How to find not in values in mongodb query - mongodb

Question

Here i am having n-number of documents, below are the examples of the documents.
{
"_id" : "ABC",
"languagesknow" : {
"lng" : [
"1",
"2"
]
}
},
{
"_id" : "CDF",
"languagesknow" : {
"lng" : [
"3",
"4"
]
}
}
My question is suppose i will pass input as a one array.suppose in that array i am passing 2 & 5 & 6 means 5 & 6 not there inside lng, so want to print the values , how can achieve this?

Answer 1

You need $setDifference operator, try:
db.collection.aggregate([
{
$match: {
"languagesknow.lng": { $type: "array" }
}
},
{
$project: {
languagesknow: {
$setDifference: [ [ "2", "5", "6" ], "$languagesknow.lng" ]
}
}
}
])
EDIT: you can put $match in front of $setDifference to filter out all the documents where languagesknow.lng is not an array (using $type operator).
EDIT(2): if you want to get single array for entire collection then you need to add $group with $unwind to gather all the languagesknow into single array and then run $setDifference on it, try:
db.collection.aggregate([
{
$match: {
"languagesknow.lng": { $type: "array" }
}
},
{
$unwind: "$languagesknow.lng"
},
{
$group: {
_id: null,
languagesknow: { $addToSet: "$languagesknow.lng" }
}
},
{
$project: {
languagesknow: {
$setDifference: [ [ "2", "5", "6" ], "$languagesknow" ]
}
}
}
])