Match Document Key Nearest to Search Value - mongodb

Question

I have the following collection:
{
"_id" : "Stats1",
"minutes" : {
"0" : [
{
"0" : {
"f" : 1,
"t" : 0,
"v" : "0"
}
}
],
"22" : [
{
"2" : "1"
}
],
"29" : [
{
"32" : "2"
}
],
"38" : [
{
"40" : "3"
}
]
}
}
and when i try:
db.stats.aggregate()
.project({"_id":"1", "minArray": {"$objectToArray": "$minutes"}})
i am getting error message:
"$objectToArray requires a document input, found: array"
and when i try:
db.stats.aggregate()
.project({"_id":"1", "minArray": {"$arrayToObject": "$minutes"}})
i am getting error message:
"$arrayToObject requires an array input, found: object"
I would like to get closest value for minute exact or lower than 30:
{ "minute" : "29", "value" : [{ "32" : "2"}] }

Answer 1

So the errors are because without a $match your pipeline is attempting to access other documents which don't have the expected structure. That's really something separate to sort out though.
To actually answer your question from it's end objective, you want a pipeline like this:
var _id = "Stats1";
var target = 30;
db.stats.aggregate([
{ "$match": { "_id" : _id } },
{ "$replaceRoot": {
"newRoot": {
"$let": {
"vars": {
"working": {
"$map": {
"input": { "$objectToArray": "$minutes" },
"in": {
"k": { "$toInt": "$$this.k" },
"v": "$$this.v",
"diff": { "$abs": { "$subtract": [ target, { "$toInt": "$$this.k" }] } }
}
}
}
},
"in": {
"$arrayToObject": {
"$map": {
"input": {
"$filter": {
"input": {
"$objectToArray": {
"$arrayElemAt": [
"$$working",
{ "$indexOfArray": [ "$$working.diff", { "$min": "$$working.diff" } ] }
]
}
},
"cond": { "$ne": [ "$$this.k", "diff" ] }
}
},
"in": {
"k": { "$cond": [{ "$eq": [ "$$this.k", "k"] }, "minute", "value" ] },
"v": { "$cond": [{ "$eq": [ "$$this.k", "k"] }, { "$toString": "$$this.v" }, "$$this.v" ] }
}
}
}
}
}
}
}}
])
Which of course returns the wanted output:
{ "minute" : "29", "value" : [ { "32" : "2" } ] }
In sequence you do the $objectToArray as you initially attempted, but then you need that key or "k" value to actually be converted to numeric for comparison. You also need to calculate the difference of that from the value you are searching for, in this case 30. That gives you a "working" copy of the data in array form, which is important for the next input stages.
The next section is basically read inwards from the levels of indentation to best understand the order.
First you basically want to extract the element from that working array where the difference ( using $abs so positive and negative are the same ) is the minimal value with $min. This gives the position of the first match from $indexOfArray and used that with $arrayElemAt to return that single selected element from the working array.
We don't want all the fields in that object, so $objectToArray converts that single object into "k" and "v" paired objects, and the first step is to $filter where that key is the difference field and remove this from that list.
Next you want to rename the fields and change some data formats, so the $map iterates the remaining array ( just two entries ) assigning readable names and setting the string format for the "minute".
Finally this can go back to an object as $arrayToObject as the final output. Since we wanted to refer to that "working" array several times, we declare in $let which allows us to do that. And since all of that was an expression that outputs what you want as a document, you use $replaceRoot to wrap this as an "expression" is basically it's single expected argument.