Make $elemMatch (projection) return all objects that match criteria

Question

I will use the example from here

{
 _id: 1,
 zipcode: 63109,
 students: [
              { name: \"john\", school: 102, age: 10 },
              { name: \"je         


        

Answer 1

Previous and incorrect answer:

This should work as of today. See https://docs.mongodb.com/v3.2/reference/operator/projection/positional/#array-field-limitations

You should get the correct result when querying using $elemMatch in the query and exposing the sub-document in the projection like following:

db.schools.find( { zipcode: 63109, students: { $elemMatch: { school: 102 } } },
                 { 'students.$': 1 } )

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New answer

Limiting the list of sub-documents to those matching the query is as of now not possible using find(). Please take aggregate() instead or take one of the following possibilities:

You could either get all the sub-documents of the matching document by adding the array-property in the projection:

db.schools.find( { zipcode: 63109, students: { $elemMatch: { school: 102 } } }, { 'students': 1 })
> { "_id" : 1, "students" : [ { "name" : "john", "school" : 102, "age" : 10 }, { "name" : "jess", "school" : 102, "age" : 11 }, { "name" : "jeff", "school" : 108, "age" : 15 } ] }
> { "_id" : 4, "students" : [ { "name" : "barney", "school" : 102, "age" : 7 } ] }

Or you can get the first item matching the $elemMatch query on the sub-documents:

db.schools.find( { zipcode: 63109, students: { $elemMatch: { school: 102 } } }, { 'students.$': 1 })
> { "_id" : 1, "students" : [ { "name" : "john", "school" : 102, "age" : 10 } ] }
> { "_id" : 4, "students" : [ { "name" : "barney", "school" : 102, "age" : 7 } ] }