Introduction to MongoDB $mergeObjects Operator

$mergeObjects is an operator in MongoDB that merges multiple documents into one. This operator can be used to merge fields and values within documents as well as elements within document arrays. Using $mergeObjects operator can quickly merge multiple documents in MongoDB.

Syntax

The syntax for the $mergeObjects operator is as follows:

{ $mergeObjects: { <expression1>, <expression2>, ... } }

The argument for the $mergeObjects operator can be one or more expressions, each of which should be a document. These expressions are merged into one document, where matching fields are overwritten and different fields are added.

Use cases

The $mergeObjects operator is typically used in the following scenarios:

• Merging two or more documents
• Merging elements within a document array

Using the $mergeObjects operator can be very useful in data aggregation and processing by merging multiple documents into one. For example, the $mergeObjects operator can be used to aggregate data from multiple documents into a new document.

Example

Assuming the following two documents:

{ _id: 1, name: "Alice", age: 25 }
{ _id: 2, name: "Bob", occupation: "Engineer" }

The $mergeObjects operator can be used to merge these two documents into a new document:

db.users.aggregate([
  {
    $project: {
      merged: {
        $mergeObjects: [
          { _id: 1, name: "Alice", age: 25 },
          { _id: 2, name: "Bob", occupation: "Engineer" }
        ]
      }
    }
  }
])

The output would be:

{ "_id" : ObjectId("..."), "merged" : { "_id" : 2, "name" : "Bob", "age" : 25, "occupation" : "Engineer" } }

In the example above, the $mergeObjects operator merges two documents into a new document, where matching fields are merged into one value and different fields are added as new keys and values to the new document.

Conclusion

The $mergeObjects operator can merge multiple documents into one. This operator can be used to merge fields and values within documents as well as elements within document arrays. Using the $mergeObjects operator in MongoDB can quickly merge multiple documents.