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Aggregation Pipeline Stages

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Operators

Aggregation Pipeline Stages

$merge (aggregation)

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Definition

Note

The following page discusses the $merge stage, which outputs the aggregation pipeline results to a collection. For a discussion of the $mergeObjects operator which merges documents into a single document, see $mergeObjects instead.

$merge

Writes the results of the aggregation pipeline to a specified collection. The $merge operator must be the last stage in the pipeline.

The $merge stage:

Can output to a collection in the same or different database.
Starting in MongoDB 4.4:
- $merge can output to the same collection that is being aggregated. For more information, see Output to the Same Collection that is Being Aggregated.
- Pipelines with the $merge stage can run on replica set secondary nodes if all the nodes in cluster have featureCompatibilityVersion set to 4.4 or higher and the Read Preference allows secondary reads.
  - Read operations of the $merge statement are sent to secondary nodes, while the write operations occur only on the primary node.
  - Not all driver versions support targeting of $merge operations to replica set secondary nodes. Check your driver documentation to see when your driver added support for $merge read operations running on secondary nodes.
Creates a new collection if the output collection does not already exist.
Can incorporate results (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) into an existing collection.
Can output to a sharded collection. Input collection can also be sharded.

For a comparison with the $out stage which also outputs the aggregation results to a collection, see $merge and $out Comparison.

Note

On-Demand Materialized Views

$merge can incorporate the pipeline results into an existing output collection rather than perform a full replacement of the collection. This functionality allows users to create on-demand materialized views, where the content of the output collection is incrementally updated when the pipeline is run.

For more information on this use case, see On-Demand Materialized Views as well as the examples on this page.

Materialized views are separate from read-only views. For information on creating read-only views, see read-only views.

Compatibility

You can use $merge for deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

$merge has the following syntax:

{ $merge: {
     into: <collection> -or- { db: <db>, coll: <collection> },
     on: <identifier field> -or- [ <identifier field1>, ...],  // Optional
     let: <variables>,                                         // Optional
     whenMatched: <replace|keepExisting|merge|fail|pipeline>,  // Optional
     whenNotMatched: <insert|discard|fail>                     // Optional
} }

For example:

{ $merge: { into: "myOutput", on: "_id", whenMatched: "replace", whenNotMatched: "insert" } }

If using all default options for $merge (including writing to a collection in the same database), you can use the simplified form:

{ $merge: <collection> } // Output collection is in the same database

The $merge takes a document with the following fields:

Field

Description

into

The output collection. Specify either:

The collection name as a string to output to a collection in the same database where the aggregation is run. For example:
into: "myOutput"
The database and collection name in a document to output to a collection in the specified database. For example:
into: { db:"myDB", coll:"myOutput" }

Note

If the output collection does not exist, $merge creates the collection:
- For a replica set or a standalone, if the output database does not exist, $merge also creates the database.
- For a sharded cluster, the specified output database must already exist.
The output collection can be a sharded collection.

Optional. Field or fields that act as a unique identifier for a document. The identifier determines if a results document matches an already existing document in the output collection. Specify either:

A single field name as a string. For example:
on: "_id"
A combination of fields in an array. For example:
on: [ "date", "customerId" ]
The order of the fields in the array does not matter, and you cannot specify the same field multiple times.

For the specified field or fields:

The aggregation results documents must contain the field(s) specified in the on, unless the on field is the _id field. If the _id field is missing from a results document, MongoDB adds it automatically.
The specified field or fields cannot contain a null or an array value.

$merge requires a unique, index with keys that correspond to the on identifier fields. Although the order of the index key specification does not matter, the unique index must only contain the on fields as its keys.

The index must also have the same collation as the aggregation's collation.
The unique index can be a sparse index.
For output collections that already exist, the corresponding index must already exist.

The default value for on depends on the output collection:

If the output collection does not exist, the on identifier must be and defaults to the _id field. The corresponding unique _id index is automatically created.
Tip
To use a different on identifier field(s) for a collection that does not exist, you can create the collection first by creating a unique index on the desired field(s). See the section on non-existent output collection for an example.
If the existing output collection is unsharded, the on identifier defaults to the _id field.
If the existing output collection is a sharded collection, the on identifier defaults to all the shard key fields and the _id field. If specifying a different on identifier, the on must contain all the shard key fields.

whenMatched

Optional. The behavior of $merge if a result document and an existing document in the collection have the same value for the specified on field(s).

