How to get the items around a specific item? - mongodb

Question

I'd like to get x items before and y items after (the neighbors of) the record with "lastseen":true:
// (_id fields omitted)
{ "msg": "hello 1" }
{ "msg": "hello 2" }
{ "msg": "hello 3", "lastseen": true }
{ "msg": "hello 4" }
{ "msg": "hello 5" }
For example, if I query with x=1 and y=1 the result should be:
// (_id fields omitted)
{ "msg": "hello 2" }
{ "msg": "hello 3", "lastseen": true }
{ "msg": "hello 4" }
What are my options in mongodb to achieve that?

Answer 1

It must be simpler to implement the logic on client side with several queries. Assuming you documents are ordered by _id:
findOne({"lastseen":true})
find({_id: {$lt: <_id from the previous query>}}).sort({_id:-1}).limit(1)
find({_id: {$gt: <_id from the first query>}}).sort({_id:1}).limit(1)
The only way to do it in a single query I can imagine is to group the documents into array, and then use $indexOfArray in combination with $slice:
db.collection.aggregate([
// ensure lastseen is present to calculate index properly
{ $addFields: {lastseen: { $ifNull: [ "$lastseen", false ] } } },
// get all documents into array
{ $group: { _id:null, docs: { $push:"$$ROOT" } } },
// get index of first matched document
{ $project: { docs:1, match: { $indexOfArray: [ "$docs.lastseen", true ] } } },
// slice the array
{ $project: { docs: { $slice: [ "$docs", { $subtract: [ "$match", 1 ] } , 3 ] } } },
// remove added lastseen
{ $project: { docs:
{ $map: {
input: "$docs",
as: "doc",
in: { $cond: {
if: "$$doc.lastseen",
then: "$$doc",
else: { $arrayToObject: { $filter: {
input: { $objectToArray: "$$doc" },
as: "field",
cond: { $ne: [ "$$field.k", "lastseen" ] }
} } }
} }
} }
} },
// un-group documents from the array
{ $unwind: "$docs" },
{ $replaceRoot: {newRoot:"$docs"}}
]);
but I doubt efficiency of such query.

Answer 2

Answer is very short you can use skip() to skip how many you want
// (_id fields omitted)
{ "msg": "hello 1" }
{ "msg": "hello 2" }`
{ "msg": "hello 3", "lastseen": true }
{ "msg": "hello 4" }
{ "msg": "hello 5" }
Command:
db.collection.find({},{_id:0}).skip(1)