Mongodb update on nested document with sort and slice - mongodb

Question

The document represents one users having images. Each image can have N images related to it. I would like to be able to update the matches list only if:
The match does exist yet.
There is less then N elements in the matches array
If they are already N element, only push if "c" parameter is higher than the lower present.
{
"user_id" : 1,
"imgs" : [
{
"img_id" : 1,
"matches" : [
{
"c" : 0.3,
"img_id" : 2
},
{
"c" : 0.2,
"img_id" : 3
}
]
},
{
"img_id" : 5,
"matches" : [
{
"c" : 0.4,
"img_id" : 6
}
]
}
]
}
Basically, "matches" is a set, but $addToSet does not provide $slice and $sort, so I am trying to use $push instead.
db.stack.updateOne(
{ "user_id" : 1, "imgs.img_id" : 1, "imgs.matches.img_id" : { "$ne" : 2 } },
{ "$push" : { "imgs.$.matches" : { "$each" : [ { "c" : 0.7, "img_id" : 2} ], "$sort" : { "c" : -1 }, "$slice" : 3 } } }
);
Does not work, since my document get inserted several times.

Answer 1

Your issue is with the filter part of the updateOne. You should use $elemMatch to make sure that the filter is applied to only one element of the "matches" list.
{"user_id": 1, "imgs": {"$elemMatch": {"img_id" : 1, "matches.img_id": {"$ne": 2}}}},
{ "$push" : { "imgs.$.matches" : { "$each" : [ { "c" : 0.7, "img_id" : 2} ], "$sort" : { "c" : -1 }, "$slice" : 3 } } })