仅当mongoDB中的外部字段不为空时才进行查找

我收集了一些文章和评论。评论可能有articleId，它是对文章的回答，或者parentId，它是对另一个评论的回答。只有两个级别，其他评论的答案不能有答案

// articles
{ "_id": 100, "text": "Article text" } 

// comments
{ "_id": 1, "text": "First text", "articleId": 100 },
{ "_id": 2: "text": "Second text", "articleId": 100 },
{ "_id": 3, "text": "Third text", "parentId": 2 }  

我想找到所有的文章，文章的评论和评论的答案

db.getCollection("articles").aggregate([
    { "$match": {} },

    // Lookup comments of article.
    { "$lookup": { "from": "comments", "localField": "_id", "foreignField": "parentId", as: "comments" } },
    { "$unwind": { "path": "$comments", "preserveNullAndEmptyArrays": true } },

    // Lookup answers to comments. There I need lookup only when foreignField is not null.
    { "$lookup": { "from": "comments", "localField": "comments._id", "foreignField": "parentId", "as": "comments.answers" } },
    { "$group": { "_id": "$_id", "comments": { "$push": "$comments" }, "text": { "first": "$text" } }
])

如果这篇文章有一些评论就行了。但如果不是，在第一次查找文章后，文章的注释看起来这个空数组是可以的：

{ "_id": 100, "text": "Article text", "comments": [] }

第二次查找后，回复评论：

{
    "_id": 100,
    "text": "Article text",
    "comments": [{
        "answers": [
            { "_id": 1, "text": "First text", "articleId": 100 },
            { "_id": 2: "text": "Second text", "articleId": 100 }
        ]
    }]
}

---

即使没有评论，也有一些评论的答案。我认为这是因为localField注释.\u id为null，这些答案的foreignField parentId也为null。只有当foreignField不为空时，才有方法查找吗

您可以在mongodb 3.6及更高版本中使用以下聚合

Article.aggregate([
  { "$lookup": {
    "from": "comments",
    "let": { "articleId": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$articleId", "$$articleId" ] } } },
      { "$lookup": {
        "from": "comments",
        "let": { "commentId": "$_id" },
        "pipeline": [
          { "$match": { "$expr": { "$eq": [ "$parentId", "$$commentId" ] } } }
        ],
        "as": "answers"
      }}
    ],
    "as": "comments"
  }}
])