result':values};}" , "verbose" : true , "out" : { "replace" : "map_reduce"} , "query" : { "$where" : "return this.items.some(function(entry){return entry.qty>5})&&this.items.some(function(entry){return entry.sku=='ggg'})"}},

but I am getting all the sku values something like this:

{ "data": [  {     "items": [     "ggg",      "ppp"     ]     }    ]}

Whereas it should give only ggg as this is the only value matching criteria.

Why would you do this with MR? You can just as easily do this with gthe aggregation framework, also why use the `$where` you should be using `$elemMatch` — Sammaye, Jul 29 '13 at 12:50

score 0 · Answer 1 · answered Jul 29 '13 at 13:07

0

Use the following command:

db.orders.aggregate(
{$unwind : "$items"},
{$match : {"items.qty": {$gt: 5 }}},
{$match : {"items.sku" : "ggg"}},
{$project : {_id:0, items:1}}
)

answered Jul 29 '13 at 13:07

Sarath Nair

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You will need to regroup the items, also you only need one match, also you can do the match first to make use of an index – Sammaye Jul 29 '13 at 13:36
@Sammaye I showed the example of one collection, but what I am actually doing is applying MR to multiple collection and storing it in a temp collection and than querying the temp for data. – Phalguni Mukherjee Jul 29 '13 at 17:20

Exact Reduce function

1 Answers1