You can specify either:

One of the pre-defined action strings:

Action	Description
"replace"	Replace the existing document in the output collection with the matching results document. When performing a replace, the replacement document cannot result in a modification of the `_id` value or, if the output collection is sharded, the shard key value. Otherwise, the operation results in an error. Tip To avoid this error, if the on field does not include the `_id` field, remove the `_id` field in the aggregation results to avoid the error, such as with a preceding `$unset` stage, etc.
"keepExisting"	Keep the existing document in the output collection.
"merge" (Default)	Merge the matching documents (similar to the `$mergeObjects` operator). If the results document contains fields not in the existing document, add these new fields to the existing document. If the results document contains fields in the existing document, replace the existing field values with those from the results document. For example, if the output collection has the document: `{ _id: 1, a: 1, b: 1 }` And the aggregation results has the document: `{ _id: 1, b: 5, z: 1 }` Then, the merged document is: `{ _id: 1, a: 1, b: 5, z: 1 }` When performing a merge, the merged document cannot result in a modification of the `_id` value or, if the output collection is sharded, the shard key value. Otherwise, the operation results in an error. Tip To avoid this error, if the on field does not include the `_id` field, remove the `_id` field in the aggregation results to avoid the error, such as with a preceding `$unset` stage, etc.
"fail"	Stop and fail the aggregation operation. Any changes to the output collection from previous documents are not reverted.

An aggregation pipeline to update the document in the collection.
[ <stage1>, <stage2> ... ]
The pipeline can only consist of the following stages:
- $addFields and its alias $set
- $project and its alias $unset
- $replaceRoot and its alias $replaceWith
The pipeline cannot modify the on field's value. For example, if you are matching on the field month, the pipeline cannot modify the month field.
The whenMatched pipeline can directly access the fields of the existing documents in the output collection using $<field>.
To access the fields from the aggregation results documents, use either:
- The built-in $$new variable to access the field, i.e. $$new.<field>. The $$new variable is only available if the let specification is omitted.
  Note
  Starting in MongoDB 4.2.2, the $$new variable is reserved, and cannot be overridden.
- The user-defined variables in the let field, i.e. $$<uservariable>.<field>.

let

Optional. Specifies variables accessible for use in the whenMatched pipeline

Specify a document with the variable name and value expression:

{ <var_1>: <expression>, ..., <var_n>: <expression> }

If unspecified, defaults to { new: "$$ROOT" }; i.e. the whenMatched pipeline can access the $$new variable.

Note

Starting in MongoDB 4.2.2, the $$new variable is reserved, and cannot be overridden.

To access the let variables in the whenMatched pipeline, use the double dollar signs ($$) prefix and variable name $$<variable>.

whenNotMatched

Optional. The behavior of $merge if a result document does not match an existing document in the out collection.

You can specify one of the pre-defined action strings:

Action	Description
"insert" (Default)	Insert the document into the output collection.
"discard"	Discard the document; i.e. `$merge` does not insert the document into the output collection.
"fail"	Stop and fail the aggregation operation. Any changes already written to the output collection are not reverted.

Considerations

`_id` Field Generation

If the _id field is not present in a document from the aggregation pipeline results, the $merge stage generates it automatically.

For example, in the following aggregation pipeline, $project excludes the _id field from the documents passed into $merge. When $merge writes these documents to the "newCollection", $merge generates a new _id field and value.

db.sales.aggregate( [
   { $project: { _id: 0 } },
   { $merge : { into : "newCollection" } }
] )

Create a New Collection if Output Collection is Non-Existent

The $merge operation creates a new collection if the specified output collection does not exist.

The output collection is created when $merge writes the first document into the collection and is immediately visible.
If the aggregation fails, any writes completed by the $merge before the error will not be rolled back.

Note

For a replica set or a standalone, if the output database does not exist, $merge also creates the database.

For a sharded cluster, the specified output database must already exist.

If the output collection does not exist, $merge requires the on identifier to be the _id field. To use a different on field value for a collection that does not exist, you can create the collection first by creating a unique index on the desired field(s) first. For example, if the output collection newDailySales201905 does not exist and you want to specify the salesDate field as the on identifier:

db.newDailySales201905.createIndex( { salesDate: 1 }, { unique: true } )
db.sales.aggregate( [
   { $match: { date: { $gte: new Date("2019-05-01"), $lt: new Date("2019-06-01") } } },
   { $group: { _id: { $dateToString: { format: "%Y-%m-%d", date: "$date" } }, totalqty: { $sum: "$quantity" } } },
   { $project: { _id: 0, salesDate: { $toDate: "$_id" }, totalqty: 1 } },
   { $merge : { into : "newDailySales201905", on: "salesDate" } }
] )

Output to a Sharded Collection

The $merge stage can output to a sharded collection. When the output collection is sharded, $merge uses the _id field and all the shard key fields as the default on identifier. If you override the default, the on identifier must include all the shard key fields:

{ $merge: {
   into: "<shardedColl>" or { db:"<sharding enabled db>", coll: "<shardedColl>" },
   on: [ "<shardkeyfield1>", "<shardkeyfield2>",... ], // Shard key fields and any additional fields
   let: <variables>,                                         // Optional
   whenMatched: <replace|keepExisting|merge|fail|pipeline>,  // Optional
   whenNotMatched: <insert|discard|fail>                     // Optional
} }

