How can paging in nested array Mongodb

Question

i want to paging in structure. params are id = 558186caa6df1a2194422949 and Question is = 55818779a6df1a219442294d and get first 2 rows in Question datasource.

How do I resolve it?

Below is MongoDB document structure:

/* 1 */
{
    "_id" : ObjectId("558186caa6df1a2194422949"),
    "Questions" : [ 
        {
            "_id" : ObjectId("55818779a6df1a219442294d"),
            "DataSource" : [ 
                {
                    "_id" : ObjectId("55818796a6df1a2194422950"),
                    "Text" : "Q1-1",
                    "Value" : "Q1-1"
                }, 
                {
                    "_id" : ObjectId("5581879aa6df1a2194422951"),
                    "Text" : "Q1-2",
                    "Value" : "Q1-2"
                }, 
                {
                    "_id" : ObjectId("5581879ea6df1a2194422952"),
                    "Text" : "Q1-3",
                    "Value" : "Q1-3"
                }
            ]
        }, 
        {
            "_id" : ObjectId("55818774a6df1a219442294c"),
            "DataSource" : [ 
                {
                    "_id" : ObjectId("55818788a6df1a219442294e"),
                    "Text" : "Q2-1",
                    "Value" : "Q2-1"
                }, 
                {
                    "_id" : ObjectId("5581878fa6df1a219442294f"),
                    "Text" : "Q2-2",
                    "Value" : "Q2-2"
                }
            ]
        }
    ],
    "Name" : "test"
}

Buzz Moschetti · Accepted Answer · 2021-09-20 14:22:47Z

The challenge is that regular find() operators on arrays that yield a match in the array will return the whole array , so querying for Questions._Id = 55818779a6df1a219442294d will yield the whole Questions array including the second item 55818774a6df1a219442294c which you probably don't want to deal with.

You can use the aggregation pipeline and $unwind to "dig through" the arrays of arrays and then ultimately use $skip and $limit stages to achieve the pagination you seek, as follows:

    db.foo.aggregate([
    {$match: { "_id": ObjectId("558186caa6df1a2194422949") }}
    ,{$unwind: "$Questions"}
    ,{$match: { "Questions._id": ObjectId("55818779a6df1a219442294d") }}  
    ,{$unwind: "$Questions.DataSource"}
    ,{$skip: 1}
    ,{$limit: 2}
                      ]);

In general, one must be careful about multiple $unwind operators because it could result in an explosion of data but here, our first $match always results in a single document because _id is unique. The second $match also seems to cut down the input set significantly so the second $unwind is probably deal with order 100 or 1000 datasource items to skip and limit, not millions.

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