Database
[{_id:"1234567890a",title:"mainTitle",country:"ITA",subDocument:[{_id:"1234567890b",mainDocumentId:"1234567890a",someField:"123",property:"car"},{_id:"1234567890b",mainDocumentId:"1234567890a",someField:"123",property:"bus"}]}]
Query
db.collection.aggregate([{"$project":{subDocument:{"$filter":{"input":"$subDocument","as":"doc","cond":{"$and":[{"$eq":["$$doc.someField","123"]},{"$eq":["$$doc.property","car"]}]}}}}}])