Differenct between update and replace

Let’s compare update and replace operations in MongoDB. This explanation is based on a review of resources.

Update Operations

db.collection.update(
   <query>,
   <update>,
   {
     upsert: <boolean>,
     multi: <boolean>,
     writeConcern: <document>,
     collation: <document>,
     arrayFilters: [ <filterdocument1>, ... ]
   }
)

db.collection.updateOne(
   <filter>,
   <update>,
   {
     upsert: <boolean>,
     writeConcern: <document>,
     collation: <document>,
     arrayFilters: [ <filterdocument1>, ... ]
   }
)

db.collection.updateMany(
   <filter>,
   <update>,
   {
     upsert: <boolean>,
     writeConcern: <document>,
     collation: <document>,
     arrayFilters: [ <filterdocument1>, ... ]
   }
)

The update functionality in MongoDB offers three distinct methods: update, updateOne, and updateMany. These methods primarily serve to modify existing data within a collection. Notably, the update method includes a multi option, which determines whether to update multiple documents or just one. The updateOne and updateMany methods implicitly handle single and multiple document updates, respectively. Therefore, the update method, when combined with the multi option, can effectively replicate the behavior of both updateOne and updateMany.

update operations function similarly to standard database update commands. They modify specific fields within a document. The upsert option allows for the insertion of a new document if no matching document is found based on the filter criteria.

Consider the following JSON document:

{
  "id": 12345,
  "name": "test",
  "age": 1
}

To update the age field to 10, you would use the $set operator:

db.collection.update( {id: 12345}, {$set: {age: 10}}, {multi: false})

This command sets the age field of the document with id: 12345 to 10, resulting in the following document:

{
  "id": 12345,
  "name": "test",
  "age": 10
}

Within the filter, operators like $gte (greater than or equal to) and $lte (less than or equal to) can be utilized, especially when combined with {multi: true} for updating multiple documents. Additionally, the $unset operator can be used to remove a field from a document.

Replace Operations

Starting from version 3.2, MongoDB introduced the replaceOne method.

db.collection.replaceOne(
   <filter>,
   <replacement>,
   {
     upsert: <boolean>,
     writeConcern: <document>,
     collation: <document>
   }
)

Unlike the update operation, replaceOne is the only available method for replacing documents. This means you cannot replace multiple documents with a single command. This distinguishes it significantly from the update operation.

Another key difference lies in the scope of the operation. While update modifies specific fields, replaceOne replaces the entire document with a new document.

Consider the following JSON document:

{
  "id": 12345,
  "name": "test",
  "age": 1
}

To replace the document’s data, setting age to 10, you would use:

db.collection.replaceOne( {id: 12345}, {age: 10});

This command replaces the entire document with id: 12345 with a new document containing only the age field set to 10, resulting in:

{
  "age": 10
}

With the introduction of replaceOne in version 3.2, the distinction between update and replace became more defined. Consequently, to modify specific fields within a document, the update operation should be used. To replace an entire document, the replaceOne operation should be employed.