MongoDB：子文档中仅存在于父文档中的返回字段

MongoDB:子文档中仅存在于父文档中的返回字段？

要将父文档中存在的字段返回到子文档，您可以遵循以下代码.它使用addFields将所需字段parent_desc添加到子文档中，如下面的输出所示:

db.getCollection("newcoll").aggregate([
  {
    $lookup: {
      from: "newcoll",
      localField: "parent_id",
      foreignField: "parent_id",
      as: "children"
    }
  },
  { $unwind: "$children" },
  { $replaceWith: "$children" },
  {
    $lookup: {
      from: "newcoll",
      localField: "parent_id",
      foreignField: "parent_id",
      as: "parent"
    }
  },
  { $unwind: "$parent" },
  {
    $addFields: {
      parent_desc: "$parent.parent_desc"
    }
  },
  {
    $group: {
      _id: "$_id",
      id: { $first: "$id" },
      parent_id: { $first: "$parent_id" },
      active: { $first: "$active" },
      parent_desc: { $first: "$parent_desc" }
    }
  },
  {
    $project: {
      _id: 1,
      id: 1,
      parent_id: 1,
      active: 1,
      parent_desc: 1
    }
  }
]).pretty()

Output:

{
        "_id" : ObjectId("662616f0af88e119ac2f49c3"),
        "id" : "doc_123_2",
        "parent_id" : "123",
        "active" : true,
        "parent_desc" : "sample parent desc"
}
{
        "_id" : ObjectId("66261744af88e119ac2f49c4"),
        "id" : "doc_123",
        "parent_id" : "123",
        "active" : true,
        "parent_desc" : "sample parent desc"
}