mongodb aggregate nested nested array - mongodb

Question

how i can find the document with $match on position 3 (only last item in array "ndr"). It is necessary that the aggreation search only in the last array-item of ndr.
{
"_id" : ObjectId("58bd5c63a3d24b4a2e4cde03"),
"name" : "great document",
"country" : "us_us",
"cdate" : ISODate("2017-03-06T12:56:03.405Z"),
"nodes" : [
{
"node" : 3244343,
"name" : "Best Node ever",
"ndr" : [
{
"position" : 7,
"cdate" : ISODate("2017-03-06T10:55:20.000Z")
},
{
"position" : 3,
"cdate" : ISODate("2017-03-06T10:55:20.000Z")
}
]
}
],
}
I need this result after aggregation
{
"name" : "great document",
"country" : "us_us",
"cdate" : ISODate("2017-03-06T12:56:03.405Z"),
"nodes" : [
{
"node" : 3244343,
"name" : "Best Node ever",
"ndr" : [
{
"position" : 3,
"cdate" : ISODate("2017-03-06T10:55:20.000Z")
}
]
}
]
}
I hope anyone can help me.

Answer 1

You can try below aggregation with Mongo 3.4 version.
The below query finds the last item (-1) using $arrayElemAt operator in the ndr array and stores the variable in last using $let operator for each nodes and compare the last variable position value using $$ notation to 3 and wraps the nbr element within array [] if entry found and else returns empty array.
$map operator to reach nbr array inside the nodes array and project the updated nbr array while mapping the rest of nodes fields.
$addFields stage will overwrite the existing nodes with new nodes while keeping the all the other fields.
db.collection.aggregate([{
$addFields: {
nodes: {
$map: {
input: "$nodes",
as: "value",
in: {
node: "$$value.node",
name: "$$value.name",
ndr: {
$let: {
vars: {
last: {
$arrayElemAt: ["$$value.ndr", -1]
}
},
in: {
$cond: [{
$eq: ["$$last.position", 3]
},
["$$last"],
[]
]
}
}
}
}
}
}
}
}]);
Update:
You can try $redact which will keep the whole document if it finds the matching position from with the given filter.
$map to project the true, false values based on the filter for each of the nodes nbr position value and $anyElementTrue will inspect the previous boolean values for each doc and return a true or false value and $redact will use the booelan value from above comparison; true value to keep and false value to remove the document.
db.collection.aggregate([{
$redact: {
$cond: [{
$anyElementTrue: {
$map: {
input: "$nodes",
as: "value",
in: {
$let: {
vars: {
last: {
$arrayElemAt: ["$$value.ndr", -1]
}
},
in: {
$cond: [{
$eq: ["$$last.position", 3]
},
true,
false
]
}
}
}
}
}
}, "$$KEEP", "$$PRUNE"]
}
}]);

Answer 2

you will need to unwind both nested arrays.
db.<collection>.aggregate([
{ $unwind: '$nodes' },
{ $unwind: '$nodes.ndr'},
{ $group: {'_id':{'_id':'$_id', 'nodeID', '$nodes.node' },
'name':{'$last':'$name'},
'country':{'$last':'$country'},
'cdate':{'$last':'$cdate'},
'nodes':{'$last':'$nodes'}
}
},
{ $match : { nodes.ndr.position: 3 } }
]);
From here you can reassemble the aggregate results with a $group on the and do a projection. I'm not sure what your ultimate end result should be.