仅当MongoDB中的外部字段不为空时查找

我收藏了一些文章和评论。评论可能有 articleId (这是对文章的答复)或 parentId (这是对另一条评论的回应)。只有两个层次,回答另一个评论不能有答案。

// 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 }

我想找到所有文章 , 评论物品和答案评论。

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,它就会起作用。但如果不是,在第一次 lookup (文章注释)文章如下(空数组可以):

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

在第二秒之后 查找 (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,评论也有一些答案。我想是因为localfield comments._id 是 null 和外域 帕伦特 这些答案中也有 无效的 . 只有当foreignfield是 not null ?

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" }} ])