Verschachtelte und Array-Felder projizieren
Verwenden Sie die Dot-Notation und den Operator $slice, um Teile von Arrays und verschachtelten Unterdokumenten zu projizieren.
Verschachtelte und Array-Felder projizieren ist eine kostenlose MongoDB Academy-Lektion auf CoddyKit. Dies ist Lektion 2 von 4. Du kannst die komplette Lektion unten kostenlos lesen – dann übst du sie direkt im Browser mit einem integrierten Code-Editor und einem KI-Tutor rund um die Uhr. Sie ist Teil des MongoDB Academy-Lernpfads, und dein Fortschritt wird über Web und CoddyKit-App synchronisiert. Der MongoDB Academy-Kurs umfasst insgesamt 4 Lektionen.
Teile dieser Lektion wurden noch nicht übersetzt und werden auf Englisch angezeigt.
Projecting Nested Fields With Dot Notation
To project a specific field from an embedded sub-document, use dot notation in the projection key. For example, if a document has an address sub-document, you can project only address.city without returning the full address object. MongoDB traverses the nested path and returns only the specified leaf field, keeping the parent field wrapper in the result.
// Document shape: { name, address: { street, city, zip }, email }
const user = await db.collection('users').findOne(
{ email: 'alice@example.com' },
{ projection: { name: 1, 'address.city': 1, _id: 0 } }
);
// Result: { name: 'Alice', address: { city: 'Austin' } }
// street and zip are excluded; only city is returned inside addressExcluding Nested Fields
Just as with top-level fields, you can exclude specific nested fields while returning the rest of the sub-document. Using 'address.zip': 0 returns the entire address object except the zip field. This is useful for hiding internal-use sub-fields while exposing the rest of a nested object to the client.
// Return everything except address.zip
const user = await db.collection('users').findOne(
{ email: 'alice@example.com' },
{ projection: { 'address.zip': 0, passwordHash: 0 } }
);
// Result includes address.street and address.city but not address.zipArray Fields in Projections
When a projected field is an array, MongoDB returns the entire array by default. If a user document has a tags array with ten elements, projecting tags: 1 returns all ten. To control how much of an array is returned, you need $slice, $, or $elemMatch—the three array-specific projection operators.
// Returns the entire tags array
const post = await db.collection('posts').findOne(
{ slug: 'intro-to-mongodb' },
{ projection: { title: 1, tags: 1, _id: 0 } }
);
// Result: { title: '...', tags: ['mongodb', 'nosql', 'tutorial', ...] }Slicing Arrays With $slice
The $slice projection operator limits the number of array elements returned. Pass a positive integer to return the first N elements, or a negative integer to return the last N elements. You can also pass a two-element array [skip, limit] to skip a number of elements and then return the next N. $slice is the simplest way to paginate or preview array contents.
// Return only the first 3 comments from a post
db.posts.findOne(
{ slug: 'intro-to-mongodb' },
{ projection: { title: 1, comments: { $slice: 3 } } }
);
// Return the last 5 comments
db.posts.findOne(
{ slug: 'intro-to-mongodb' },
{ projection: { title: 1, comments: { $slice: -5 } } }
);
// Skip 10 comments, then return the next 5
db.posts.findOne(
{ slug: 'intro-to-mongodb' },
{ projection: { title: 1, comments: { $slice: [10, 5] } } }
);Projecting Nested Array Sub-Fields
You can project a field inside an array of sub-documents using dot notation in an inclusion projection. For example, an orders document might have an items array where each element has productId, qty, and price. Projecting 'items.qty': 1 returns the entire items array but with only the qty field inside each element.
// Return items array with only the qty field in each element
db.orders.findOne(
{ _id: ObjectId('o1') },
{ projection: { 'items.qty': 1, 'items.productId': 1, _id: 0 } }
);
// Result:
// { items: [{ productId: ObjectId('p1'), qty: 2 }, { productId: ObjectId('p2'), qty: 1 }] }Excluding a Nested Array Sub-Field
Similarly, you can exclude specific sub-fields from array elements. For example, an employees collection might store salary inside a compensation sub-document within each element of an array. Projecting 'positions.compensation': 0 returns the positions array with the sensitive compensation data stripped from each element.
// Exclude compensation from every element of the positions array
db.employees.findOne(
{ name: 'Alice' },
{ projection: { 'positions.compensation': 0 } }
);Projecting Multiple Nested Paths
You can project multiple nested fields from the same sub-document or array in one projection. Each dot-notation path is independent. This composability lets you extract precisely the sub-tree of a document that your query needs, without bringing back irrelevant nested data that would inflate the response.
