How do i 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 join two documents in MongoDB?For performing MongoDB Join two collections, you must use the $lookup operator. It is defined as a stage that executes a left outer join with another collection and aids in filtering data from joined documents. For example, if a user requires all grades from all students, then the below query can be written: Students.
How do I merge objects in MongoDB?$mergeObjects overwrites the field values as it merges the documents. If documents to merge include the same field name, the field, in the resulting document, has the value from the last document merged for the field.
Can we join two collections in MongoDB?Hi, Yes, you can join 2 collections with Aggregation Framework and $unionWith stage. Here are the docs with syntax and examples, so you can check how to do it.
How can I combine data from multiple collections into one collection?You need to have some key in both collections that you can use as an _id.. mongodump collection1.. collection2. rename(collection1). mongorestore.. |