Retrieve only the queried element in an object array in MongoDB collection

Question

Suppose you have the following documents in my collection:

{      "_id":ObjectId("562e7c594c12942f08fe4192"),    "shapes":[         {            "shape":"square",          "color":"blue"       },       {            "shape":"circle",          "color":"red"       }    ] }, {      "_id":ObjectId("562e7c594c12942f08fe4193"),    "shapes":[         {            "shape":"square",          "color":"black"       },       {            "shape":"circle",          "color":"green"       }    ] } 

---

Do query:

db.test.find({"shapes.color": "red"}, {"shapes.color": 1}) 

Or

db.test.find({shapes: {"$elemMatch": {color: "red"}}}, {"shapes.color": 1}) 

---

Returns matched document (Document 1), but always with ALL array items in shapes:

{ "shapes":    [     {"shape": "square", "color": "blue"},     {"shape": "circle", "color": "red"}   ]  } 

However, I'd like to get the document (Document 1) only with the array that contains color=red:

{ "shapes":    [     {"shape": "circle", "color": "red"}   ]  } 

How can I do this?

Answer 1

MongoDB 2.2's new $elemMatch projection operator provides another way to alter the returned document to contain only the first matched shapes element:

db.test.find(     {"shapes.color": "red"},      {_id: 0, shapes: {$elemMatch: {color: "red"}}}); 

Returns:

{"shapes" : [{"shape": "circle", "color": "red"}]} 

In 2.2 you can also do this using the $ projection operator, where the $ in a projection object field name represents the index of the field's first matching array element from the query. The following returns the same results as above:

db.test.find({"shapes.color": "red"}, {_id: 0, 'shapes.$': 1}); 

MongoDB 3.2 Update

Starting with the 3.2 release, you can use the new $filter aggregation operator to filter an array during projection, which has the benefit of including all matches, instead of just the first one.

db.test.aggregate([     // Get just the docs that contain a shapes element where color is 'red'     {$match: {'shapes.color': 'red'}},     {$project: {         shapes: {$filter: {             input: '$shapes',             as: 'shape',             cond: {$eq: ['$$shape.color', 'red']}         }},         _id: 0     }} ]) 

Results:

[      {         "shapes" : [              {                 "shape" : "circle",                 "color" : "red"             }         ]     } ] 

Answer 2

The new Aggregation Framework in MongoDB 2.2+ provides an alternative to Map/Reduce. The $unwind operator can be used to separate your shapes array into a stream of documents that can be matched:

db.test.aggregate(   // Start with a $match pipeline which can take advantage of an index and limit documents processed   { $match : {      "shapes.color": "red"   }},   { $unwind : "$shapes" },   { $match : {      "shapes.color": "red"   }} ) 

Results in:

{     "result" : [         {             "_id" : ObjectId("504425059b7c9fa7ec92beec"),             "shapes" : {                 "shape" : "circle",                 "color" : "red"             }         }     ],     "ok" : 1 }