使用filter和limit连接objectid的数组字段 - MongoDB

Question

Does anyone know how to get the desired result below with some requirements? I can't think of much efficient way and hoping that experts can help me. :)

Thank you very much in advance.

---

Playlist Document

"_id" : "playlist1", "title" : "Sample Playlist", 
"items" : ["item3", "item2", "item1", "item5", "item4"] // These are ObjectId type

Items Collection

{ "_id" : "item1", "type" : "video", "playlist": "playlist1" }
{ "_id" : "item2", "type" : "music", "playlist": "playlist1" }
{ "_id" : "item3", "type" : "video", "playlist": "playlist1" }
{ "_id" : "item4", "type" : "music", "playlist": "playlist1" }
{ "_id" : "item5", "type" : "video", "playlist": "playlist1" }
{ "_id" : "item6", "type" : "video", "playlist": "playlist2" } // Different playlist

Requirements

• Limit to 2 with skip
• Only type of video
• Maintain order as in items array field in playlist document
• Efficient way as items can be huge array (like 1000+)

Desired Result

Page 1:

{ "_id" : "item3", "type" : "video", "playlist": "playlist1" } // Maintain order
{ "_id" : "item1", "type" : "video", "playlist": "playlist1" }

Page 2:

{ "_id" : "item5", "type" : "video", "playlist": "playlist1" }

Answer 1

This seems to work for me:

db.test.aggregate([
    {$match: {_id: "playlist1"}},
    {$unwind: {path: "$items", includeArrayIndex: 'item_index'}},
    {$limit: 2},
    {
        $lookup: {
            from: "items",
            localField: "items",
            foreignField: "_id",
            as: "items"
        }
    },
    {$sort: {item_index: 1}},
    {$replaceRoot: {newRoot: {$arrayElemAt: ["$items", 0]}}}
])

Answer 2

you can use $sort $skip $limit of mongodb to get the desired object

db.Items.aggregate({
            $match: {"type":"video"}
        }, {
            $sort: {
                KEY_TO_SORT: -1
            }
        }, {
            $skip: parseInt(skip)
        }, {
            $limit: parseInt(limit)
        },
        function (err, success) {
       // Error and success comes here
        });