Model One-to-Many Relationships with Embedded Documents与嵌入文档建立一对多关系模型

Overview概述

This page describes a data model that uses embedded documents to describe a one-to-many relationship between connected data. 本页描述了一个数据模型，该模型使用嵌入式文档来描述连接数据之间的一对多关系。Embedding connected data in a single document can reduce the number of read operations required to obtain data. 在单个文档中嵌入连接数据可以减少获取数据所需的读取操作数。In general, you should structure your schema so your application receives all of its required information in a single read operation.一般来说，您应该构造模式，以便应用程序在一次读取操作中接收所有所需信息。

Embedded Document Pattern嵌入式文档模式

Consider the following example that maps patron and multiple address relationships. 考虑以下映射用户和多地址关系的示例。The example illustrates the advantage of embedding over referencing if you need to view many data entities in context of another. 如果需要在另一个数据实体的上下文中查看多个数据实体，该示例说明了嵌入优于引用的优点。In this one-to-many relationship between patron and address data, the patron has multiple address entities.在patron和address数据之间的这种一对多关系中，patron具有多个address实体。

In the normalized data model, the address documents contain a reference to the patron document.在规范化数据模型中，address文档包含对patron文档的引用。

// patron document
{
   _id: "joe",
   name: "Joe Bookreader"
}
// address documents
{
   patron_id: "joe", // reference to patron document
   street: "123 Fake Street",
   city: "Faketon",
   state: "MA",
   zip: "12345"
}
{
   patron_id: "joe",
   street: "1 Some Other Street",
   city: "Boston",
   state: "MA",
   zip: "12345"
}

If your application frequently retrieves the address data with the name information, then your application needs to issue multiple queries to resolve the references. 如果应用程序经常使用name信息检索address数据，则应用程序需要发出多个查询以解析引用。A more optimal schema would be to embed the address data entities in the patron data, as in the following document:更理想的方案是将address数据实体嵌入到patron数据中，如以下文档所示：

{
   "_id": "joe",
   "name": "Joe Bookreader",
   "addresses": [
                {
                  "street": "123 Fake Street",
                  "city": "Faketon",
                  "state": "MA",
                  "zip": "12345"
                },
                {
                  "street": "1 Some Other Street",
                  "city": "Boston",
                  "state": "MA",
                  "zip": "12345"
                }
              ]
 }

With the embedded data model, your application can retrieve the complete patron information with one query.使用嵌入式数据模型，您的应用程序可以通过一次查询检索完整的用户信息。

Subset Pattern子集模式

A potential problem with the embedded document pattern is that it can lead to large documents, especially if the embedded field is unbounded. 嵌入式文档模式的一个潜在问题是，它可能导致大文档，特别是如果嵌入式字段是无界的。In this case, you can use the subset pattern to only access data which is required by the application, instead of the entire set of embedded data.在这种情况下，您可以使用子集模式仅访问应用程序所需的数据，而不是整个嵌入式数据集。

Consider an e-commerce site that has a list of reviews for a product:考虑一个电子商务网站，它有一个产品评论列表：

{
  "_id": 1,
  "name": "Super Widget",
  "description": "This is the most useful item in your toolbox.",
  "price": { "value": NumberDecimal("119.99"), "currency": "USD" },
  "reviews": [
    {
      "review_id": 786,
      "review_author": "Kristina",
      "review_text": "This is indeed an amazing widget.",
      "published_date": ISODate("2019-02-18")
    },
    {
      "review_id": 785,
      "review_author": "Trina",
      "review_text": "Nice product. Slow shipping.",
      "published_date": ISODate("2019-02-17")
    },
    ...
    {
      "review_id": 1,
      "review_author": "Hans",
      "review_text": "Meh, it's okay.",
      "published_date": ISODate("2017-12-06")
    }
  ]
}

The reviews are sorted in reverse chronological order. 这些评论按相反的时间顺序排序。When a user visits a product page, the application loads the ten most recent reviews.当用户访问产品页面时，应用程序将加载十个最新的评论。

Instead of storing all of the reviews with the product, you can split the collection into two collections:您可以将集合拆分为两个集合，而不是将所有评论与产品一起存储：

• The product collection stores information on each product, including the product's ten most recent reviews:product集合存储每个产品的信息，包括产品的十个最新评论：

  {
    "_id": 1,
    "name": "Super Widget",
    "description": "This is the most useful item in your toolbox.",
    "price": { "value": NumberDecimal("119.99"), "currency": "USD" },
    "reviews": [
      {
        "review_id": 786,
        "review_author": "Kristina",
        "review_text": "This is indeed an amazing widget.",
        "published_date": ISODate("2019-02-18")
      }
      ...
      {
        "review_id": 777,
        "review_author": "Pablo",
        "review_text": "Amazing!",
        "published_date": ISODate("2019-02-16")
      }
    ]
  }

• The review collection stores all reviews. review集合存储所有评论。Each review contains a reference to the product for which it was written.每一篇评论都包含对其撰写的产品的引用。

  {
    "review_id": 786,
    "product_id": 1,
    "review_author": "Kristina",
    "review_text": "This is indeed an amazing widget.",
    "published_date": ISODate("2019-02-18")
  }
  {
    "review_id": 785,
    "product_id": 1,
    "review_author": "Trina",
    "review_text": "Nice product. Slow shipping.",
    "published_date": ISODate("2019-02-17")
  }
  ...
  {
    "review_id": 1,
    "product_id": 1,
    "review_author": "Hans",
    "review_text": "Meh, it's okay.",
    "published_date": ISODate("2017-12-06")
  }

By storing the ten most recent reviews in the product collection, only the required subset of the overall data is returned in the call to the product collection. 通过在product集合中存储十个最近的评论，在对product集合的调用中只返回总体数据的所需子集。If a user wants to see additional reviews, the application makes a call to the review collection.如果用户希望查看其他评论，应用程序将调用review集合。