嵌套在数组中的数组的MongoDB投影

Question

（请注意：尽管this question的标题相似，但实际上与过滤相比，与投影有关的更多；我无法理解它如何适用于我的用例。
如果相关，那么我先向您道歉:)）

给出以下文档结构：

{
    "id" : "1",
    "groups" : [
        {
            "name" : "group1.1",
            "contacts" : [
                { "localId" : "c1.1", "address" : "some address 1" },
                { "localId" : "c1.2", "address" : "some address 2" },
                { "localId" : "c1.3", "address" : "some address 3" }
            ]
        },
        {
            "name" : "group1.2",
            "contacts" : [
                { "localId" : "c1.1", "address" : "some address 1" },
                { "localId" : "c1.4", "address" : "some address 3" }
            ]
        }
    ]
},
{
    "id" : "2",
    "groups" : [
        {
            "name" : "group2.1",
            "contacts" : [
                { "localId" : "c2.1", "address" : "some address 1" },
                { "localId" : "c2.2", "address" : "some address 2" },
                { "localId" : "c2.3", "address" : "some address 3" }
            ]
        },
        {
            "name" : "group2.2",
            "contacts" : [
                { "localId" : "c2.1", "address" : "some address 1" },
                { "localId" : "c2.3", "address" : "some address 3" }
            ]
        }
    ]
}

如何创建投影以仅返回{em> contacts数组中的groups数组以及这些数组中的 all 元素？ br /> 我仍然想保持原始结构（即数组不应展平）

即要求的输出为：

{
    "groups" : [
      {
        "contacts" : [
          { "localId" : "c1.1", "address" : "some address 1" },
          { "localId" : "c1.2", "address" : "some address 2" },
          { "localId" : "c1.3", "address" : "some address 3" }
        ]
      },
      {
        "contacts" : [
          { "localId" : "c1.1", "address" : "some address 1" },
          { "localId" : "c1.4", "address" : "some address 3" }
        ]
      }
    ]
},
{
    "groups" : [
      {
        "contacts" : [
          { "localId" : "c2.1", "address" : "some address 1" },
          { "localId" : "c2.2", "address" : "some address 2" },
          { "localId" : "c2.3", "address" : "some address 3" }
        ]
      },
      {
        "contacts" : [
          { "localId" : "c2.1", "address" : "some address 1" },
          { "localId" : "c2.3", "address" : "some address 3" }
        ]
      }
    ]
}

对于非数组属性，类似{'groups.name': 1}的东西可以正常工作。

我也尝试过类似db.collection.findOne({}, { 'groups.contacts': 1 })的方法来尝试获取联系人，但这根本没有返回contacts字段。.