MongoDB中的条件$查找? [英] Conditional $lookup in MongoDB?

问题描述

我在 MongoDB 3.6 中有两个集合:

I have two collections in MongoDB 3.6:

users: [
  {name: "John", allowedRoles: [1, 2, 3]},
  {name: "Charles", allowedRoles: [1]},
  {name: "Sarah", isAdmin: true}
]

roles: [
  {_id: 1, name: "Foo", description: "This role allows to foo the blargs"},
  {_id: 2, name: "Bar", description: "..."},
  {_id: 3, name: "Doh", descripcion: "..."}
]

我对 MongoDB 很陌生；我刚刚想出了如何使用 $lookup 聚合阶段:

I'm very new to MongoDB; I just figured out how to query an user and join all the data from his roles, using the $lookup aggregation stage:

db.users.aggregate([{
  "$match": { "name": "John" }          // Or without this $match part, to get all users
},{                                     //
  "$lookup": {
    "from": "roles",
    "localField": "allowedRoles",
    "foreignField": "_id",
    "as": "roles"
  }
}]);

它适用于我的普通用户，他们有一组允许的角色 ID.我也有管理员用户，他们可以访问所有现有角色，但没有 allowedRoles 数组(维护起来会很麻烦，因为会频繁创建新角色).因此，我没有指定连接字段，而是使用空管道执行 $lookup，以获取两个集合的笛卡尔积:

It works for my regular users, who have an array of allowed roles IDs. I have also administrator users, which can access all existing roles, but don't have the allowedRoles array (it would be a burden to maintain, since new roles will be created frequently). So, instead of specifying the join fields, I do a $lookup with an empty pipeline, to get the cartesian product of both collections:

db.users.aggregate([{
  "$match": { "name": "Sarah" }
},{
  "$lookup": {
    "from": "roles",
    "pipeline": [],
    "as": "roles"
  }
}]);

有什么方法可以同时使用一个查询吗?使用条件表达式之类的?

Is there any way to have both, with a single query? With a conditional expression or something?

在 SQL 数据库中，我会简单地将条件包含在连接中:

In an SQL database, I would simply include the condition in the join:

select users.*, roles.*
from users
left join users_x_roles inter on users.id = inter.user_id
left join roles on inter.role_id = roles.id or users.is_admin = 1;
--                                          ^^^^^^^^^^^^^^^^^^^^^

推荐答案

你可以使用下面的聚合

$expr 允许您在其中使用聚合运算符.因此您可以轻松使用 $cond 聚合具有 allowedRoles 和没有

$expr allows you use aggregation operator inside it. So you can easily use $cond aggregation for the users who has allowedRoles and who hasn't

db.users.aggregate([
  { "$match": { "name": "Charles" }},
  { "$lookup": {
    "from": "roles",
    "let": { "ar": "$allowedRoles" },
    "pipeline": [
      { "$match": {
        "$expr": {
          "$cond": [
            { "$eq": [{ "$type": "$$ar" }, "missing"] },
            {},
            { "$in": ["$_id", "$$ar"] }
          ]
        }
      }}
    ],
    "as": "roles"
  }}
])

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