$elemMatch query in MongoDB - mongodb

Question

I have a collection 'name' with 2 documents of the structure :
doc 1:
{
a: 1
b : [{name:"AAA",age:10},
{name:"BBB",age:12},
{name:"CCC",age:13}]
}
doc 2 :
{
a: 2
b : [{name:"DDD",age:14},
{name:"EEE",age:15},
{name:"FFF",age:16}]
}
Since I am new to MongoDB, I am trying to find the difference of using the $elemMatch operator and not using it. Basically I am trying to query and find the first doc ( doc 1) with name AAA and age 10. So using the $elemMatch, my query looks like this :
db.name.find({b: {$elemMatch :{name:"AAA",age:10}}})
This query works perfectly fine, but my question is that what's the need to use this query when I can query like this :
db.name.find({b:{name:"AAA",age:10}})
I am sure there should be some reason for $elemMatch, just trying to find the difference. Thanks in advance for the reply !!!

Answer 1

The key difference is that the second query (without the $elemMatch) would only match elements in the b array that only contained those two fields, and only in that order.
So the first query would match the following two documents, but the second query wouldn't:
{
a: 1
b: [{name: "AAA", age: 10, city: 'New York'},
{name: "BBB", age: 12, city: 'Paris'},
{name: "CCC", age: 13, city: 'London'}]
}
{
a: 1,
b: [{age: 10, name: "AAA"},
{name: "BBB", age: 12},
{name: "CCC", age: 13}]
}

Answer 2

Another important difference is that how Mongo uses indexes.
If we have declared a multi-key-compound index:
db.name.createIndex({ "b.name": 1, "b.age": 1 })
And we execute this:
db.name.explain().find({
b: {
name: "DDD",
age: 14
}
})
we get:
"winningPlan" : {
"stage" : "COLLSCAN",
If we execute this:
db.name.explain().find({
b: {
$elemMatch: {
name: "DDD",
age: 14
}
}
})
we get:
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
But if we have a simple multi-key index in the array:
db.name.createIndex({b: 1})
The above index will be used in this query:
db.name.explain().find({
b: {
name: "DDD",
age: 14
}
})
And under my very un-professional profiling tests looks like this is faster.