聚合流水线操作（aggregation pipline）

Stages

Operators

Attenttion

aggregation pipline 的 stage 顺序非常的重要，对聚合操作的性能有非常大的影响。

Example

// Requires official MongoShell 3.6+
use revdol;
db.getCollection("forums").aggregate(
    [
        { 
            "$lookup" : {
                "localField" : "user_id", 
                "from" : "users", 
                "foreignField" : "id", 
                "as" : "user_info"
            }
        }, 
        { 
            "$addFields" : {
                "user_info" : {
                    "$arrayElemAt" : [
                        "$user_info", 
                        0.0
                    ]
                }, 
                "forum_picture.images" : {
                    "$ifNull" : [
                        "$images", 
                        "$forum_picture.images"
                    ]
                }, 
                "forum_picture.thumbs" : {
                    "$ifNull" : [
                        "$thumbs", 
                        "$forum_picture.thumbs"
                    ]
                }
            }
        }, 
        { 
            "$project" : {
                "_id" : 0.0, 
                "forum_id" : "$id", 
                "idol_id" : 1.0, 
                "user_id" : 1.0, 
                "title" : 1.0, 
                "content" : 1.0, 
                "images" : 1.0, 
                "nickname" : "$user_info.nickname", 
                "forum_picture" : 1.0
            }
        }
    ], 
    { 
        "allowDiskUse" : false
    }
);