For example, in a database that has sharding enabled, use the sh.shardCollection() method to create a new sharded collection newrestaurants with the postcode field as the shard key.

sh.enableSharding("exampledb");     // Sharding must be enabled in the database
sh.shardCollection(
   "exampledb.newrestaurants",      // Namespace of the collection to shard
   { postcode: 1 },                 // Shard key
);

The newrestaurants collection will contain documents with information on new restaurant openings by month (date field) and postcode (shard key); specifically, the on identifier is ["date", "postcode"] (the ordering of the fields does not matter). Because $merge requires a unique, index with keys that correspond to the on identifier fields, create the unique index (the ordering of the fields do not matter): [1]

use exampledb
db.newrestaurants.createIndex( { postcode: 1, date: 1 }, { unique: true } )

With the sharded collection restaurants and the unique index created, you can use $merge to output the aggregation results to this collection, matching on [ "date", "postcode" ] as in the following example:

use exampledb
db.openings.aggregate([
   { $group: {
      _id: { date: { $dateToString: { format: "%Y-%m", date: "$date" } }, postcode: "$postcode" },
      restaurants: { $push: "$restaurantName" } } },
   { $project: { _id: 0, postcode: "$_id.postcode", date: "$_id.date", restaurants: 1 } },
   { $merge: { into: "newrestaurants", "on": [ "date", "postcode" ], whenMatched: "replace", whenNotMatched: "insert" } }
])

[1] The sh.shardCollection() method can also create a unique index on the shard key when passed the { unique: true } option if: the shard key is range-based; the collection is empty; and a unique index on the shard key doesn't already exist.In the example above, because the on identifier is the shard key and another field, a separate operation to create the corresponding index is required.

Replace Documents (`$merge`) vs Replace Collection (`$out`)

$merge can replace an existing document in the output collection if the aggregation results contain a document or documents that match based on the on specification. As such, $merge can replace all documents in the existing collection if the aggregation results include matching documents for all existing documents in the collection and you specify "replace" for whenMatched.

However, to replace an existing collection regardless of the aggregation results, use $out instead.

Existing Documents and `_id` and Shard Key Values

The $merge errors if the $merge results in a change to an existing document's _id value.

Tip

To avoid this error, if the on field does not include the _id field, remove the _id field in the aggregation results to avoid the error, such as with a preceding $unset stage, etc.

Additionally, for a sharded collection, $merge also errors if it results in a change to the shard key value of an exising document.

Any writes completed by the $merge before the error will not be rolled back.

Unique Index Constraints

If the unique index used by $merge for on field(s) is dropped mid-aggregation, there is no guarantee that the aggregation will be killed. If the aggregation continues, there is no guarantee that documents do not have duplicate on field values.

If the $merge attempts to write a document that violates any unique index on the output collection, the operation errors; for example:

Insert a non-matching document that violates a unique index other than the index on the on field(s).
Fail if there is a matching document in the collection. Specifically, the operation attempts to insert the matching document which violates the unique index on the on field(s).
Replace an existing document with a new document that violates a unique index other than the index on the on field(s).
Merge the matching documents that results in a document that violates a unique index other than the index on the on field(s).

`whenMatched` Pipeline Behavior

Starting in MongoDB 4.2.2, if all of the following are true for a $merge stage:

The value of whenMatched is an aggregation pipeline,
The value of whenNotMatched is insert, and
There is no match for a document in the output collection,

$merge inserts the document directly into the output collection.

Prior to MongoDB 4.2.2, when these conditions for a $merge stage are met, the pipeline specified in the whenMatched field is executed with an empty input document. The resulting document from the pipeline is inserted into the output collection.

Tip

4.2.2 Release Notes

`$merge` and `$out` Comparison

With the introduction of $merge, MongoDB provides two stages, $merge and $out, for writing the results of the aggregation pipeline to a collection. The following summarizes the capabilities of the two stages:

`$merge`	`$out`
Can output to a collection in the same or different database.	Can output to a collection in the same or, starting in MongoDB 4.4, different database.
Creates a new collection if the output collection does not already exist.	Creates a new collection if the output collection does not already exist.
Can incorporate results (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) into an existing collection. See also Replace Documents (`$merge`) vs Replace Collection (`$out`).	Replaces the output collection completely if it already exists.
Can output to a sharded collection. Input collection can also be sharded.	Cannot output to a sharded collection. Input collection, however, can be sharded.
Corresponds to SQL statements: `MERGE`. `INSERT INTO T2 SELECT FROM T1`. `SELECT INTO T2 FROM T1`. Create/Refresh Materialized Views.	Corresponds to SQL statement: `INSERT INTO T2 SELECT FROM T1`. `SELECT INTO T2 FROM T1`.

