mongodb $elemMatch in query return all sub docs - mongodb

Question

db.aaa.insert({"_id":1, "m":[{"_id":1,"a":1},{"_id":2,"a":2}]})
db.aaa.find({"_id":1,"m":{$elemMatch:{"_id":1}}})
{ "_id" : 1, "m" : [ { "_id" : 1, "a" : 1 }, { "_id" : 2, "a" : 2 } ] }
Using $elemMatch as query operator, it return all sub docs in 'm' !! Strange!
Use it as project operator:
db.aaa.find({"_id":1},{"m":{$elemMatch:{"_id":1}}})
{ "_id" : 1, "m" : [ { "_id" : 1, "a" : 1 } ] }
This is OK. Following this logic, use it as query operator in update will change all sub docs in 'm'. So I do:
db.aaa.update({"_id":1,"m":{$elemMatch:{"_id":1}}},{$set:{"m.$.a":3}})
db.aaa.find()
{ "_id" : 1, "m" : [ { "_id" : 1, "a" : 3 }, { "_id" : 2, "a" : 2 } ] }
It works in manner of as second example(project operator). This really confuse me.
Give me a explain

Answer 1

It Isn't strange, it's how it works.
You are using $elemMatch to match an element within an array contained in your document. That means it mactches the "document" and not the "array element", so it does not just selectively display only the array element that was matched.
What you can do, and how you used it in with the $set operator, is use a positional $ operator to indicate the matched "position" from your query side:
db.aaa.find({"_id":1},{"m":{$elemMatch:{"_id":1}}},{ "m.$": 1 })
And that will show you only one element of the array. But it is of course *still an array in the result shown, and you cannot cast it to a different type.
The other part of the usage is that this will only match once. And only the first match will be assigned to the positional operator.
So perhaps the most succinct explaination is you matching the "document that contains" the properties of the sub-document your specified in your query, and not just the "sub-document" itself.
See the documentation for more:
http://docs.mongodb.org/manual/reference/operator/projection/positional/
http://docs.mongodb.org/manual/reference/operator/query/elemMatch/