Mongodb Aggregation : Project an array without the last element

Question

Using aggregation pipeline in spring Data, I have documents with nested arrays, and I want to project an array except the last item of it. for example for each document like :

{
    "_id" : ObjectId("59ce411c2708c97154d13150"),
    "doc1" : [ 
        {
            "nodeValue" : "AAA"
        }, 
        {
            "nodeValue" : "BBB"
        }, 
        {
            "nodeValue" : "CCC"
        }, 
        {
            "nodeValue" : "DDD"
        }
    ],
    "field2" : 20170102,
    "field3" : 4,
}

I want as result :

{
    "_id" : ObjectId("59ce411c2708c97154d13150"),
    "doc1" : [ 
        {
            "nodeValue" : "AAA"
        }, 
        {
            "nodeValue" : "BBB"
        }, 
        {
            "nodeValue" : "CCC"
        }, 
        {
            "nodeValue" : "DDD"
        }
    ],
    "doc1_without_last" : [ 
        {
            "nodeValue" : "AAA"
        }, 
        {
            "nodeValue" : "BBB"
        }, 
        {
            "nodeValue" : "CCC"
        }
    ],
    "field2" : 20170102,
    "field3" : 4,
}

I tried something like this, but I didn't find a operator that can $pop the array and remove the last item from it.

Aggregation agg = Aggregation.newAggregation(
                      Aggregation.project()
                        .andInclude("doc1","field2","field3"),
                       Aggregation.project().and(ArrayOperators.arrayOf("doc1")..).as("doc1_without_last")

                       new OutOperation("newCollection")
                ).withOptions(Aggregation.newAggregationOptions().allowDiskUse(true).build());

Thank you for your help

the $slice just controls the number of items of an array to project. $slice(-n) project just the last n elements and $slice(n) for the first n elements. But how to project all datas of the array except the last one. — Dr. Mza
– Dr. Mza, Commented Nov 22, 2017 at 13:35
Sorry, forgot to mention that there is also $size. All but the last is $slice($size - 1). Just in case docs.mongodb.com/manual/reference/operator/aggregation-array has all of them. — Alex Blex
– Alex Blex, Commented Nov 22, 2017 at 13:40
there is no way to do it by spring data in pipeline $slice in java get only a numeric integer parameter. I had the idea to reverse the array to get simply the last one as the first like this .and(ArrayOperators.arrayOf(doc1).reverse()).as("Reversed_doc1) But how to exclude the Reversed_doc1.0.nodeValue from the projection — Dr. Mza
– Dr. Mza, Commented Nov 22, 2017 at 14:11
I am pretty sure you can build a custom $slice with serverside calculated $size - 1 , see stackoverflow.com/questions/39393672/… for inspiration — Alex Blex
– Alex Blex, Commented Nov 22, 2017 at 15:05

Alex Blex · Accepted Answer · 2017-11-23 09:36:52Z

3

The shell query can be like this:

db.collection.aggregate([
    { $project: {
        doc1: 1,
        doc1_without_last: { $slice: [ 
            "$doc1", 
            { $subtract: [ { $size: "$doc1" }, 1 ] } 
        ] }
    } }
])

Docs:

If there are any limitations in spring that don't let you build such query, you may employ $let to use query variables:

db.collection.aggregate([
    { $project: {
        doc1: 1,
        doc1_without_last: { $let: {
            vars: { size: { $subtract: [ { $size: "$doc1" }, 1 ] } },
            in: { $slice: [ "$letters", "$$size"] }
        } }    
    } }
])

answered Nov 23, 2017 at 9:36

Alex Blex

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1 Comment

Dr. Mza Over a year ago

Thank you Alex for your help, I tried something like that :

.and(VariableOperators                               .define(VariableOperators.Let.ExpressionVariable.newVariable("SSIZE")                                  .forExpression(AggregationFunctionExpressions.SUBTRACT.of(Fields.field("pathLengthInInteraction"),1)))                            .andApply(ArrayOperators.Slice.sliceArrayOf("doc").offset(?))).as("WITHOUT_LAST")

"pathLengthInInteraction is a the size already projected, the offset function support juste integer value, I can"t recuperate the SSIZE value

Dr. Mza · Accepted Answer · 2017-11-27 00:24:41Z

2

Finally, After Blowing my mind, the trick was simply that :

Aggregation agg = Aggregation.newAggregation(
Aggregation.project()
.andInclude("doc1","field1","field2")
.andExpression("slice(doc1, size(doc1)-1)").as("doc1_without_last"),
new OutOperation("newCollection")
                ).withOptions(Aggregation.newAggregationOptions().allowDiskUse(true).build());

the andExpression in projections serve to include some mongo operators in a String format.

answered Nov 27, 2017 at 0:24

Dr. Mza

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