MongoDB min/max aggregation - mongodb

Question

I've got documents with this simplified schema :
{
positon: 10,
value: 5,
count: 3
}
What I'd like to compute, is to group those documents by position and find the maximum value where the count is greater than 4 but with value less than the minimum value where the count is less than 4.
Here what I've done, but it does not work :
{ $group: {
_id: {
position: "$position",
},
result: {$max: { $cond: [ {$and: [ {$gte: ["$count", 4]},
{$lt: ["$value", {$min: { $cond: [ {$lt: ["$count", 4]},
{ value: "$value" },
10]
}
}]
}]},
{ value: "$value", nb: "$count"},
0]
}
}
}
}
I am said that $minis an invalid operator and I cant figure out how to write the right aggregation function. Would it be better to run a mapreduce ?
If for example I have those documents
{Position: 10, value: 1, count 5}
{Position: 10, value: 3, count 3}
{Position: 10, value: 4, count 5}
{Position: 10, value: 7, count 4}
I'd like the reslt to be
{Position: 10, value: 1, count 4}
As it is the maximum of 'value' where count is greater than 4 but also as there is a value of 3 that has only 3 counts so that the value 4 is not what I'm looking for.

Answer 1

That is a bit of a mouthful to say the least but I'll have another crack at explaining it:
You want:
For each "Position" value find the document whose "value" is less than the the largest "value" of the document with a "count" of less than four, whose own "count" is actually greater than 4.
Which reads like a math exam problem designed to confuse you with the logic. But catching that meaning then you perform the aggregation with the following steps:
db.positions.aggregate([
// Separate the values greater than and less than 4 by "Position"
{ "$group": {
"_id": "$Position",
"high": { "$push": {
"$cond": [
{ "$gt": ["$count", 4] },
{ "value": "$value", "count": "$count" },
null
]
}},
"low": { "$push": {
"$cond": [
{ "$lt": ["$count", 4] },
{ "value": "$value", "count": "$count" },
null
]
}}
}},
// Unwind the "low" counts array
{ "$unwind": "$low" },
// Find the "$max" value from the low counts
{ "$group": {
"_id": "$_id",
"high": { "$first": "$high" },
"low": { "$min": "$low.value" }
}},
// Unwind the "high" counts array
{ "$unwind": "$high" },
// Compare the value to the "low" value to see if it is less than
{ "$project": {
"high": 1,
"lower": { "$lt": [ "$high.value", "$low" ] }
}},
// Sorting, $max won't work over multiple values. Want the document.
{ "$sort": { "lower": -1, "high.value": -1 } },
// Group, get the highest order document which was on top
{ "$group": {
"_id": "$_id",
"value": { "$first": "$high.value" },
"count": { "$first": "$high.count" }
}}
])
So from the set of documents:
{ "Position" : 10, "value" : 1, "count" : 5 }
{ "Position" : 10, "value" : 3, "count" : 3 }
{ "Position" : 10, "value" : 4, "count" : 5 }
{ "Position" : 10, "value" : 7, "count" : 4 }
Only the first is returned in this case as it's value is less than the "count of three" document where it's own count is greater than 4.
{ "_id" : 10, "value" : 1, "count" : 5 }
Which I am sure is what you actually meant.
So the application of $min and $max really only applies when getting discrete values from documents out of a grouping range. If you are interested in more than one value from the document or indeed the whole document, then you are sorting and getting the $first or $last entries on the grouping boundary.
And aggregate is much faster than mapReduce as it uses native code without invoking a JavaScript interpreter.