What is the difference between save and using update in mongodb

Mongoose is an Object-Document Mapping (ODM) library for MongoDB that enables operating on MongoDB databases within Node.js environments using object-document mapping. Both the .save() and update() methods are used to persist document data in the database, but they have several key differences:

.save() Method

1. Create or Update: The .save() method is typically used to save a new document instance or update an existing document. If the document instance has an _id field and a matching record exists in the database, it performs an update operation. If no _id field is present or the _id does not match any record in the database, it creates a new record.

2. Full Document Operation: When using .save(), you generally operate on the entire document. Whether creating a new document or updating an existing one, you send the complete document data to the database.

3. Middleware Trigger: The .save() method triggers Mongoose middleware (such as pre and post hooks), allowing custom logic to be executed during the save process (e.g., password hashing, data validation).

4. Return Value: After executing .save(), it returns the saved document object.

5. Example:

javascript
const user = new UserModel({
  name: 'Alice',
  email: 'alice@example.com'
});
user.save(function(err, savedUser) {
  if (err) throw err;
  // savedUser is the saved document object
});

update() Method

1. Only for Update: The .update() method is exclusively used to update existing documents and cannot create new ones.

2. Partial Document Operation: When using .update(), you can update only specific fields of the document, not the entire document. This is often employed for performance optimization, as it transmits only the necessary fields.

3. No Middleware Trigger: Using .update() typically does not trigger Mongoose middleware. If specific logic needs to be executed before or after the update, it must be handled manually.

4. Return Value: After executing .update(), it returns an object containing operation results, such as the number of updated documents, rather than the updated document object.

5. Example:

javascript
UserModel.update({ _id: userId }, { $set: { name: 'Bob' } }, function(err, result) {
  if (err) throw err;
  // result is the operation result object; use result.nModified to get the number of modified documents
});

Summary

The .save() method is used for creating new documents or replacing entire documents, while .update() is used for modifying specific fields of existing documents. .save() triggers middleware and returns the saved document, whereas .update() does not trigger middleware and returns operation results. Depending on the specific application scenario and performance considerations, developers should choose the most suitable method for database operations.

Application Scenario Comparison

.save() Method Application Scenarios:

• New Document Scenario: Use .save() when adding a completely new document to the database, such as when a user registers in your application.
• Full Document Update Scenario: Use .save() when updating a document with multiple fields or when the entire document has been loaded and modified in the application layer.
• Middleware Handling Scenario: Use .save() when save logic requires middleware, such as data validation, automatic timestamp setting, or password hashing.

.update() Method Application Scenarios:

• Partial Update Scenario: Use .update() when updating one or several fields without loading the entire document, common in responsive web applications.
• Bulk Update Scenario: Use .update() to update multiple matching documents in a single operation, which is more efficient than individually loading and saving each document.
• No Middleware Scenario: Use .update() when middleware is unnecessary, such as in batch operations or background tasks, to avoid performance overhead.

Direct Modification and Replacement of Documents

• .save() Replaces Documents: When using .save(), if the document has an _id field and a matching record exists, Mongoose replaces the original document. Fields not specified in the new document are removed from the database.
• .update() Modifies Fields: Unlike .save(), .update() only modifies specified fields and leaves others unchanged, making it safer for preserving existing data.

Performance Considerations

• Performance Optimization: In large applications, .update() generally has less performance impact than .save(), especially for partial updates, as it avoids sending full document data and reduces network/memory usage.
• Atomic Operations: .update() supports MongoDB's atomic update operators (e.g., $set, $inc, $push), ensuring atomicity and preventing data inconsistencies in concurrent scenarios.

Summary

Depending on your needs, choose between .save() or .update()—or other Mongoose methods like .updateOne(), .updateMany(), or .findOneAndUpdate()—based on whether you need full document handling, middleware triggering, performance optimization, or atomicity. The specific requirements, such as document scope, middleware needs, and concurrency considerations, should guide your decision.

An often-overlooked detail: Concurrency

As mentioned earlier, when using doc.save(), you must first load the document into memory, modify it, and then send the changes to the MongoDB server.

When editing documents concurrently in this manner, issues arise:

• A loads the document (v1)
• B loads the document (v1)
• B saves changes to the document (now v2)
• A saves changes to the outdated (v1) document
• A encounters a VersionError from Mongoose because the document has been modified since it was last loaded from the collection.

When performing such atomic operations like Model.updateOne(), concurrency is not an issue because the operation is fully executed on the MongoDB server, which handles concurrency control to some extent.

Therefore, be cautious!

Mongoose offers a useful feature called middleware, which includes pre and post hooks. Middleware executes when you call save but not during update. For example, if you want to hash the password in the User schema whenever it is modified, you can use the pre middleware to perform this operation, as shown below.

shell
UserSchema.pre('save', function(next) {
    var user = this;
    // only hash the password if it has been modified (or is new)
    if (!user.isModified('password')) {
        console.log('password not modified');
        return next();
    }
    console.log('password modified');
    // generate a salt
    bcrypt.genSalt(10, function(err, salt) {
        if (err) {
            return next(err);
        }

        // hash the password along with our new salt
        bcrypt.hash(user.password, salt, function(err, hash) {
            if (err) {
                return next(err);
            }
            // override the cleartext password with the hashed one
            user.password = hash;
            next();
        });
    });
});

Another useful example is setting the lastModified field for each document. You can find this documentation at http://mongoosejs.com/docs/middleware.html.

Key differences:

• As noted elsewhere, save is more efficient than update because it avoids loading the entire document. Unlike find and save, it does not load the document.
• Mongoose update translates to MongoDB update, but Mongoose save translates to MongoDB insert (for new documents) or update.
• Note that for save, Mongoose internally compares the document as shown here and only sends the fields that have actual changes. This is beneficial for atomicity.
• By default, validation does not run for update as mentioned here, but it can be enabled.
• The middleware API (pre and post hooks) is different.

First, let's introduce two concepts. Your application is the client, MongoDB is the server.

The main difference is that .save() is used when you already have an object in your client code, or you must retrieve data from the server before writing it back, and you are writing back the entire document.

On the other hand, .update() does not require loading data from the server to the client. All interactions occur on the server side without retrieving data to the client. Therefore, .update() is very efficient when adding fields to existing documents.

Additionally, the multi parameter in .update() allows performing operations on multiple documents that match the query.

When used as a method call, some convenience methods are not available with .update(), but the benefits of certain operations come with a trade-off you must accept. For more details on this and available options, see the documentation here.

In summary, .save() is the client-side interface, while .update() is the server-side interface.