// Get shipping address city and order total only
db.orders.find(
{ status: 'shipped' },
{
projection: {
'shippingAddress.city': 1,
'shippingAddress.country': 1,
total: 1,
_id: 0
}
}
);Deeply Nested Dot Notation
Dot notation can go multiple levels deep. If you have a document with meta.seo.keywords, you can project exactly that path. MongoDB will return the outer wrapper fields (meta and seo) but only include the keywords leaf, stripping all other sibling fields at each level.
// Document: { title, meta: { seo: { keywords: [...], description: '...' }, author: '...' } }
db.articles.findOne(
{ slug: 'my-article' },
{ projection: { title: 1, 'meta.seo.keywords': 1, _id: 0 } }
);
// Result: { title: '...', meta: { seo: { keywords: [...] } } }
// meta.seo.description and meta.author are excludedProjection and Index Coverage for Nested Fields
For a query to be covered (served entirely from the index without touching documents), the projection must include only fields present in the index, including nested fields. A compound index on { 'address.city': 1, name: 1 } can cover a query that filters on address.city and projects only address.city and name (with _id: 0). Covered queries achieve the lowest possible latency.
// Create an index on nested field
db.users.createIndex({ 'address.city': 1, name: 1 });
// Covered query — both filter and projection are inside the index
db.users.find(
{ 'address.city': 'Austin' },
{ projection: { 'address.city': 1, name: 1, _id: 0 } }
).explain('executionStats');
// totalDocsExamined should be 0$slice in Mongoose
Mongoose does not have a native $slice shorthand on the query builder, but you can pass a projection object directly. Use { fieldName: { $slice: N } } inside the Mongoose .select() call if you pass an object, or pass the projection as the second argument to .find(). The driver forwards it to MongoDB unchanged.
// Mongoose: return title and first 5 comments using $slice
const post = await Post.findOne(
{ slug: 'intro-to-mongodb' },
{ title: 1, comments: { $slice: 5 }, _id: 0 }
);
// Alternatively as a Mongoose query
const post2 = await Post
.findOne({ slug: 'intro-to-mongodb' })
.select({ title: 1, comments: { $slice: 5 }, _id: 0 });Nested Projection Gotchas
A common gotcha: projecting a nested field in an array requires you to use dot notation, but MongoDB returns the full array wrapper. You cannot project individual array elements by index (e.g., items.0 doesn't filter to just the first element in the projection—use $slice: 1 for that). Also, you cannot mix projecting a parent field and a child field in the same path: either project address or address.city, not both.
Quick Check
Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.
Lesson Recap
In this lesson you learned: dot notation projects specific nested fields from sub-documents and arrays, $slice limits the number of array elements returned, and projecting nested array sub-fields includes only the specified leaf inside each array element. Next up we explore the $ and $elemMatch array projections for returning a single matching element from an array.
Häufig gestellte Fragen
Ist die Lektion „Verschachtelte und Array-Felder projizieren“ kostenlos?
Ja — der vollständige Text von „Verschachtelte und Array-Felder projizieren“ ist hier im Web kostenlos zu lesen. Um sie interaktiv zu üben (integrierter Code-Editor und 24/7 KI-Tutor) und den Rest des MongoDB Academy-Kurses freizuschalten, upgrade auf CoddyKit PRO. Der MongoDB Academy-Kurs umfasst insgesamt 4 Lektionen.
Was lerne ich in „Verschachtelte und Array-Felder projizieren“?
Verwenden Sie die Dot-Notation und den Operator $slice, um Teile von Arrays und verschachtelten Unterdokumenten zu projizieren. Du übst MongoDB Academy mit praktischem Code, den du direkt im Browser ausführst, und ein 24/7 KI-Tutor beantwortet deine Fragen während du die Lektion bearbeitest.
Brauche ich Erfahrung, um MongoDB Academy zu starten?
Keine Vorkenntnisse erforderlich. MongoDB Academy auf CoddyKit ist für Anfänger bis fortgeschrittene Lernende strukturiert, sodass du hier starten oder von Anfang an beginnen und in deinem eigenen Tempo voranschreiten kannst. Dies ist Lektion 2 von 4.
Wie lange dauert die Lektion „Verschachtelte und Array-Felder projizieren“?
Die meisten CoddyKit-Lektionen dauern etwa 5–10 Minuten. Jede ist kompakt und interaktiv, sodass du stetig Fortschritte machst und genau dort weitermachst, wo du aufgehört hast – im Web und in der App.
Kann ich in dieser MongoDB Academy-Lektion Code schreiben und ausführen?
Ja. Jede MongoDB Academy-Lektion enthält einen integrierten Code-Editor, sodass du echten Code direkt in deinem Browser schreibst und ausführst und sofort KI-Feedback erhältst — ohne lokale Einrichtung erforderlich.
Alle Lektionen in diesem Kurs
- Projektionen mit Ein- und Ausschluss
- Verschachtelte und Array-Felder projizieren
- Die Array-Projektionen $ und $elemMatch
- Best Practices für Projektionen in API-Antworten