How do i merge two mongodb databases?

Note

This page describes the $merge stage, which outputs the aggregation pipeline results to a collection. For the $mergeObjects operator, which merges documents into a single document, see $mergeObjects.

New in version 4.2.

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.

$merge has the following syntax:

{ $merge: {
     into:  -or- { db: , coll:  },
     on:  -or- [ , ...],  // Optional
     let: ,                                         // Optional
     whenMatched: ,  // Optional
     whenNotMatched:                      // 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:  } // Output collection is in the same database

The $merge takes a document with the following fields:

on

Optional. Field or fields that act as a unique identifier for a document. The identifier determines if a results document matches an 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.

• The unique index cannot be a partial 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.