foreach item in an array find largest record using mongodbcsharp driver - mongodb-.net-driver

Question

Suppose there is an document like:
{
"_id": "1cf2080c-ce9a-367b-93ba-dbf7b2bf8a2c",
"template_id" : "1cf2080c-ce9a-367b-93ba-dbf7b2bf8a2c",
"some_value" : 12
}
mapping to a class as Template, and given a templateid array array_id = ["aa", "bb", "cc"]
question: how can I get a Template List using mongodb C# driver2.1 up, which each item in list have largest some_value ?
for example there is a collection:
{"_id": "1", "template_id": "aa", "some_value":1}
{"_id": "2", "template_id": "aa", "some_value":2}
{"_id": "3", "template_id": "aa", "some_value":3}
{"_id": "4", "template_id": "bb", "some_value":4}
{"_id": "5", "template_id": "bb", "some_value":5}
and a template_id_array = ["aa", "bb"]
expect result is:
{"_id": "3", "template_id": "aa", "some_value":3}
{"_id": "5", "template_id": "bb", "some_value":5}
I think it would be some "aggregate" or "map-reduce" like procedure, first match result and then group by template id and then find max somevalue for each group.

Answer 1

I have not tested this but should work with some probable modifications
var result = new List<Template>();
foreach (var element in template_id_array)
{
var filter = builder.Eq("TemplateId", element);
var doc = await collection.Find(filter)
.SortByDescending(x => x.SomeValue)
.Limit(1)
.FirstOrDefaultAsync();
result.Add(doc);
}
Then you can serialize it to the final form:
var expectedResult = JsonConvert.SerializeObject(result);