Mongo find value with unknown parent key - mongodb

Question

I am looking for a value in a Mongo table where its parent key might not have a descriptive or known name. Here is an example of what one of our documents looks like.
{
"assetsId": {
"0": "546cf2f8585ffa451bb68369"
},
"slotTypes": {
"0": { "usage": "json" },
"1": { "usage": "image" }
}
}
I am looking to see if this contains "usage": "json" in slotTypes, but I can't guarantee that the parent key for this usage will be "0".
I tried using the following query without any luck:
db.documents.find(
{
slotTypes:
{
$elemMatch:
{
"usage": "json"
}
}
}
)
Sorry in advance if this is a really basic question, but I'm not used to working in a nosql database.

Answer 1

I'm not sure you're going to be able to solve elegantly this with your current schema; slotTypes should be an array of sub-documents, which would allow your $elemMatch query to work. Right now, it's an object with numeric-ish keys.
That is, your document schema should be something like:
{
"assetsId": {
"0": "546cf2f8585ffa451bb68369"
},
"slotTypes": [
{ "usage": "json" },
{ "usage": "image" }
]
}
If changing the data layout isn't an option, then you're going to need to basically scan through every document to find matches with $where. This is slow, unindexable, and awkward.
db.objects.find({$where: function() {
for(var key in this.slotTypes) {
if (this.slotTypes[key].usage == "json") return true;
}
return false;
}})
You should read the documentation on $where to make sure you understand the caveats of it, and for the love of all that is holy, sanitize your inputs to the function; this is live code that is executing in the context of your database.