Avoid Unbounded Arrays避免无边界数组

Subset Pattern子集模式

Subsetting data is best for when you need quick access to data that is not frequently updated. Using the subset pattern, you can embed three of the reviews in the book document to return all required information in a single operation. 子集数据最适合您需要快速访问不经常更新的数据。使用子集模式，您可以在图书文档中嵌入三条评论，在一次操作中返回所有必需的信息。The other reviews are stored in a separate reviews collection. This schema design pattern provides the following benefits:其他评论存储在单独的reviews集合中。此模式设计模式具有以下优点：

• Eliminate the unbounded array消除无界数组
• Control the document size控制文档大小
• Avoid use of multiple queries避免使用多个查询

The books collection:books集合：

db.books.insertOne( [
   {
      title: "Harry Potter",
      author: "J.K. Rowling",
      publisher: "Scholastic",
      reviews: [
        {
           reviewer: "Alice",
           review: "Great book!",
           rating: 5
        },
        {
           reviewer: "Charlie",
           review: "Didn't like it.",
           rating: 1
        },
        {
           reviewer: "Bob",
           review: "Not bad, but could be better.",
           rating: 3
        }
      ],
   }
] )

The reviews collection:reviews（评论）集合：

db.reviews.insertMany( [
   {
      reviewer: "Jason",
      review: "Did not enjoy!",
      rating: 1
   },
   {
      reviewer: "Pam",
      review: "Favorite book!",
      rating: 5
   },
   {
      reviewer: "Bob",
      review: "Not bad, but could be better.",
      rating: 3
   }
] )

This approach duplicates data which causes updates to be expensive. This approach is best if reviews are not frequently updated.这种方法复制数据，导致更新成本高昂。如果评论不经常更新，这种方法是最好的。

Reference Data参考数据

Referencing data is best for when you need to manage large or frequently updated datasets without inflating document sizes.当您需要管理大型或频繁更新的数据集而不扩大文档大小时，引用数据是最好的选择。

To reference data, store reviews in a separate collection and add a review_id field to the documents in the reviews collection. Use the review_id field to reference the reviews in the books collection.要引用数据，请将评论存储在单独的集合中，并将review_id字段添加到评论集合中的文档中。使用review_id字段引用books集合中的评论。

This approach solves the problem of the unbounded array, but it introduces latency because you need to query the reviews collection to retrieve review information for the books collection. Depending on your use case, this additional latency may be an acceptable trade-off to avoid the issues caused by unbounded arrays.这种方法解决了无界数组的问题，但它引入了延迟，因为您需要查询reviews集合来检索books集合的评论信息。根据使用情况，这种额外的延迟可能是可以接受的权衡，以避免无界数组造成的问题。

The books collection:books集合：

db.books.insertMany( [
   {
      title: "Harry Potter",
      author: "J.K. Rowling",
      publisher: "Scholastic",
      reviews: ["review1", "review2", "review3"]
   },
   {
      title: "Pride and Prejudice",
      author: "Jane Austen",
      publisher: "Penguin",
      reviews: ["review4", "review5"]
   }
] )

The reviews collection:reviews集合：

db.reviews.insertMany( [
   {
      review_id: "review1",
      reviewer: "Jason",
      review: "Did not enjoy!",
      rating: 1
   },
   {
      review_id: "review2",
      reviewer: "Pam",
      review: "Favorite book!",
      rating: 5
   },
   {
      review_id: "review3",
      reviewer: "Bob",
      review: "Not bad, but could be better.",
      rating: 3
   },
   {
      review_id: "review4",
      reviewer: "Tina",
      review: "Amazing!",
      rating: 5
   },
   {
      review_id: "review5",
      reviewer: "Jacob",
      review: "A little overrated",
      rating: 4,
   }
] )

db.books.aggregate( [
   {
      $lookup: {
         from: "reviews",
         localField: "reviews",
         foreignField: "review_id",
         as: "reviewDetails"
      }
   }
] )

[
   {
      _id: ObjectId('665de81eeda086b5e22dbcc9'),
      title: 'Harry Potter',
      author: 'J.K. Rowling',
      publisher: 'Scholastic',
      reviews: [ 'review1', 'review2', 'review3' ],
      reviewDetails: [
      {
         _id: ObjectId('665de82beda086b5e22dbccb'),
         review_id: 'review1',
         reviewer: 'Jason',
         review: 'Did not enjoy!',
         rating: 1
      },
      {
         _id: ObjectId('665de82beda086b5e22dbccc'),
         review_id: 'review2',
         reviewer: 'Pam',
         review: 'Favorite book!',
         rating: 5
      },
      {
         _id: ObjectId('665de82beda086b5e22dbccd'),
         review_id: 'review3',
         reviewer: 'Bob',
         review: 'Not bad, but could be better.',
         rating: 3
      } ]
   },
   {
      _id: ObjectId('665de81eeda086b5e22dbcca'),
      title: 'Pride and Prejudice',
      author: 'Jane Austen',
      publisher: 'Penguin',
      reviews: [ 'review4', 'review5' ],
      reviewDetails: [
      {
         _id: ObjectId('665de82beda086b5e22dbcce'),
         review_id: 'review4',
         reviewer: 'Tina',
         review: 'Amazing!',
         rating: 5
      },
      {
         _id: ObjectId('665de82beda086b5e22dbccf'),
         review_id: 'review5',
         reviewer: 'Jacob',
         review: 'A little overrated',
         rating: 4
      } ]
   }
]