Output to the Same Collection that is Being Aggregated

Warning

When $merge outputs to the same collection that is being aggregated, documents may get updated multiple times or the operation may result in an infinite loop. This behavior occurs when the update performed by $merge changes the physical location of documents stored on disk. When the physical location of a document changes, $merge may view it as an entirely new document, resulting in additional updates. For more information on this behavior, see Halloween Problem.

Starting in MongoDB 4.4, $merge can output to the same collection that is being aggregated. You can also output to a collection which appears in other stages of the pipeline, such as $lookup.

Versions of MongoDB prior to 4.4 did not allow $merge to output to the same collection as the collection being aggregated.

Restrictions

Restrictions	Description
Transactions	An aggregation pipeline cannot use `$merge` inside a transaction.
View definition Separate from materialized view	View definition cannot include the `$merge` stage. If the view definition includes nested pipeline (e.g. the view definition includes `$facet` stage), this `$merge` stage restriction applies to the nested pipelines as well.
`$lookup` stage	`$lookup` stage's nested pipeline cannot include the `$merge` stage.
`$facet` stage	`$facet` stage's nested pipeline cannot include the `$merge` stage.
`$unionWith` stage	`$unionWith` stage's nested pipeline cannot include the `$merge` stage.
`"linearizable"` read concern	The `$merge` stage cannot be used in conjunction with read concern `"linearizable"`. That is, if you specify `"linearizable"` read concern for `db.collection.aggregate()`, you cannot include the `$merge` stage in the pipeline.

Examples

On-Demand Materialized View: Initial Creation

If the output collection does not exist, the $merge creates the collection.

For example, a collection named salaries in the zoo database is populated with the employee salary and department history:

db.getSiblingDB("zoo").salaries.insertMany([
   { "_id" : 1, employee: "Ant", dept: "A", salary: 100000, fiscal_year: 2017 },
   { "_id" : 2, employee: "Bee", dept: "A", salary: 120000, fiscal_year: 2017 },
   { "_id" : 3, employee: "Cat", dept: "Z", salary: 115000, fiscal_year: 2017 },
   { "_id" : 4, employee: "Ant", dept: "A", salary: 115000, fiscal_year: 2018 },
   { "_id" : 5, employee: "Bee", dept: "Z", salary: 145000, fiscal_year: 2018 },
   { "_id" : 6, employee: "Cat", dept: "Z", salary: 135000, fiscal_year: 2018 },
   { "_id" : 7, employee: "Gecko", dept: "A", salary: 100000, fiscal_year: 2018 },
   { "_id" : 8, employee: "Ant", dept: "A", salary: 125000, fiscal_year: 2019 },
   { "_id" : 9, employee: "Bee", dept: "Z", salary: 160000, fiscal_year: 2019 },
   { "_id" : 10, employee: "Cat", dept: "Z", salary: 150000, fiscal_year: 2019 }
])

You can use the $group and $merge stages to initially create a collection named budgets (in the reporting database) from the data currently in the salaries collection:

Note

For a replica set or a standalone deployment, if the output database does not exist, $merge also creates the database.

For a sharded cluster deployment, the specified output database must already exist.

db.getSiblingDB("zoo").salaries.aggregate( [
   { $group: { _id: { fiscal_year: "$fiscal_year", dept: "$dept" }, salaries: { $sum: "$salary" } } },
   { $merge : { into: { db: "reporting", coll: "budgets" }, on: "_id",  whenMatched: "replace", whenNotMatched: "insert" } }
] )

$group stage to group the salaries by the fiscal_year and dept.
$merge stage writes the output of the preceding $group stage to the budgets collection in the reporting database.

To view the documents in the new budgets collection:

db.getSiblingDB("reporting").budgets.find().sort( { _id: 1 } )

The budgets collection contains the following documents:

{ "_id" : { "fiscal_year" : 2017, "dept" : "A" }, "salaries" : 220000 }
{ "_id" : { "fiscal_year" : 2017, "dept" : "Z" }, "salaries" : 115000 }
{ "_id" : { "fiscal_year" : 2018, "dept" : "A" }, "salaries" : 215000 }
{ "_id" : { "fiscal_year" : 2018, "dept" : "Z" }, "salaries" : 280000 }
{ "_id" : { "fiscal_year" : 2019, "dept" : "A" }, "salaries" : 125000 }
{ "_id" : { "fiscal_year" : 2019, "dept" : "Z" }, "salaries" : 310000 }

Tip

On-Demand Materialized Views

On-Demand Materialized View: Update/Replace Data

The following example refers to the collections from the previous example.

