MongoDB: Project to array item with minimum value of field - mongodb

Question

Suppose my collection consists of items that looks like this:
{
"items" : [
{
"item_id": 1,
"item_field": 10
},
{
"item_id": 2,
"item_field": 15
},
{
"item_id": 3,
"item_field": 3
},
]
}
Can I somehow select the entry of items with the lowest value of item_field, in this case the one with item_id 3?
I'm ok with using the aggregation framework. Bonus point if you can give me the code for the C# driver.

Answer 1

You can use $reduce expression in the following way.
The below query will set the initialValue to the first element of $items.item_field and followed by $lt comparison on the item_field and if true set $$this to $$value, if false keep the previous value and $reduce all the values to find the minimum element and $project to output min item.
db.collection.aggregate([
{
$project: {
items: {
$reduce: {
input: "$items",
initialValue:{
item_field:{
$let: {
vars: { obj: { $arrayElemAt: ["$items", 0] } },
in: "$$obj.item_field"
}
}
},
in: {
$cond: [{ $lt: ["$$this.item_field", "$$value.item_field"] }, "$$this", "$$value" ]
}
}
}
}
}
])

Answer 2

You can use $unwind to seperate items entries.
Then $sort by item_field asc and then $group.
db.coll.find().pretty()
{
"_id" : ObjectId("58edec875748bae2cc391722"),
"items" : [
{
"item_id" : 1,
"item_field" : 10
},
{
"item_id" : 2,
"item_field" : 15
},
{
"item_id" : 3,
"item_field" : 3
}
]
}
db.coll.aggregate([
{$unwind: {path: '$items', includeArrayIndex: 'index'}},
{$sort: { 'items.item_field': 1}},
{$group: {_id: '$_id', item: {$first: '$items'}}}
])
{ "_id" : ObjectId("58edec875748bae2cc391722"), "item" : { "item_id" : 3, "item_field" : 3 } }

Answer 3

We can get expected result using following query
db.testing.aggregate([{$unwind:"$items"}, {$sort: { 'items.item_field': 1}},{$group: {_id: "$_id", minItem: {$first: '$items'}}}])
Result is
{ "_id" : ObjectId("58edf28c73fed29f4b741731"), "minItem" : { "item_id" : 3, "item_field" : 3 } }
{ "_id" : ObjectId("58edec3373fed29f4b741730"), "minItem" : { "item_id" : 3, "item_field" : 3 } }