如何投影一些嵌入式文档？ Mongodb

Question

我的文档如下：

{"_id":{"$_id":"5dd315bc717872581c5071a8"},
"answered":[
         {"waiting":false,"_id":{"$_id":"5dd315fe717872581c5071a9"},"user":"5dd2f023b6c5075758c06cb5"}, 
         {"waiting":true,"_id":{"$_id":"5dd31605717872581c5071aa"},"user":"5dd2ed7ab6c5075758c06ca9"}, 
         {"waiting":true,"_id":{"$_id":"5dd31608717872581c5071ab"},"user":"5dd2ed97b6c5075758c06cab"}]

什么是查询，仅将“ waiting”为真的文档返回给我？ 即

{"_id":{"$_id":"5dd315bc717872581c5071a8"},
"answered":[
         {"waiting":true,"_id":{"$_id":"5dd31605717872581c5071aa"},"user":"5dd2ed7ab6c5075758c06ca9"}, 
         {"waiting":true,"_id":{"$_id":"5dd31608717872581c5071ab"},"user":"5dd2ed97b6c5075758c06cab"}]