embedding 和 referencing 之间有什么区别？

Question

Accepted Answer

**Embedding** 在一个文档*内*存储相关数据（嵌套）；**Referencing** 在*单独的*文档中存储数据，通过 id 链接。这是 MongoDB 中的核心建模决策，在读取性能、数据大小和更新复杂性方面有不同的权衡。

## Embedding — 嵌套在文档内

```js
{
  _id: ObjectId("..."),
  title: "My Post",
  author: { name: "Ann", email: "ann@x.com" },   // EMBEDDED author
  comments: [                                      // EMBEDDED comments
    { user: "Bob", text: "Great!" },
    { user: "Carol", text: "Thanks" }
  ]
}
// ✓ ONE query gets the post + author + comments (fast, atomic)
// ✗ if author info changes, must update it in every post embedding it (duplication)
// ✗ document grows with comments → could hit the 16MB limit if unbounded
```

## Referencing — 通过 id 链接的单独文档

```js
// posts collection
{ _id: ObjectId("p1"), title: "My Post", authorId: ObjectId("u1") }
// users collection
{ _id: ObjectId("u1"), name: "Ann", email: "ann@x.com" }
// comments collection
{ _id: ObjectId("c1"), postId: ObjectId("p1"), user: "Bob", text: "Great!" }

// ✓ author/comments stored ONCE, updated in one place; no document-size issues
// ✓ each entity managed independently
// ✗ need multiple queries or $lookup to combine (slower reads for related data)
```

## 权衡对比

```text
                  Embedding                    Referencing
Reads             ✓ fast (one query)           ✗ multiple queries / $lookup
Data duplication  ✗ duplicated (update many)   ✓ stored once
Document size     ✗ grows (16MB limit risk)    ✓ no size concern
Updates           ✗ update in every copy       ✓ update in one place
Atomicity         ✓ atomic (single document)   ✗ across documents
Best for          contains, accessed-together  shared, large, independent, many
```

## 混合方法（常见）

```js
// embed a SUBSET for fast reads, reference for the full data
{ title: "Post", author: { _id: ObjectId("u1"), name: "Ann" } }  // enough for display
// → reference (author._id) for full details when needed; embed name for the common read
```

## 为什么这很重要

理解 embedding 与 referencing 是 **MongoDB 数据建模的核心技能**，对于设计高效的 MongoDB 应用程序至关重要。

这个决策——在文档内存储相关数据（embedding）还是在单独的链接文档中（referencing）——直接决定了应用程序的读取性能、数据一致性、文档大小和更新复杂性，使其成为最关键的 MongoDB 设计选择。

理解**权衡**是关键：**embedding** 提供快速的单查询读取相关数据和原子更新（适合