db={"prop_type":[{"_id":1,"prop_type":"A","price":12},{"_id":2,"prop_type":"A","price":20},{"_id":3,"prop_type":"B","price":30}],"prop_for_rent":[{"_id":1,"prop_type":"B","description":"product 1"},{"_id":2,"prop_type":"B","description":"product 2"}]}

db.prop_type.aggregate([{"$lookup":{"from":"prop_for_rent","localField":"prop_type","foreignField":"prop_type","as":"prop_for_rent_docs"}},{"$unwind":{path:"$prop_for_rent_docs",preserveNullAndEmptyArrays:true}},{"$group":{"_id":"$prop_type","no_of_properties":{"$sum":1}}}])