Mongodb aggregation, finding within an array of values - mongodb

Question

I have a schemea that creates documents using the following structure:
{
"_id" : "2014-07-16:52TEST",
"date" : ISODate("2014-07-16T23:52:59.811Z"),
"name" : "TEST"
"values" : [
[
1405471921000,
0.737121
],
[
1405471922000,
0.737142
],
[
1405471923000,
0.737142
],
[
1405471924000,
0.737142
]
]
}
In the values, the first index is a timestamp. What I'm trying to do is query a specific timestamp to find the closest value ($gte).
I've tried the following aggregate query:
[
{ "$match": {
"values": {
"$elemMatch": { "0": {"$gte": 1405471923000} }
},
"name" : 'TEST'
}},
{ "$project" : {
"name" : 1,
"values" : 1
}},
{ "$unwind": "$values" },
{ "$match": { "values.0": { "$gte": 1405471923000 } } },
{ "$limit" : 1 },
{ "$sort": { "values.0": -1 } },
{ "$group": {
"_id": "$name",
"values": { "$push": "$values" },
}}
]
This seems to work, but it doesn't pull the closest value. It seems to pull anything greater or equal to and the sort doesn't seem to get applied, so it will pull a timestamp that is far in the future.
Any suggestions would be great!
Thank you

Answer 1

There are a couple of things wrong with the approach here even though it is a fair effort. You are right that you need to $sort here, but the problem is that you cannot "sort" on an inner element with an array. In order to get a value that can be sorted you must $unwind the array first as it otherwise will not sort on an array position.
You also certainly do not want $limit in the pipeline. You might be testing this against a single document, but "limit" will actually act on the entire set of documents in the pipeline. So if more than one document was matching your condition then they would be thrown away.
The key thing you want to do here is use $first in your $group stage, which is applied once you have sorted to get the "closest" element that you want.
db.collection.aggregate([
// Documents that have an array element matching the condition
{ "$match": {
"values": { "$elemMatch": { "0": {"$gte": 1405471923000 } } }
}},
// Unwind the top level array
{ "$unwind": "$values" },
// Filter just the elements that match the condition
{ "$match": { "values.0": { "$gte": 1405471923000 } } },
// Take a copy of the inner array
{ "$project": {
"date": 1,
"name": 1,
"values": 1,
"valCopy": "$values"
}},
// Unwind the inner array copy
{ "$unwind": "$valCopy" },
// Filter the inner elements
{ "$match": { "valCopy": { "$gte": 1405471923000 } }},
// Sort on the now "timestamp" values ascending for nearest
{ "$sort": { "valCopy": 1 } },
// Take the "first" values
{ "$group": {
"_id": "$_id",
"date": { "$first": "$date" },
"name": { "$first": "$name" },
"values": { "$first": "$values" },
}},
// Optionally push back to array to match the original structure
{ "$group": {
"_id": "$_id",
"date": { "$first": "$date" },
"name": { "$first": "$name" },
"values": { "$push": "$values" },
}}
])
And this produces your document with just the "nearest" timestamp value matching the original document form:
{
"_id" : "2014-07-16:52TEST",
"date" : ISODate("2014-07-16T23:52:59.811Z"),
"name" : "TEST",
"values" : [
[
1405471923000,
0.737142
]
]
}