In MongoDB, how do I search in an array of sub-documents?

Question

I have a survey document in mongodb, each survey have surveyRefId for unique identification. I am not able to understand how to find sub-documents having questionType = hard in documents whose surveyRefid = 377 or 360.

Here is a sample document:

{
  "json": {
    "surveyRefId": 377,
    "surveyName": "survey on sociology",
    "questionsVoList": [
      {
        "questionId": "556708425215763c64b8af3d",
        "questionText": "question no 1",
        "questionTitle": "",
        "questionType": "hard",
        "preQuestion": true,
        "questionOptions": [
          {
            "questionRefId": 0,
            "optionType": "RADIOBUTTON",
            "isanswer": true,
            "optionText": "ch1"
          }
        ]
      },
      {
        "questionId": "556708425215763c64b8af3d",
        "questionText": "question no 2",
        "questionTitle": "",
        "questionType": "simple",
        "question": true,
        "questionOptions": [
          {
            "questionRefId": 0,
            "optionType": "RADIOBUTTON",
            "isanswer": true,
            "optionText": "ch1"
          }
        ],
      },
      {
       "questionId": "556708425215763c64b8af3d",
        "questionText": "question no 3",
        "questionTitle": "",
        "questionType": "hard",
        "questionOptions": [
          {
            "questionRefId": 0,
            "optionType": "RADIOBUTTON",
            "isanswer": true,
            "optionText": "ch1"
          },
          {
            "questionRefId": 0,
            "optionType": "RADIOBUTTON",
            "isanswer": false,
            "optionText": "ch2"
          }
        ],
       }
    ]
  }
}

EDIT-- Solution by using Java driver as per Sylvain Leroux

    BasicDBObject matchSurvey = new BasicDBObject();
    matchSurvey.put("$match", new BasicDBObject("json.surveyRefId", new BasicDBObject("$in", new Integer[]{377,360})));
    BasicDBObject unwind = new BasicDBObject();
    unwind.put("$unwind", "$json.questionsVoList");
    BasicDBObject matchQuestion = new BasicDBObject();
    matchQuestion.put("$match", new BasicDBObject("json.questionsVoList.questionType", "hard"));
    HashMap map = new HashMap();
    map.put("_id", "$_id");
    map.put("questionsVoList", new BasicDBObject("$push", "$json.questionsVoList"));
    BasicDBObject group = new BasicDBObject();
    group.put("$group",map);        
    List<BasicDBObject> list = new ArrayList<BasicDBObject>();
    list.add(matchSurvey);
    list.add(unwind);
    list.add(matchQuestion);
    list.add(group);
    AggregateIterable output = collection.aggregate(list, DBObject.class);

Answer 1

"find sub-documents having questionType = "hard"" can be understood in three different ways:

All documents having a "hard" query

If you only want all documents having an "hard query, you will use find and $elemMatch:

db.test.find({"json.surveyRefId": { $in: [377, 360]},
              "json.questionsVoList": {$elemMatch: {"questionType":"hard"}}})

First "hard" query of a document

If you need to find the first "hard" query of a document, you will combine the above query with the $ projection operator:

db.test.find({"json.surveyRefId": { $in: [377, 360]}, 
              "json.questionsVoList": {$elemMatch: {"questionType":"hard"}}}
             {"json.surveyRefId":1, "json.questionsVoList.$":1})

All hard queries

If you need to find all "hard" queries of the document, you will have to use the aggregation framework:

db.test.aggregate({$match: { "json.surveyRefId": { $in: [377, 360]} }},
                  {$unwind: "$json.questionsVoList"},
                  {$match: { "json.questionsVoList.questionType": "hard"}},
                  {$group: {_id: "$_id", questionsVoList: {$push: "$json.questionsVoList"}}}
)

• The first $match step will filter out unwanted documents based on their surveyRefId
• Then $unwind will produce a document for each sub-document
• An other $match filters out unwanted documents based on questionType
• Finally, a $group will combine all sub-documents back as one for a given _id

Producing:

{
    "_id" : ObjectId("556828d002509ae174742d11"),
    "questionsVoList" : [
        {
            "questionId" : "556708425215763c64b8af3d",
            "questionText" : "question no 1",
            "questionTitle" : "",
            "questionType" : "hard",
            "preQuestion" : true,
            "questionOptions" : [
                {
                    "questionRefId" : 0,
                    "optionType" : "RADIOBUTTON",
                    "isanswer" : true,
                    "optionText" : "ch1"
                }
            ]
        },
        {
            "questionId" : "556708425215763c64b8af3d",
            "questionText" : "question no 3",
            "questionTitle" : "",
            "questionType" : "hard",
            "questionOptions" : [
                {
                    "questionRefId" : 0,
                    "optionType" : "RADIOBUTTON",
                    "isanswer" : true,
                    "optionText" : "ch1"
                },
                {
                    "questionRefId" : 0,
                    "optionType" : "RADIOBUTTON",
                    "isanswer" : false,
                    "optionText" : "ch2"
                }
            ]
        }
    ]
}