Create an Index on an Embedded Field in an Array
You can create indexes on embedded document fields within arrays. These indexes improve performance for queries on specific embedded fields that appear in arrays. When you create an index on a field inside an array, MongoDB stores that index as a multikey index.
To create an index, use the db.collection.createIndex()
method. Your operation should resemble this prototype:
db.<collection>.createIndex( { <field>: <sortOrder> } )
About this Task
The example on this page uses an inventory
collection that contains
these documents:
db.inventory.insertMany( [ { "item": "t-shirt", "stock": [ { "size": "small", "quantity": 8 }, { "size": "large", "quantity": 10 }, ] }, { "item": "sweater", "stock": [ { "size": "small", "quantity": 4 }, { "size": "large", "quantity": 7 }, ] }, { "item": "vest", "stock": [ { "size": "small", "quantity": 6 }, { "size": "large", "quantity": 1 } ] } ] )
You need to order more inventory any time you have less than five of an
item in stock. To find which items to reorder, you query for documents
where an element in the stock
array has a quantity
less than
5
. To improve performance for this query, you can create an index on
the stock.quantity
field.
Procedure
The following operation creates an ascending multikey index on the
stock.quantity
field of the inventory
collection:
db.inventory.createIndex( { "stock.quantity": 1 } )
Because stock
contains an array value, MongoDB stores this
index as a multikey index.
Results
The index contains a key for each individual value that appears in the
stock.quantity
field. The index is ascending, meaning the keys are
stored in this order: [ 1, 4, 6, 7, 8, 10 ]
.
The index supports queries that select on the stock.quantity
field. For
example, the following query returns documents where at least one
element in the stock
array has a quantity
less than 5
:
db.inventory.find( { "stock.quantity": { $lt: 5 } } )
Output:
[ { _id: ObjectId("63449793b1fac2ee2e957ef3"), item: 'vest', stock: [ { size: 'small', quantity: 6 }, { size: 'large', quantity: 1 } ] }, { _id: ObjectId("63449793b1fac2ee2e957ef2"), item: 'sweater', stock: [ { size: 'small', quantity: 4 }, { size: 'large', quantity: 7 } ] } ]
Sort Results
The index also supports sort operations on the stock.quantity
field,
such as this query:
db.inventory.find().sort( { "stock.quantity": -1 } )
Output:
[ { _id: ObjectId("63449793b1fac2ee2e957ef1"), item: 't-shirt', stock: [ { size: 'small', quantity: 8 }, { size: 'large', quantity: 10 } ] }, { _id: ObjectId("63449793b1fac2ee2e957ef2"), item: 'sweater', stock: [ { size: 'small', quantity: 4 }, { size: 'large', quantity: 7 } ] }, { _id: ObjectId("63449793b1fac2ee2e957ef3"), item: 'vest', stock: [ { size: 'small', quantity: 6 }, { size: 'large', quantity: 1 } ] } ]
When sorting an array of objects, in a descending sort, MongoDB sorts based on the field with the highest-valued element first.
Note
Index Sort Order
Using a descending single-field index may negatively impact index performance. For best performance, only use ascending single-field indexes.