Two MongoDB queries same result, what is the difference? - mongodb

Question

I'm playing around with mongoDB and I can't figure out what is the difference between my two queries.
I use following collection:
{
"_id" : ObjectId("520b79869971eb1a0fdd0ad4"),
"created" : 1376483718636,
"updated" : 1376483718636,
"firstName" : "Jakob",
"lastName" : "D",
"email" : "jakob.d#test.com",
"emailValidated" : false,
"phoneNumber" : "",
"lastLogin" : 1376483718624,
"linkedProviders" : [
{
"userId" : "1XXXXXXXX6",
"providerId" : "facebook",
"password" : "",
"salt" : "",
"authMethod" : "oauth2",
"avatarUrl" : ""
}
],
"userRoles" : [
"ADMIN"
]
},
{
"_id" : ObjectId("520b7dd09971ebcd35dd0ad6"),
"created" : 1376484816666,
"updated" : 1376484816666,
"firstName" : "Jakob",
"lastName" : "D",
"email" : "jakob.d#test.com",
"emailValidated" : false,
"phoneNumber" : "",
"lastLogin" : 1376484816666,
"linkedProviders" : [
{
"userId" : "jakob.d#test.com",
"providerId" : "userpass",
"password" : "7e4aff9e0d90db2318ffcc689c11b66d",
"salt" : "N1GgNvy3NnS0i5GFDyglQZ9s4CeFNndn",
"authMethod" : "userPassword",
"avatarUrl" : ""
}
],
"userRoles" : [
"ADMIN"
]
}
The two queries that gives me the correct and same result(the one with objectId 520b79869971eb1a0fdd0ad4) are:
db.users.find({"linkedProviders.userId":"1XXXXXXXX6","linkedProviders.providerId":"facebook"})
db.users.find({"linkedProviders": {"$elemMatch": {"userId":"1XXXXXXXX6" },"$elemMatch": {"providerId":"facebook" }}})
So what is the difference between these two?

Answer 1

The difference is that $elemMatch finds items for one single array element.
This solution:
db.users.find({
"linkedProviders.userId": "1XXXXXXXX6",
"linkedProviders.providerId": "facebook"
})
Finds any user that has that userId and that providerId, but possibly in different items in linkedProviders, e.g., if linkedProviders[0].userId matches the first and linkedProviders[1].providerId matches the second part of the query, the full document (i.e., the user) will match that query.
On the other hand,
db.users.find({
"linkedProviders": {
"$elemMatch": {
"userId": "1XXXXXXXX6",
"providerId": "facebook"
}
}
})
will match only if the index values (0 and 1 in the previous example) are the same in the document, i.e., only if one array element matches both.
When there's only one key-value mapping in the $elemMatch, it should do the same as the query with the key-value mapping applied directly.
More information:
http://docs.mongodb.org/manual/reference/operator/elemMatch/#op._S_elemMatch