mongo: how to project array elements excluding some values? - mongodb

Question

I'm using Mongo 3.6. How do I return array items which are NOT in a certain list of values?
Here's the doc:
{
"_id" : "myId",
"myList" : [
"a",
"b",
"c"
]
}
here's the query, which returns only the FIRST match:
db.getCollection('Item').find({_id:"myId"}, {
myList:{ $elemMatch:{
$not: { $in:["a"] }
}}
})
result:
{
"_id" : "myId",
"myList" : [ "b" ]
}
Expected result - how do I change the query above to return ALL matches for 'not in ["a"]'? Something that yields:
{
"_id" : "myId",
"myList" : [ "b", "c" ]
}
Any idea why items are missing?

Answer 1

User aggregate with
$filter
as below to get expected output:
db.getCollection("Item").aggregate([
{
$project: {
list: {
$filter: {
input: "$myList",
as: "item",
cond: { $ne: ["$$item", "a"] },
},
},
},
},
]);

Answer 2

It can be done with an aggregate
[
{
$match: {
_id: "myId"
}
},
{
$project: {
myList: {
$filter: {
input: "$myList",
as: "item",
cond: {
$not: {
$in: [
"$$item",
[
"a"
]
]
}
}
}
}
}
}
]
Try it on mongoplayground

Answer 3

You can get the expected result using Aggregation Pipeline...
db.getCollection("Item").aggregate([
{
$project: {
list: {
$filter: {
input: "$myList",
as: "item",
cond: {
$ne: [
"$$item",
"a"
]
},
},
},
},
},
])

Answer 4

this seems to be the only thing that works on Mongo 3.6 for me, although #AlexisG's solution works in the Mongo v5 playground as well but not on my Mongo 3.6 :-/
db.getCollection('Item').aggregate([
{$match:{_id:"myId"}},
{$project:{myList:1}},
{$unwind:{path:"$myList"}},
{$match:{myList:{$not:{$in:["a"]}}}}
])