如何限制在mongodb中返回的嵌套数组字段

Question

这是课程集：

{
  "_id": "RtPA6Cxs3fzJcGpgP",
  "Seasons": [
    {
      "title": "intro",
      "Episodes": [
        {
          "title": "what is c++?",
          "length": "12:52",
          "free_url": "free-episode-1.mp4"
        },
        {
          "title": "why c++?",
          "length": "05:20",
          "paid_url": "premium-episode-1.mp4"
        }
      ]
    },
    {
      "title": "first season",
      "Episodes": [
        {
          "title": "declare variables",
          "length": "12:35",
          "paid_url": "premium-episode-2.mp4"
        },
        {
          "title": "pointers",
          "length": "04:00",
          "free_url": "free-episode-2.mp4"
        }
      ]
    }
  ]
}

我正在尝试获取（除了 paid_urls 之外的所有内容）：

{
  "_id": "RtPA6Cxs3fzJcGpgP",
  "Seasons": [
    {
      "title": "intro",
      "Episodes": [
        {
          "title": "what is c++?",
          "length": "12:52",
          "free_url": "free-episode-1.mp4"
        },
        {
          "title": "why c++?",
          "length": "05:20"
        }
      ]
    },
    {
      "title": "first season",
      "Episodes": [
        {
          "title": "declare variables",
          "length": "12:35"
        },
        {
          "title": "pointers",
          "length": "04:00",
          "free_url": "free-episode-2.mp4"
        }
      ]
    }
  ]
}

尝试了这些查询： 1. 这适用于客户端（chrome 控制台），但不适用于“meteor mongo”：

db.courses.find({_id: "RtPA6Cxs3fzJcGpgP"}, {fields: {"Seasons.Episodes.paid_url": 0}}).fetch()

我在“meteor mongo”上遇到的错误：

"$err" : "无法规范化查询：BadValue 不支持的投影选项：字段：{ Seasons.$.Episodes.$.paid_url: 0.0 }"`

1. 也试过了：

   Courses.find({_id: this.params.id}, {fields: {"Seasons.$.Episodes.$.paid_url": 0}});
   

Answer 1

这是正确的查询：

db.courses.find({_id: "RtPA6Cxs3fzJcGpgP"}, {"Seasons.Episodes.paid_url": 0}})

不需要字段