collection.aggregate()
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Definition
collection.aggregate()¶
Execute an aggregation pipeline and return a handle object that allows you to access the pipeline's output documents.
Usage
Example
To call the collection.aggregate() action from a
Function, get a collection handle with
database.collection() then call the handle's
aggregate() method.
const pipeline = [ { "$group": { "_id": "$customerId", "numPurchases": { "$sum": 1 }, "numItemsPurchased": { "$sum": { "$size": "$items" } } } }, { "$addFields": { "averageNumItemsPurchased": { "$divide": ["$numItemsPurchased", "$numPurchases"] } } } ] return purchasesCollection.aggregate(pipeline).toArray() .then(customers => { console.log(`Successfully grouped purchases for ${customers.length} customers.`) for(const customer of customers) { console.log(`customer: ${customer._id}`) console.log(`num purchases: ${customer.numPurchases}`) console.log(`total items purchased: ${customer.numItemsPurchased}`) console.log(`average items per purchase: ${customer.averageNumItemsPurchased}`) } return customers }) .catch(err => console.error(`Failed to group purchases by customer: ${err}`))
Parameters
The collection.aggregate() action has the following form:
aggregate(pipeline)
Parameter | Description |
|---|---|
Aggregation Pipeline pipeline: Array<document> | An array of one or more aggregation pipeline stage documents. Note Supported Aggregation Stages MongoDB Realm supports nearly all MongoDB aggregation pipeline stages and operators, but some stages and operators must be executed within a system function. See Aggregation Framework Limitations for more information. |
Return Value
The collection.aggregate() action returns a cursor object
that points to any documents output from the final stage of the
aggregation pipeline. You can manipulate and access documents
in the aggregation result set with the following methods:
Method | Description | ||
|---|---|---|---|
cursor.next() | Iterates the cursor and returns a Promise that resolves to the
next document in the cursor. If the cursor is exhausted, the
promise resolves to Example | ||
cursor.toArray() | Iterates the cursor to exhaustion and returns a Promise that resolves to an array that contains all of the iterated documents. Example | ||
cursor.skip(amount) | Specifies a number of matching documents to omit from the aggregation result set. MongoDB omits documents from the result set in sort order until it has skipped the specified number. Note You cannot call this method after retrieving one or more
documents using | ||
Client Session session: <ClientSession> | Optional. A client session that specifies the transaction context
in which the operation occurs. To learn more, see
Transactions. |
You cannot return a cursor from a Function.
Instead, evaluate the cursor using cursor.next() or
cursor.toArray() and return the result.