How to find a max value of a specific key in a dictionary in a list in a document in mongodb? - database

Question

I want to use an aggregation to get the highest value of a specified key that's in a dict field that's in a list field that's in a document that's in a mongodb collection.
Here's some example data
[
{
"name": "hi",
"hist": [
{
"username": "bill",
},
{
"username": "jack",
"changed_from": 127
}
]
},
{
"name": "member1",
"hist": [
{
"username": "asdf",
"changed_from": 123
},
{
"username": "duhby",
"changed_from": 126
}
]
},
{
"name": "member5",
"hist": [
{
"username": "duhby",
"changed_from": 150
},
{
"username": "test",
"changed_from": 123
},
{
"username": "duhby",
"changed_from": 125
}
]
}
]
I want to be able to put in duhby as the username, for example, and get at least a list of results I can then easily get the maximum value of, with the maximum value in this case being 150.
I tried using an aggregate group but got stuck when trying to only get the data from the specific username, and not just all documents that had that username in the hist field.
db.collection.aggregate([
{
$group: {
"_id": "$hist.duhby",
update_time: {
$max: "$hist.changed_from"
}
}
}
])
With the example data shown earlier, this returns:
[
{
"_id": [],
"update_time": [
150,
123,
125
]
}
]
However, this isn't useful because it shows every changed_from value when I want it to only show (and sort by) the ones with the username specified. Expected result:
[
{
"_id": [],
"update_time": [
150,
125
]
}
]
I also want to be able to get the original document and maybe have the name field as the id in the aggregation, but the id is currently returned as an empty list.

Answer 1

I realized here that what I needed is just a find, not an aggregation, because I was just trying to see if any document existed that had the embedded document with a key value pair of a certain value. The following does what I was looking for:
db.collection.find({
"hist": {
$elemMatch: {
username: "duhby",
changed_from: {
"$lte": 123
}
}
}
})