MongoDB 学习笔记之 Aggregation Pipeline实战实现inner join

Posted 2020-10-06 AK47Sonic

 Aggregation Pipeline实战实现inner join:

 

 leftT集合：

 comments集合：

 

现在我们要用aggregation实现inner join

db.comments.aggregate([{
$lookup:
{
from:"leftT",
localField:"timestamp",
foreignField:"timestamp",
as: "timestampCol"
}},
{$unwind: "$timestampCol"},
{$project: {"ratingEQ": {"$eq": ["$rating", "$timestampCol.rating"]}, "timestamp" : 1, "rating" : 1,  "timestampCol.timestamp": 1}},
{$match: {"ratingEQ":true}}
])

 

是不是很神奇？

步骤解析：

1. $lookup:实现timestamp的等值连接，相等的leftT结果被放在timestampCol集合中
2. $unwind:实现把leftT数组拉平，出现多条
3. $project:实现对第二个等值连接rating的判断，并放在ratingEQ字段中
4. $match:只取ratingEQ是true，说明第二个rating字段相等