Proyeksi Array $ dan $elemMatch
Kembalikan hanya elemen array pertama yang cocok atau subarray yang difilter menggunakan proyeksi $ dan $elemMatch.
Proyeksi Array $ dan $elemMatch adalah pelajaran MongoDB Academy gratis di CoddyKit. Ini adalah pelajaran 3 dari 4. Kamu bisa membaca pelajaran lengkapnya di bawah secara gratis — lalu praktikkan langsung di browser dengan editor kode bawaan dan tutor AI 24/7. Ini adalah bagian dari jalur belajar MongoDB Academy, dan progresmu tersinkronisasi di web dan aplikasi CoddyKit. Kursus MongoDB Academy mencakup 4 pelajaran total.
Bagian dari pelajaran ini belum diterjemahkan dan ditampilkan dalam bahasa Inggris.
The Problem: Returning Only One Array Element
Sometimes a query matches a document based on an array element—for example, finding an order that contains a specific product—but you only want the matching array element returned, not the entire array. The standard inclusion projection returns all elements of the array. MongoDB provides two special array projection operators to solve this: the positional $ operator and $elemMatch.
The $ Positional Operator
The $ positional projection operator returns the first element of an array that matches the query condition. It is placed in the projection where the array field name normally goes. The condition that matched in the filter is automatically used to determine which element to project. Only one $ can appear in a single projection.
// Find the order and return only the matching line item
db.orders.findOne(
{ 'items.productId': ObjectId('p1') },
{ projection: { 'items.$': 1 } }
);
// Result: { _id: ..., items: [{ productId: ObjectId('p1'), qty: 2, price: 9.99 }] }
// Only the FIRST matching element is returnedHow $ Matches the Filter Condition
The $ operator uses the filter condition on the array field from the query to identify which element to return. The query filter must include a condition on the same array field that appears in the projection. If the filter matches multiple elements, only the first match (by array order) is returned. This is an important limitation to keep in mind.
// Filter on items.qty, project only that matching item
db.orders.find(
{ 'items.qty': { $gt: 1 } },
{ projection: { 'items.$': 1, total: 1 } }
);
// Returns the first item in the items array where qty > 1
// If multiple items have qty > 1, only the first one is projected$ Operator Limitations
The $ positional operator has two key limitations: (1) it only returns the first matching element, even if multiple elements satisfy the filter; (2) the array field must appear in the query filter (you cannot project a field with $ that was not part of the filter condition). For multi-condition or multi-element matching, use $elemMatch in the projection instead.
The $elemMatch Projection Operator
$elemMatch in a projection returns only the first array element that matches the specified conditions, similar to the $ operator but with one key difference: the filter conditions are specified directly in the projection, not in the query filter. This lets you project a matching element from an array even when the main query filter uses different criteria.
// Find all orders, but from the items array return only the item with qty > 1
db.orders.find(
{ status: 'shipped' }, // main filter on a different field
{
projection: {
total: 1,
items: { $elemMatch: { qty: { $gt: 1 } } } // array filter in projection
}
}
);
// items array is present only if an element matches; absent if none match$ vs $elemMatch: The Key Difference
The core difference:
$in projection: the match condition comes from the query filter. Requires the array field to appear in the filter.$elemMatchin projection: you write a separate condition inside the projection itself. The main filter can be on any field.
$ when the filter condition is already on the array field; use $elemMatch projection when you need a different condition or the filter is on a different field.$elemMatch With Multiple Conditions
$elemMatch in the projection can apply multiple conditions to a sub-document within the array—conditions that must all be satisfied by the same array element. This is critical for sub-documents: without $elemMatch, MongoDB evaluates conditions across different elements and can return false positives.
// From orders, return only items where qty > 1 AND price < 20
db.orders.find(
{ status: 'delivered' },
{
projection: {
items: {
$elemMatch: {
qty: { $gt: 1 },
price: { $lt: 20 }
}
}
}
}
);
// Both conditions must match the SAME array elementAbsent Array Field When No Element Matches
When using $elemMatch in a projection and no array element satisfies the condition, the array field is entirely absent from the result document (not present as an empty array). This means your application code must handle the case where the projected array field may be undefined. Always check for the field's existence before accessing its elements.
const order = await db.collection('orders').findOne(
{ status: 'shipped' },
{ projection: { items: { $elemMatch: { qty: { $gt: 100 } } } } }
);
// Safe access — items may be absent if no element matched
const matchedItem = order.items ? order.items[0] : null;
console.log('Matched item:', matchedItem);Combining $elemMatch With Other Projections
You can combine an $elemMatch array projection with regular field projections in the same query. Project scalar fields with their usual inclusion syntax and apply $elemMatch only on the array field. Remember the cannot-mix rule: all non-array fields must use the same mode (inclusion or exclusion).
// Project total and status (inclusion) + first matching item
db.orders.findOne(
{ customerId: ObjectId('c1') },
{
projection: {
total: 1,
status: 1,
_id: 0,
items: { $elemMatch: { qty: { $gt: 0 } } }
}
}
);$elemMatch in the Query Filter vs Projection
Be careful not to confuse $elemMatch in the query filter (which matches documents) with $elemMatch in the projection (which selects which element to return). The filter form determines which documents are returned; the projection form determines which array elements appear in those documents. They are often used together but serve different purposes.
// $elemMatch in FILTER: find orders containing a specific item
db.orders.find({
items: { $elemMatch: { productId: ObjectId('p1'), qty: { $gt: 1 } } }
});
// $elemMatch in PROJECTION: from all shipped orders, return only that matching item
db.orders.find(
{ status: 'shipped' },
{ projection: { items: { $elemMatch: { productId: ObjectId('p1'), qty: { $gt: 1 } } } } }
);Real-World Use Case: User Scores
A gaming leaderboard document stores all scores for a player in an embedded array. When displaying a player's score for a specific game, you only want that game's score element, not the entire score history. $elemMatch in the projection targets exactly that element, keeping the response small even if the player has thousands of game scores recorded.
db.players.find(
{ username: 'gamer42' },
{
projection: {
username: 1,
scores: { $elemMatch: { gameId: 'chess_blitz' } },
_id: 0
}
}
);
// Result: { username: 'gamer42', scores: [{ gameId: 'chess_blitz', score: 1540 }] }Quick Check
Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.
Lesson Recap
In this lesson you learned: the $ positional operator returns the first element that matched the query filter condition, $elemMatch in projection lets you specify match conditions independently of the query filter, and if no element matches $elemMatch, the field is absent from the result. Next up we look at projection best practices for API responses.
Pertanyaan yang Sering Diajukan
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Semua pelajaran dalam kursus ini
- Proyeksi Inklusi vs Eksklusi
- Memproyeksikan Bidang Bertingkat dan Array
- Proyeksi Array $ dan $elemMatch
- Praktik Terbaik Proyeksi untuk Respons API