Mongoose - finding subdocuments by criteria

Question

I've just got stuck with this problem. I've got two Mongoose schemas:

var childrenSchema = mongoose.Schema({     name: {         type: String     },     age: {         type: Number,         min: 0     } });  var parentSchema = mongoose.Schema({     name : {         type: String     },     children: [childrenSchema] }); 

Question is, how to fetch all subdocuments (in this case, childrenSchema objects) from every parent document? Let's suppose I have some data:

var parents = [     { name: "John Smith",     children: [         { name: "Peter", age: 2 }, { name: "Margaret", age: 20 }     ]},     { name: "Another Smith",     children: [         { name: "Martha", age: 10 }, { name: "John", age: 22 }     ]} ]; 

I would like to retrieve - in a single query - all children older than 18. Is it possible? Every answer will be appreciated, thanks!

Answer 1

You can use $elemMatch as a query-projection operator in the most recent MongoDB versions. From the mongo shell:

db.parents.find(     {'children.age': {$gte: 18}},     {children:{$elemMatch:{age: {$gte: 18}}}}) 

This filters younger children's documents out of the children array:

{ "_id" : ..., "children" : [ { "name" : "Margaret", "age" : 20 } ] } { "_id" : ..., "children" : [ { "name" : "John", "age" : 22 } ] } 

As you can see, children are still grouped inside their parent documents. MongoDB queries return documents from collections. You can use the aggregation framework's $unwind method to split them into separate documents:

> db.parents.aggregate({     $match: {'children.age': {$gte: 18}} }, {     $unwind: '$children' }, {     $match: {'children.age': {$gte: 18}} }, {     $project: {         name: '$children.name',         age:'$children.age'     } }) {     "result" : [         {             "_id" : ObjectId("51a7bf04dacca8ba98434eb5"),             "name" : "Margaret",             "age" : 20         },         {             "_id" : ObjectId("51a7bf04dacca8ba98434eb6"),             "name" : "John",             "age" : 22         }     ],     "ok" : 1 } 

I repeat the $match clause for performance: the first time through it eliminates parents with no children at least 18 years old, so the $unwind only considers useful documents. The second $match removes $unwind output that doesn't match, and the $project hoists children's info from subdocuments to the top level.