MongoDB 使用来自另一个 collection 的另一个字段更新字段

Question

我有两个 collection：书籍和类别。类别 collections 表示树结构，使用 parents 和 children.

使它们成为嵌套类别

这本书可以有多个类别，并将它们存储在一个数组中。

示例：图书有类别，我想停用它并将其设置为 parent 类别。

这就是类别 collection 的填充方式。

db.categories.insertMany([
  {
    _id: "Space Opera",
    ancestors: ["Science Fiction", "Fiction", "Science Fiction & Fantasy"],
    parent: ["Science Fiction"],
  },
  {
    _id: "Dystopian",
    ancestors: ["Science Fiction", "Fiction", "Science Fiction & Fantasy"],
    parent: ["Science Fiction"],
  },
  {
    _id: "Cyberpunk",
    ancestors: ["Science Fiction", "Fiction", "Science Fiction & Fantasy"],
    parent: ["Science Fiction"],
  },
  {
    _id: "Science Fiction",
    ancestors: ["Fiction", "Science Fiction & Fantasy"],
    parent: ["Fiction", "Science Fiction & Fantasy"],
  },
  {
    _id: "Fantasy",
    ancestors: ["Science Fiction & Fantasy"],
    parent: ["Science Fiction & Fantasy"],
  },
  {
    _id: "Science Fiction & Fantasy",
    ancestors: [],
    parent: [],
  },
  {
    _id: "Fiction",
    ancestors: [],
    parent: [],
  },
]);

还有，如何查询这个，只得到值“Science Fiction”（注意是存储在数组中）？

db.categories.find({_id : "Space Opera"}, {_id : 0, parent : 1})[0].parent // Did not work


db.categories.find({_id : "Space Opera"}, {_id : 0, parent : 1}) // find parent

// result 

[
  {
    "parent": [
      "Science Fiction"
    ]
  }
]

db.books.update(
    {title : "Book1"}, 
    {$set : {category : [**PARENT CATEGORY**]}}
)

我相信我可以在 books.update()

中使用上面的代码

我可以将它存储在一个单独的变量中，但在 vscode 中它给了我未定义的。内部查询并没有像之前那样给我正确的值，但我想你明白了。

db.books.update(
    {title : "Book1"}, 
    {$set : {category : [db.categories.find({_id : "Space Opera"}, {_id : 0, parent : 1})]}}
)

Answer 1

您可以通过此聚合管道获得的父项：

db.categories.aggregate([
   { $match: { _id: "Space Opera" } },
   { $project: { _id: 0, parent: { $first: "$parent" } } }
])

甚至

db.categories.aggregate([
   { $match: { _id: "Space Opera" } },
   { $project: { _id: 0, parent: { $first: "$parent" } } }
]).toArray().shift().parent

要加入集合，您必须使用 $lookup 运算符。请记住，像 MongoDB 这样的 NoSQL 数据库并未针对 join/lookup 进行优化。在现实生活中你应该寻找更好的设计。

db.books.aggregate([
   { $match: { title: "Book1" } },
   {
      $lookup:
         {
            from: "categories",
            pipeline: [
               { $match: { _id: "Space Opera" } },
               { $project: { _id: 0, parent: { $first: "$parent" } } }
            ],
            as: "category"
         }
   },
   { $set: { category: { $first: "$category.parent" } } }
])

如果您想更新现有集合，则必须为其创建一个循环：

db.books.aggregate([...]).forEach(function (doc) {
   db.books.updateOne({ _id: doc._id }, { $set: { category: doc.category } });
})

MongoDB 使用来自另一个 collection 的另一个字段更新字段

MongoDB update field with another field from another collection

collections

mongodb

nosql