Unexpected result when trying to filter fields in mongo db - mongodb

Question

I have a document structured as follows:
{
_id: "someid",
games: [{
slug: "world-of-warcraft",
class: 'shaman'
}, {
slug: 'starcraft-ii',
class: 'zerg'
}],
roles: {
'starcraft-ii': ['player'],
'world-of-warcraft': ['player']
}
}
I am trying to filter it so that only starcraft-ii within the games array will show up for all players in the role of player in starcraft-ii. I do the following:
function getUsersInGame(slug) {
return Users.find({
'games.slug': slug,
[`roles.${slug}`]: 'player'
}, {
fields: {
'games.$': 1,
'roles': 1
}
});
}
However, this does not match within the games array, and instead returns a 1-element array with world-of-warcraft instead.
What is the appropriate way to filter this array in mongo?

Answer 1

Use $elemMatch in your fields, since the $ will return the first element of the array. So your query should look like this:
function getUsersInGame(slug) {
return Users.find(
{
'"roles.'+ slug + '"': { $in : ['player']}
},
{
'games': {
$elemMatch: {'slug': slug}
},
'roles': 1
});
Please note the difference from the docs:
"The $ operator projects the array elements based on some condition from the query statement.
The $elemMatch projection operator takes an explicit condition argument. This allows you to project based on a condition not in the query..."