I'm trying to query an array of nested documents in mongodb, and also get the highest value of a particular field in that nested document. (In java)
Here is an example of the document structure, where I want to find the largest value of the "value" field in the array.
{
"_id" : ObjectId("526d89571cd72ce9dbb6b443"),
"array" : [
{"text" : "this is a nested document", "value" : 1 },
{"text" : "this is another nested document", "value" : 2 }
]
}
You can also try modern approach - aggregation framework:
1) Find maximum array 'value' for all elements in collection:
db.collection.aggregate([
{ $unwind: "$array" },
{ $group: { _id: "$_id", value: { $max: "$array.value" } } }
]);
2) Find maximum array 'value' for specified element:
db.collection.aggregate([
{ $match: { _id: new ObjectId("526d89571cd72ce9dbb6b443") } },
{ $unwind: "$array" },
{ $group: { _id: null, value: { $max: "$array.value" } } }
]);
Use real collection name instead of collection in these examples.
Some information on how to use aggregation in Java MongoDB driver: Java Driver and Aggregation Framework.
You can do this pretty easily with a MongoDB map/reduce. Here's the map/reduce I'd write:
map = function() {
for (var a in this.array) {
emit('value', a.value);
}
};
reduce_max = function(key, values) {
var max = values[0];
values.forEach(function(val) {
if (val > max) max = val;
})
return max;
};
and, while I don't have a java dev environment ready to go, here's an article on how to do Map/Reduce queries in Java.