The example salaries collection contains the employee salary and department history:

{ "_id" : 1, employee: "Ant", dept: "A", salary: 100000, fiscal_year: 2017 },
{ "_id" : 2, employee: "Bee", dept: "A", salary: 120000, fiscal_year: 2017 },
{ "_id" : 3, employee: "Cat", dept: "Z", salary: 115000, fiscal_year: 2017 },
{ "_id" : 4, employee: "Ant", dept: "A", salary: 115000, fiscal_year: 2018 },
{ "_id" : 5, employee: "Bee", dept: "Z", salary: 145000, fiscal_year: 2018 },
{ "_id" : 6, employee: "Cat", dept: "Z", salary: 135000, fiscal_year: 2018 },
{ "_id" : 7, employee: "Gecko", dept: "A", salary: 100000, fiscal_year: 2018 },
{ "_id" : 8, employee: "Ant", dept: "A", salary: 125000, fiscal_year: 2019 },
{ "_id" : 9, employee: "Bee", dept: "Z", salary: 160000, fiscal_year: 2019 },
{ "_id" : 10, employee: "Cat", dept: "Z", salary: 150000, fiscal_year: 2019 }

The example budgets collection contains the cumulative yearly budgets:

{ "_id" : { "fiscal_year" : 2017, "dept" : "A" }, "salaries" : 220000 }
{ "_id" : { "fiscal_year" : 2017, "dept" : "Z" }, "salaries" : 115000 }
{ "_id" : { "fiscal_year" : 2018, "dept" : "A" }, "salaries" : 215000 }
{ "_id" : { "fiscal_year" : 2018, "dept" : "Z" }, "salaries" : 280000 }
{ "_id" : { "fiscal_year" : 2019, "dept" : "A" }, "salaries" : 125000 }
{ "_id" : { "fiscal_year" : 2019, "dept" : "Z" }, "salaries" : 310000 }

During the current fiscal year (2019 in this example), new employees are added to the salaries collection and new head counts are pre-allocated for the next year:

db.getSiblingDB("zoo").salaries.insertMany([
   { "_id" : 11,  employee: "Wren", dept: "Z", salary: 100000, fiscal_year: 2019 },
   { "_id" : 12,  employee: "Zebra", dept: "A", salary: 150000, fiscal_year: 2019 },
   { "_id" : 13,  employee: "headcount1", dept: "Z", salary: 120000, fiscal_year: 2020 },
   { "_id" : 14,  employee: "headcount2", dept: "Z", salary: 120000, fiscal_year: 2020 }
])

To update the budgets collection to reflect the new salary information, the following aggregation pipeline uses:

$match stage to find all documents with fiscal_year greater than or equal to 2019.
$group stage to group the salaries by the fiscal_year and dept.
$merge to write the result set to the budgets collection, replacing documents with the same _id value (in this example, a document with the fiscal year and dept). For documents that do not have matches in the collection, $merge inserts the new documents.

db.getSiblingDB("zoo").salaries.aggregate( [
   { $match : { fiscal_year:  { $gte : 2019 } } },
   { $group: { _id: { fiscal_year: "$fiscal_year", dept: "$dept" }, salaries: { $sum: "$salary" } } },
   { $merge : { into: { db: "reporting", coll: "budgets" }, on: "_id",  whenMatched: "replace", whenNotMatched: "insert" } }
] )

After the aggregation is run, view the documents in the budgets collection:

db.getSiblingDB("reporting").budgets.find().sort( { _id: 1 } )

The budgets collection incorporates the new salary data for fiscal year 2019 and adds new documents for fiscal year 2020:

{ "_id" : { "fiscal_year" : 2017, "dept" : "A" }, "salaries" : 220000 }
{ "_id" : { "fiscal_year" : 2017, "dept" : "Z" }, "salaries" : 115000 }
{ "_id" : { "fiscal_year" : 2018, "dept" : "A" }, "salaries" : 215000 }
{ "_id" : { "fiscal_year" : 2018, "dept" : "Z" }, "salaries" : 280000 }
{ "_id" : { "fiscal_year" : 2019, "dept" : "A" }, "salaries" : 275000 }
{ "_id" : { "fiscal_year" : 2019, "dept" : "Z" }, "salaries" : 410000 }
{ "_id" : { "fiscal_year" : 2020, "dept" : "Z" }, "salaries" : 240000 }

Tip

On-Demand Materialized Views

Only Insert New Data

To ensure that the $merge does not overwrite existing data in the collection, set whenMatched to keepExisting or fail.

The example salaries collection in the zoo database contains the employee salary and department history:

{ "_id" : 1, employee: "Ant", dept: "A", salary: 100000, fiscal_year: 2017 },
{ "_id" : 2, employee: "Bee", dept: "A", salary: 120000, fiscal_year: 2017 },
{ "_id" : 3, employee: "Cat", dept: "Z", salary: 115000, fiscal_year: 2017 },
{ "_id" : 4, employee: "Ant", dept: "A", salary: 115000, fiscal_year: 2018 },
{ "_id" : 5, employee: "Bee", dept: "Z", salary: 145000, fiscal_year: 2018 },
{ "_id" : 6, employee: "Cat", dept: "Z", salary: 135000, fiscal_year: 2018 },
{ "_id" : 7, employee: "Gecko", dept: "A", salary: 100000, fiscal_year: 2018 },
{ "_id" : 8, employee: "Ant", dept: "A", salary: 125000, fiscal_year: 2019 },
{ "_id" : 9, employee: "Bee", dept: "Z", salary: 160000, fiscal_year: 2019 },
{ "_id" : 10, employee: "Cat", dept: "Z", salary: 150000, fiscal_year: 2019 }

A collection orgArchive in the reporting database contains historical departmental organization records for the past fiscal years. Archived records should not be modified.

