How to use $elemMatch on aggregate's projection?
问题 This is my object: { "_id" : ObjectId("53fdcb6796cb9b9aa86f05b9"), "list" : [ "a", "b" ], "complist" : [ { "a" : "a", "b" : "b" }, { "a" : "c", "b" : "d" } ] } And this is what I want to accomplish: check if "list" contains a certain element and get only the field "a" from the objects on "complist" while reading the document regardless of any of these values. I'm building a forum system, this is the query that will return the details of a forum. I need to read the forum information while