Mongodb use slice and elementmatch - mongodb

Question

I am trying to retrieve elements in an array in mongo db. I would like to retrieve the 15 first elements which do not match a pattern
So let's imagine I have
{
"_id" : ObjectId("s4dcsd5s4d6c54s6d"),
"items" : [
{
type : "TYPE_1",
text : "blablabla"
},
{
type : "TYPE_2",
text : "blablabla"
},
{
type : "TYPE_3",
text : "blablabla"
},
{
type : "TYPE_1",
text : "blablabla"
},
{
type : "TYPE_2",
text : "blablabla"
},
{
type : "TYPE_1",
text : "blablabla"
}
]
}
So currently I have more element to match compared to the element to not match that's why I use nin. but it is to simplifiy
If I use
db.history.find({ "_id" : ObjectId("s4dcsd5s4d6c54s6d")}, { "items" : { "$elemMatch" : { "type" : { "$nin" : [ "TYPE_2" , "TYPE_3"]}}}, "items" : { $slice : [0, 2]}}).pretty()
It seems that the element match is not taken into account (inverse as well if i put element match after slice)
Then if I do:
db.history.find({ "_id" : ObjectId("s4dcsd5s4d6c54s6d")}, { "items" : { "$elemMatch" : { "type" : { "$nin" : [ "TYPE_2" , "TYPE_3"]}}, $slice : [0, 2]}}).pretty()
An error is thrown by mongo
Do you know how I can do?
Thanks a lot

Answer 1

You can't use $elemMatch for your case since it will only return the first element. From documentation :
$elemMatch The $elemMatch operator limits the contents of an
field from the query results to contain only the first element
matching the $elemMatch condition.
You can do an aggregation query which will do the following:
match your _id
unwind your items array to have one record per items in the array
match the types $nin your array [ "TYPE_2" , "TYPE_3"]
limit the number of result
The query is :
db.history.aggregate([{
$match: {
_id: ObjectId("s4dcsd5s4d6c54s6d")
}
}, {
$unwind: '$items'
}, {
$match: {
'items.type': { '$nin': ["TYPE_2", "TYPE_3"] }
}
},
{ $limit: 2 }
])
It gives :
{ "_id" : "s4dcsd5s4d6c54s6d", "items" : { "type" : "TYPE_1", "text" : "blablabla" } }
{ "_id" : "s4dcsd5s4d6c54s6d", "items" : { "type" : "TYPE_1", "text" : "blablabla" } }

Answer 2

You will need to use aggregation for restricting the array in the form you have. Use $filter to apply the condition and $slice to limit the array elements.
db.history.aggregate([{
$match: {
_id: ObjectId("586309d6772c68234445f2a5")
}
}, {
"$project": {
"items": {
"$slice": [{
"$filter": {
"input": "$items",
"as": "item",
"cond": {
"$and": [{
$ne: ["$$item.type", "TYPE_2"]
}, {
$ne: ["$$item.type", "TYPE_3"]
}]
}
}
},
2
]
}
}
}])
Sample Output:
{ "_id" : ObjectId("586309d6772c68234445f2a5"), "items" : [ { "type" : "TYPE_1", "text" : "blablabla" }, { "type" : "TYPE_1", "text" : "blablabla" } ] }