{ "_id" : ObjectId("5cd8c68261baa09e9f3622be"), "employees" : [ "Ant", "Gecko" ], "dept" : "A", "fiscal_year" : 2018 }
{ "_id" : ObjectId("5cd8c68261baa09e9f3622bf"), "employees" : [ "Ant", "Bee" ], "dept" : "A", "fiscal_year" : 2017 }
{ "_id" : ObjectId("5cd8c68261baa09e9f3622c0"), "employees" : [ "Bee", "Cat" ], "dept" : "Z", "fiscal_year" : 2018 }
{ "_id" : ObjectId("5cd8c68261baa09e9f3622c1"), "employees" : [ "Cat" ], "dept" : "Z", "fiscal_year" : 2017 }

The orgArchive collection has a unique compound index on the fiscal_year and dept fields; i.e. there should be at most one record for the same fiscal year and department combination:

db.getSiblingDB("reporting").orgArchive.createIndex ( { fiscal_year: 1, dept: 1 }, { unique: true } )

At the end of current fiscal year (2019 in this example), the salaries collection contain the following documents:

{ "_id" : 1, "employee" : "Ant", "dept" : "A", "salary" : 100000, "fiscal_year" : 2017 }
{ "_id" : 2, "employee" : "Bee", "dept" : "A", "salary" : 120000, "fiscal_year" : 2017 }
{ "_id" : 3, "employee" : "Cat", "dept" : "Z", "salary" : 115000, "fiscal_year" : 2017 }
{ "_id" : 4, "employee" : "Ant", "dept" : "A", "salary" : 115000, "fiscal_year" : 2018 }
{ "_id" : 5, "employee" : "Bee", "dept" : "Z", "salary" : 145000, "fiscal_year" : 2018 }
{ "_id" : 6, "employee" : "Cat", "dept" : "Z", "salary" : 135000, "fiscal_year" : 2018 }
{ "_id" : 7, "employee" : "Gecko", "dept" : "A", "salary" : 100000, "fiscal_year" : 2018 }
{ "_id" : 8, "employee" : "Ant", "dept" : "A", "salary" : 125000, "fiscal_year" : 2019 }
{ "_id" : 9, "employee" : "Bee", "dept" : "Z", "salary" : 160000, "fiscal_year" : 2019 }
{ "_id" : 10, "employee" : "Cat", "dept" : "Z", "salary" : 150000, "fiscal_year" : 2019 }
{ "_id" : 11, "employee" : "Wren", "dept" : "Z", "salary" : 100000, "fiscal_year" : 2019 }
{ "_id" : 12, "employee" : "Zebra", "dept" : "A", "salary" : 150000, "fiscal_year" : 2019 }
{ "_id" : 13, "employee" : "headcount1", "dept" : "Z", "salary" : 120000, "fiscal_year" : 2020 }
{ "_id" : 14, "employee" : "headcount2", "dept" : "Z", "salary" : 120000, "fiscal_year" : 2020 }

To update the orgArchive collection to include the fiscal year 2019 that has just ended, the following aggregation pipeline uses:

$match stage to find all documents with fiscal_year equal to 2019.
$group stage to group the employees by the fiscal_year and dept.
$project stage to suppress the _id field and add separate dept and fiscal_year field. When the documents are passed to $merge, $merge automatically generates a new _id field for the documents.
$merge to write the result set to orgArchive.
The $merge stage matches documents on the dept and fiscal_year fields and fails when matched. That is, if a document already exists for the same department and fiscal year, the $merge errors.

db.getSiblingDB("zoo").salaries.aggregate( [
    { $match: { fiscal_year: 2019 }},
    { $group: { _id: { fiscal_year: "$fiscal_year", dept: "$dept" }, employees: { $push: "$employee" } } },
    { $project: { _id: 0, dept: "$_id.dept", fiscal_year: "$_id.fiscal_year", employees: 1 } },
    { $merge : { into : { db: "reporting", coll: "orgArchive" }, on: [ "dept", "fiscal_year" ], whenMatched: "fail" } }
] )

After the operation, the orgArchive collection contains the following documents:

