查找后从嵌入的投影文档中删除字段答案

【问题标题】：Remove field from embedded projection document after lookup查找后从嵌入的投影文档中删除字段
【发布时间】：2019-05-31 11:56:52
【问题描述】：

我有以下 mongo 聚合查询：

    return db.collection('projects').aggregate([
  {
    $match: {
      agents: ObjectId(agent)
    }
  },
  {
    $lookup: {
      from: "agents",
      localField: "agents",
      foreignField: "_id",
      as: "agents"
    }
  },
  {
    $lookup: {
      from: "roles",
      localField: "roles",
      foreignField: "_id",
      as: "roles"
    }
  },
  {
    $lookup: {
      from: "agencies",
      localField: "agency",
      foreignField: "_id",
      as: "agency"
    }
  },
  {
    $lookup: {
      from: "resources",
      localField: "roles.applicants",
      foreignField: "_id",
      as: "roles.applicants"
    }
  }
])

它可以正常工作，嵌入正确的文档。但是，每个申请人都会显示“password_hash”字段。我想删除该字段。如果我尝试投影和设置roles.applicants.password_hash: 0，我实际上最终会得到整个申请人而没有密码哈希，但其余的roles 字段不再存在。所以我得到的东西看起来像：

roles: {
 applicants: {
  name: "Josh"
 }
}

应该是这样的

roles: {
 title: "Super Hero",
 applicants: {
  name: "Josh"
 }
}

【问题讨论】：

标签： mongodb aggregation projection

【解决方案1】：

我想通了。这是我的做法。首先，投影不是角色文档缺少字段的问题。这是查找。

我所做的是将角色查找更改为在嵌套文档上使用管道方法，然后对申请人进行另一个管道，首先匹配资源集合中的申请人，然后处理投影。看起来像这样。

 {
    $lookup: {
      from: "roles",
      let: { "roles": "$roles" },
      pipeline: [
        { $match: { $expr: { $in: [ "$_id", "$$roles" ] } } },
        {
          $lookup: {
            from: "resources",
            let: { "applicants": "$applicants" },
            pipeline: [
              { $match: { $expr: { $in: [ "$_id", "$$applicants" ] } } },
              {
                $project: {
                  first_name: 1
                }
              }
            ],
            as: "applicants"
          }
        }
      ],
      as: "roles"
    }
  }

整个聚合看起来像这样

return db.collection('projects').aggregate([
  {
    $match: {
      agents: ObjectId(agent)
    }
  },
  {
    $lookup: {
      from: "agents",
      localField: "agents",
      foreignField: "_id",
      as: "agents"
    }
  },
  {
    $lookup: {
      from: "agencies",
      localField: "agency",
      foreignField: "_id",
      as: "agency"
    }
  },      
  {
    $lookup: {
      from: "roles",
      let: { "roles": "$roles" },
      pipeline: [
        { $match: { $expr: { $in: [ "$_id", "$$roles" ] } } },
        {
          $lookup: {
            from: "resources",
            let: { "applicants": "$applicants" },
            pipeline: [
              { $match: { $expr: { $in: [ "$_id", "$$applicants" ] } } },
              {
                $project: {
                  first_name: 1
                }
              }
            ],
            as: "applicants"
          }
        }
      ],
      as: "roles"
    }
  }
])

【讨论】：