MongoDB Academy · レッスン

包含プロジェクションと除外プロジェクション

クエリ結果で特定のフィールドを選択または非表示にし、1つのプロジェクションで包含と除外を混在できない理由を理解します。

「包含プロジェクションと除外プロジェクション」はCoddyKit上の無料MongoDB Academyレッスンです。これはレッスン1/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはMongoDB Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 MongoDB Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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.

よくある質問

「包含プロジェクションと除外プロジェクション」レッスンは無料ですか？

はい。「包含プロジェクションと除外プロジェクション」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、MongoDB Academyコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 MongoDB Academyコースには全4レッスンが含まれています。

「包含プロジェクションと除外プロジェクション」で何を学びますか？

クエリ結果で特定のフィールドを選択または非表示にし、1つのプロジェクションで包含と除外を混在できない理由を理解します。ブラウザで直接実行するハンズオンコードでMongoDB Academyを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

MongoDB Academyを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのMongoDB Academyは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン1/4です。

「包含プロジェクションと除外プロジェクション」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このMongoDB Academyレッスンでコードを書いて実行できますか？

はい。すべてのMongoDB Academyレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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