{ "_id" : ObjectId("5caccc6a66b22dd8a8cc419f"), "employees" : [ "Ahn", "Bess" ], "dept" : "A", "fiscal_year" : 2017 }
{ "_id" : ObjectId("5caccc6a66b22dd8a8cc419e"), "employees" : [ "Ahn", "Gee" ], "dept" : "A", "fiscal_year" : 2018 }
{ "_id" : ObjectId("5caccd0b66b22dd8a8cc438e"), "employees" : [ "Ahn", "Zeb" ], "dept" : "A", "fiscal_year" : 2019 }
{ "_id" : ObjectId("5caccc6a66b22dd8a8cc41a0"), "employees" : [ "Carl" ], "dept" : "Z", "fiscal_year" : 2017 }
{ "_id" : ObjectId("5caccc6a66b22dd8a8cc41a1"), "employees" : [ "Bess", "Carl" ], "dept" : "Z", "fiscal_year" : 2018 }
{ "_id" : ObjectId("5caccd0b66b22dd8a8cc438d"), "employees" : [ "Bess", "Carl", "Wen" ], "dept" : "Z", "fiscal_year" : 2019 }

If the orgArchive collection already contained a document for 2019 for department "A" and/or "B", the aggregation fails because of the duplicate key error. However, any document inserted before the error will not be rolled back.

If you specify keepExisting for the matching document, the aggregation does not affect the matching document and does not error with duplicate key error. Similarly, if you specify replace, the operation would not fail; however, the operation would replace the existing document.

Merge Results from Multiple Collections

By default, if a document in the aggregation results matches a document in the collection, the $merge stage merges the documents.

An example collection purchaseorders is populated with the purchase order information by quarter and regions:

db.purchaseorders.insertMany( [
   { _id: 1, quarter: "2019Q1", region: "A", qty: 200, reportDate: new Date("2019-04-01") },
   { _id: 2, quarter: "2019Q1", region: "B", qty: 300, reportDate: new Date("2019-04-01") },
   { _id: 3, quarter: "2019Q1", region: "C", qty: 700, reportDate: new Date("2019-04-01") },
   { _id: 4, quarter: "2019Q2", region: "B", qty: 300, reportDate: new Date("2019-07-01") },
   { _id: 5, quarter: "2019Q2", region: "C", qty: 1000, reportDate: new Date("2019-07-01") },
   { _id: 6, quarter: "2019Q2", region: "A", qty: 400, reportDate: new Date("2019-07-01") },
] )

Another example collection reportedsales is populated with the reported sales information by quarter and regions:

db.reportedsales.insertMany( [
   { _id: 1, quarter: "2019Q1", region: "A", qty: 400, reportDate: new Date("2019-04-02") },
   { _id: 2, quarter: "2019Q1", region: "B", qty: 550, reportDate: new Date("2019-04-02") },
   { _id: 3, quarter: "2019Q1", region: "C", qty: 1000, reportDate: new Date("2019-04-05") },
   { _id: 4, quarter: "2019Q2", region: "B", qty: 500, reportDate: new Date("2019-07-02") },
] )

Assume that, for reporting purposes, you want to view the data by quarter in the following format:

{ "_id" : "2019Q1", "sales" : 1950, "purchased" : 1200 }
{ "_id" : "2019Q2", "sales" : 500, "purchased" : 1700 }

You can use the $merge to merge in results from the purchaseorders collection and the reportedsales collection to create a new collection quarterlyreport.

To create the quarterlyreport collection, you can use the following pipeline:

db.purchaseorders.aggregate( [
   { $group: { _id: "$quarter", purchased: { $sum: "$qty" } } },  // group purchase orders by quarter
   { $merge : { into: "quarterlyreport", on: "_id",  whenMatched: "merge", whenNotMatched: "insert" } }
])

First stage:

The $group stage groups by the quarter and uses $sum to add the qty fields into a new purchased field. For example:

{ "_id" : "2019Q2", "purchased" : 1700 }
{ "_id" : "2019Q1", "purchased" : 1200 }

Second stage:

The $merge stage writes the documents to the quarterlyreport collection in the same database. If the stage finds an existing document in the collection that matches on the _id field, the stage merges the matching documents. Otherwise, the stage inserts the document. For the initial creation, no documents should match.

To view the documents in the collection, run the following operation:

db.quarterlyreport.find().sort( { _id: 1 } )

The collection contains the following documents:

{ "_id" : "2019Q1", "sales" : 1200, "purchased" : 1200 }
{ "_id" : "2019Q2", "sales" : 1700, "purchased" : 1700 }

Similarly, run the following aggregation pipeline against the reportedsales collection to merge the sales results into the quarterlyreport collection.

db.reportedsales.aggregate( [
   { $group: { _id: "$quarter", sales: { $sum: "$qty" } } },  // group sales by quarter
   { $merge : { into: "quarterlyreport", on: "_id",  whenMatched: "merge", whenNotMatched: "insert" } }
])

First stage:

The $group stage groups by the quarter and uses $sum to add the qty fields into a new sales field. For example:

{ "_id" : "2019Q2", "sales" : 500 }
{ "_id" : "2019Q1", "sales" : 1950 }

Second stage:

The $merge stage writes the documents to the quarterlyreport collection in the same database. If the stage finds an existing document in the collection that matches on the _id field (the quarter), the stage merges the matching documents. Otherwise, the stage inserts the document.

