[{"ID":"1","attr1":"123","array":[{"ID":"2","attribute1":"456"},{"ID":"3","attribute1":"567"}]},{"ID":"4","attr1":"123","array":[{"ID":"5","attr1":"456"}]}]

db.collection.aggregate([{"$facet":{"1":[{$match:{"ID":"2"}}],"2":[{$match:{"array.ID":"2"}}],"3":[{$match:{"array.ID":"4"}}],"4":[{$match:{"ID":"4"}}]}},{"$project":{_id:0,"1":{"$concatArrays":["$1","$2"]},"2":{"$concatArrays":["$3","$4"]}}}])