$elemMatch dosen't work after $unwind in MongoDB Aggregation Framework

Question

I have a collection of the following data:

{
  "_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 am using the query:

cmd { "aggregate" : "orders" , "pipeline" : [ 
    { "$unwind" : "$items"} , 
    { "$match" : { "items" : { "$elemMatch" : { "qty" : { "$in" : [ 7]}}}}} , 
    { "$group" : { "price" : { "$first" : "$price"} , "items" : { "$push" : { "sku" : "$items.sku"}} , "_id" : { "items" : "$items"}}} , 
    { "$sort" : { "price" : -1}} , 
    { "$project" : { "_id" : 0 , "price" : 1 , "items" : 1}}
]}

Not able to understand what is going wrong

Answer 1

It's because you're doing $match after $unwind. $unwind generates a new stream of documents where items is no longer an array (see docs).

It emits each document as many times as there are items in it.

If you want to select documents with desired element in it and then process all of its documents, you should call $match first:

db.orders.aggregate(
  { "$match" : { "items" : { "$elemMatch" : { "qty" : { "$in" : [ 7]}}}}},
  { "$unwind" : "$items"},
  ...
);

If you want to select items to be processed after $unwind, you shoul remove $elemMatch:

db.orders.aggregate(
  { "$unwind" : "$items"},
  { "$match" : { "items.qty" : { "$in" : [7]}}},
  ...
);

In first case you'll get two documents:

{ 
  "price" : 25,
  "items" : [
    {"sku" : "ppp"}
  ]
},
{
  "price" : 25,
  "items" : [
    {"sku" : "ggg"}
  ]
}

and in second case you'll get one:

{
  "price" : 25,
  "items" : [
    {"sku" : "ggg"}
  ]
}

Update. After $unwind your documents will look like:

{
  "_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
  }
}

Answer 2

For small number of documents, unwind and match is fine. But large number of documents, it better to do - match ($elemMatch), unwind, and match again.

db.orders.aggregate(
  { "$match" : { "items" : { "$elemMatch" : { "qty" : { "$in" : [ 7]}}}}},
  { "$unwind" : "$items"},
  { "$match" : { "items.qty" : { "$in" : [7]}}}
  ...
  ...
);

The first match will filter only documents that match qty criteria. Among the selected documents, the second match will remove the subdocuments not matching the qty criteria.