To view the documents in the quarterlyreport collection after the data has been merged, run the following operation:

db.quarterlyreport.find().sort( { _id: 1 } )

The collection contains the following documents:

{ "_id" : "2019Q1", "sales" : 1950, "purchased" : 1200 }
{ "_id" : "2019Q2", "sales" : 500, "purchased" : 1700 }

Use the Pipeline to Customize the Merge

The $merge can use a custom update pipeline when documents match. The whenMatched pipeline can have the following stages:

$addFields and its alias $set
$project and its alias $unset
$replaceRoot and its alias $replaceWith

An example collection votes is populated with the daily vote tally. Create the collection with the following documents:s

db.votes.insertMany([
   { date: new Date("2019-05-01"), "thumbsup" : 1, "thumbsdown" : 1 },
   { date: new Date("2019-05-02"), "thumbsup" : 3, "thumbsdown" : 1 },
   { date: new Date("2019-05-03"), "thumbsup" : 1, "thumbsdown" : 1 },
   { date: new Date("2019-05-04"), "thumbsup" : 2, "thumbsdown" : 2 },
   { date: new Date("2019-05-05"), "thumbsup" : 6, "thumbsdown" : 10 },
   { date: new Date("2019-05-06"), "thumbsup" : 13, "thumbsdown" : 16 }
])

Another example collection monthlytotals has the up-to-date monthly vote totals. Create the collection with the following document:

db.monthlytotals.insertOne({ "_id" : "2019-05", "thumbsup" : 26, "thumbsdown" : 31 } )

At the end of each day, that day's votes is inserted into the votes collection:

db.votes.insertOne(
   { date: new Date("2019-05-07"), "thumbsup" : 14, "thumbsdown" : 10 }
)

You can use $merge with an custom pipeline to update the existing document in the collection monthlytotals:

db.votes.aggregate([
   { $match: { date: { $gte: new Date("2019-05-07"), $lt: new Date("2019-05-08") } } },
   { $project: { _id: { $dateToString: { format: "%Y-%m", date: "$date" } }, thumbsup: 1, thumbsdown: 1 } },
   { $merge: {
         into: "monthlytotals",
         on: "_id",
         whenMatched:  [
            { $addFields: {
                thumbsup: { $add:[ "$thumbsup", "$$new.thumbsup" ] },
                thumbsdown: { $add: [ "$thumbsdown", "$$new.thumbsdown" ] }
            } } ],
         whenNotMatched: "insert"
   } }
])

First stage:

The $match stage finds the specific day's votes. For example:

{ "_id" : ObjectId("5ce6097c436eb7e1203064a6"), "date" : ISODate("2019-05-07T00:00:00Z"), "thumbsup" : 14, "thumbsdown" : 10 }

Second stage:

The $project stage sets the _id field to a year-month string. For example:

{ "thumbsup" : 14, "thumbsdown" : 10, "_id" : "2019-05" }

Third stage:

The $merge stage writes the documents to the monthlytotals collection in the same database. If the stage finds an existing document in the collection that matches on the _id field, the stage uses a pipeline to add the thumbsup votes and the thumbsdown votes.

This pipeline cannot directly accesses the fields from the results document. To access the thumbsup field and the thumbsdown field in the results document, the pipeline uses the $$new variable; i.e. $$new.thumbsup and $new.thumbsdown.
This pipeline can directly accesses the thumbsup field and the thumbsdown field in the existing document in the collection; i.e. $thumbsup and $thumbsdown.

The resulting document replaces the existing document.

To view documents in the monthlytotals collection after the merge operation, run the following operation:

db.monthlytotals.find()

The collection contains the following document:

{ "_id" : "2019-05", "thumbsup" : 40, "thumbsdown" : 41 }

Back

$match (aggregation)

$out (aggregation)

Definition

Note

Note

On-Demand Materialized Views

Compatibility

Syntax

Note

Tip

Tip

Tip

Note

Note

Considerations

_id Field Generation

Create a New Collection if Output Collection is Non-Existent

Note

Output to a Sharded Collection

Replace Documents ($merge) vs Replace Collection ($out)

Existing Documents and _id and Shard Key Values

Tip

Unique Index Constraints

whenMatched Pipeline Behavior

Tip

$merge and $out Comparison

Output to the Same Collection that is Being Aggregated

Warning

Restrictions

Examples

On-Demand Materialized View: Initial Creation

Note

Tip

On-Demand Materialized View: Update/Replace Data

Tip

Only Insert New Data

Merge Results from Multiple Collections

Use the Pipeline to Customize the Merge

`_id` Field Generation

Replace Documents (`$merge`) vs Replace Collection (`$out`)

Existing Documents and `_id` and Shard Key Values

`whenMatched` Pipeline Behavior

`$merge` and `$out` Comparison