仅当 mongoDB 中外来字段不为空时才查找

Question

我收集了文章和评论。评论可能有articleId（这是对文章的回答）或parentId（这是对另一条评论的回答）。只有2级，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、文章评论和评论答案。

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" } }
])

如果文章有一些评论，它就会起作用。但如果没有，在第一篇lookup（文章评论）文章之后看起来像这样（空数组就可以了）：

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

第二次之后lookup（评论的答案）：

{
    "_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，评论也有一些回复。我认为这是因为这些答案的 localFieldcomments._id是null和foreignFieldparentId也是null。有没有办法只在foreignField为时查找not null？

Answer 1

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"
  }}
])