Question

我在Mongo DB中有3个模型。

User 

Posts - Linked to userid

Comments - Linked to userid

我需要获取以下几列作为最终结果。

Username  |  UserEmail  |  TotalPostsbyUser  |  TotalCommentsbyUser

现在要实现此目的，我正在使用多个异步循环。无论如何，我可以在单个查询中获取所需的结果吗？类似于MySQL中的联接吗？

Answer 1

使用应使用Aggregation 但这可能不是执行此操作的最佳方法。如果您需要多次查询，则应将这些号码缓存在其他地方。

db.users.aggregate([
{
    $match: {
        username: "USERNAME" // or any other condition here
    }
},
{
    $lookup: {
       from: "posts",
       let: { "userId": "$_id" }, 
       pipeline: [ 
           { $match: { $expr: { $eq: ["$owner", "$$userId"] }}}, 
           { $project: {_id: 1}} ] // I assume that the foreign key is "owner"
       as: "posts"
    }
},
{
    $lookup: {
        from: "comments",
        let: { "userId": "$_id" }, 
        pipeline: [ 
           { $match: { $expr: { $eq: ["$owner", "$$userId"] }}}, 
           { $project: {_id: 1}} ] // I assume that the foreign key is "owner"
        as: "comments"
    }
},
{
 $project: {
   Username: "$username",
   UserEmail: "$userEmail",
   TotalPostsbyUser: { $size: "$posts"},
   TotalCommentsbyUser: { $size: "$comments" }
 }
}])

Answer 2

除了在项目阶段进行计数之外，您无需加入集合，而只需在查找阶段进行计数，否则不适用于大型数据集：

db.users.aggregate([
{
    $match: {
        username: "USERNAME" // or any other condition here
    }
},
{
    $lookup: {
       from: "posts",
       let: { username: "$username"},
       pipeline: [
        {$match : {"username" : "$$username"}},
        {$group : {_id : "$username", "post_count" : {"$sum : 1"}}}
       ],
       as: "posts"
    }
},
{
    $lookup: {
        from: "comments",
        pipeline: [
          {$match : {"username" : "$$username"}},
          {$group : {_id : "$username", "comment_count" : {"$sum : 1"}}}
        ], 
        as: "comments"
    }
} 
])

卡在猫鼬中一个复杂的查询

2 个答案: