MongoDB Academy · 课时

包含式与排除式投影

您将选择或隐藏查询结果中的特定字段，并理解为什么不能在同一个投影中混用包含和排除。

包含式与排除式投影 是 CoddyKit 上的免费 MongoDB Academy 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 MongoDB Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 MongoDB Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What Is a Projection?

A projection is the second argument to find() or findOne() that tells MongoDB which fields to include or exclude from the result. Instead of returning the entire document, you specify exactly the fields you need. This reduces bandwidth, decreases memory usage in your application, and keeps API responses lean. Projections are one of the simplest and most impactful query optimisations available.

Inclusion Mode: Specify What to Return

In inclusion mode, you list the fields you want to receive and set their value to 1. MongoDB returns only those fields plus _id (which is included by default). This is the most common form of projection because it explicitly declares the fields your query depends on, making the intent clear to anyone reading the code.

// Return only name and email — everything else is excluded
const user = await db.collection('users').findOne(
  { email: 'alice@example.com' },
  { projection: { name: 1, email: 1 } }
);
// Result: { _id: ObjectId('...'), name: 'Alice', email: 'alice@example.com' }

Excluding _id From Results

The _id field is always included in inclusion projections unless you explicitly exclude it. Set _id: 0 alongside inclusion fields to suppress it. This is useful when building API responses that use a different identifier field, or when you want to avoid leaking internal MongoDB IDs to clients.

// Include name and email but suppress _id
const user = await db.collection('users').findOne(
  { email: 'alice@example.com' },
  { projection: { _id: 0, name: 1, email: 1 } }
);
// Result: { name: 'Alice', email: 'alice@example.com' }
// _id is not present

Exclusion Mode: Specify What to Omit

In exclusion mode, you set fields to 0 to remove them from the result. Every other field is returned. This is useful when you want almost all fields but need to suppress a few sensitive or large ones—for example, hiding a passwordHash field from query results in a general-purpose user query.

// Return all user fields EXCEPT passwordHash and internalNotes
const user = await db.collection('users').findOne(
  { email: 'alice@example.com' },
  { projection: { passwordHash: 0, internalNotes: 0 } }
);
// Result: everything except passwordHash and internalNotes

Cannot Mix Inclusion and Exclusion

MongoDB does not allow mixing inclusion (1) and exclusion (0) fields in a single projection, except for the _id field. Attempting to do so results in a Projection cannot have a mix of inclusion and exclusion fields error. Choose one mode per query. The only legal combination is inclusion fields (1) with _id: 0.

// VALID: inclusion mode with _id suppressed
{ projection: { name: 1, email: 1, _id: 0 } }

// VALID: exclusion mode
{ projection: { passwordHash: 0, secret: 0 } }

// INVALID: mixing 1 and 0 (throws an error)
// { projection: { name: 1, passwordHash: 0 } }  <- ERROR

Projections in find() vs Aggregation

In find(), projection is the second argument. In the aggregation pipeline, you use the $project stage, which supports the same inclusion/exclusion syntax plus computed fields and expressions. Both behave the same way with respect to the cannot-mix rule, but $project is more powerful because it can rename fields and add calculated values.

// find() projection
db.users.find({}, { name: 1, email: 1, _id: 0 });

// Equivalent in aggregation with $project
db.users.aggregate([
  { $project: { name: 1, email: 1, _id: 0 } }
]);

Projections and Covered Queries

A covered query is a query where both the filter fields and the projection fields are entirely within a single index. MongoDB can answer a covered query from the index alone without reading the actual document from disk. Projections are essential for covered queries: if you project a field not in the index, MongoDB must fetch the document, breaking the coverage benefit.

// Index on { email: 1, name: 1 }
db.users.createIndex({ email: 1, name: 1 });

// Covered query — filter + projection both satisfied by the index
db.users.find(
  { email: 'alice@example.com' },
  { name: 1, email: 1, _id: 0 }  // _id must be excluded for full coverage
).explain('executionStats');
// Look for 'totalDocsExamined: 0' in the output

Projections With Mongoose

In Mongoose, you can pass a projection string or object to .select(). A space-separated string with + for inclusion and - for exclusion is common. Fields marked with select: false in the schema (like passwordHash) are excluded from results by default and must be explicitly included with +passwordHash.

// Mongoose: string projection (+ include, - exclude)
const users = await User.find({}).select('name email -_id');

// Object projection
const user = await User.findOne({ email: 'a@b.com' }).select({ name: 1, email: 1, _id: 0 });

// Schema-level default exclusion
const userSchema = new mongoose.Schema({
  email: String,
  passwordHash: { type: String, select: false }  // excluded by default
});

Projection Performance Benefits

Projections improve performance in three ways: (1) they reduce the amount of data read from disk when the projection aligns with an index; (2) they reduce network bandwidth between the MongoDB server and your application; (3) they reduce the amount of memory your application must allocate to hold query results. For large documents or high-throughput APIs, projections can reduce response size by 80% or more.

Projecting Computed Values in Aggregation

The $project stage in aggregation goes beyond simple inclusion/exclusion. You can compute new fields using expressions—for example, concatenating strings, performing arithmetic, or extracting parts of a date. This lets you transform the shape of documents server-side before sending results to the application, reducing client-side processing.

db.orders.aggregate([
  {
    $project: {
      orderId: '$_id',
      _id: 0,
      totalWithTax: { $multiply: ['$subtotal', 1.1] },  // computed field
      year: { $year: '$createdAt' }                       // date extraction
    }
  }
]);

Always Project Sensitive Fields Out

Never return sensitive fields like passwordHash, apiKey, ssn, or creditCardNumber in general-purpose queries. Use exclusion projections or schema-level select: false to ensure these fields never accidentally appear in API responses. Defence in depth means applying projections at the data layer as a backstop, even if the application already strips these fields from responses.

Quick Check

Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.

Lesson Recap

In this lesson you learned: inclusion projections (value 1) return only listed fields, exclusion projections (value 0) return all fields except the listed ones, and you cannot mix inclusion and exclusion except for _id: 0 with inclusion fields. Next up we explore projecting nested fields and arrays using dot notation and $slice.

常见问题解答

「包含式与排除式投影」课时是免费的吗？

是的 — 「包含式与排除式投影」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 MongoDB Academy 课程的其余内容，请升级到 CoddyKit PRO。 MongoDB Academy 课程共包含 4 节课。

「包含式与排除式投影」这节课中我会学到什么？

您将选择或隐藏查询结果中的特定字段，并理解为什么不能在同一个投影中混用包含和排除。你通过在浏览器中直接运行的动手代码来练习 MongoDB Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 MongoDB Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 MongoDB Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 1 节课，共 4 节。

「包含式与排除式投影」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 MongoDB Academy 课中编写并运行代码吗？

能。每节 MongoDB Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 MongoDB Academy