How to merge two documents in mongodb
Docs Home → MongoDB Manual Show NoteThis page describes the $merge New in version 4.2. Writes the results of the aggregation pipeline to a specified collection. The
The
For a comparison with the NoteOn-Demand Materialized Views
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.
For example:
If using all default options for
The
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:
For the specified field or fields:
The default value for on depends on the output collection:
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 If the existing output collection is a sharded collection, the on identifier defaults to all the shard key fields and the whenMatched Optional. The behavior of You can specify either:
"keepExisting" Keep the existing document in the output collection. "merge" (Default) Merge the matching documents (similar to the
For example, if the output collection has the document:
And the aggregation results has the document:
Then, the merged document is:
When performing a merge, the merged document cannot result in a modification of the TipTo
avoid this error, if the on field does not include the How do I merge two documents in MongoDB?For performing MongoDB Join two collections, you must use the $lookup operator. It is defined as a stage that executes a left outer join with another collection and aids in filtering data from joined documents.
How do I merge two MongoDB databases?Use mongodump --db [dbname] on the source computer to export all collections of the database into files which are stored in the directory dump/[collection]. bson . Copy the files to the target computer and then use mongorestore --db [dbname] [collection].
How do I join a collection in MongoDB?Join Collections
MongoDB is not a relational database, but you can perform a left outer join by using the $lookup stage. The $lookup stage lets you specify which collection you want to join with the current collection, and which fields that should match.
How do I merge two arrays in MongoDB aggregate?MongoDB provides different types of array expression operators that are used in the aggregation pipeline stages and $concatArrays operator is one of them. This operator is used to concatenate two or more arrays and return a concatenated array.
|