What's the difference between replaceOne() and updateOne() in MongoDB?

MongoDB bulk operations have two options:

1. Bulk.find.updateOne() > Adds a single document update operation to a bulk operations list. The operation can either replace an existing document or update specific fields in an existing document.

2. Bulk.find.replaceOne() > Adds a single document replacement operation to a bulk operations list. Use the Bulk.find() method to specify the condition that determines which document to replace. The Bulk.find.replaceOne() method limits the replacement to a single document.

According to the documentation, both of these two methods can replace a matching document. Do I understand correctly, that updateOne() is more general purpose method, which can either replace the document exactly like replaceOne() does, or just update its specific fields?

With replaceOne() you can only replace the entire document, while updateOne() allows for updating fields.

Since replaceOne() replaces the entire document - fields in the old document not contained in the new will be lost. With updateOne() new fields can be added without losing the fields in the old document.

For example if you have the following document:

{
   "_id" : ObjectId("0123456789abcdef01234567"),
   "my_test_key3" : 3333
}

Using:

replaceOne({"_id" : ObjectId("0123456789abcdef01234567")}, { "my_test_key4" : 4})

results in:

{
   "_id" : ObjectId("0123456789abcdef01234567"),
   "my_test_key4" : 4.0
}

Using:

updateOne({"_id" : ObjectId("0123456789abcdef01234567")}, {$set: { "my_test_key4" : 4}})

results in:

{
   "_id" : ObjectId("0123456789abcdef01234567"),
   "my_test_key3" : 3333.0,
   "my_test_key4" : 4.0
}

Note that with updateOne() you can use the update operators on documents.

replaceOne() replaces the entire document, while updateOne() allows for updating or adding fields. When using updateOne() you also have access to the update operators which can reliably perform updates on documents. For example two clients can "simultaneously" increment a value on the same field in the same document and both increments will be captured, while with a replace the one may overwrite the other potentially losing one of the increments.

Since replaceOne() replaces the entire document - fields in the old document not contained in the new will be lost. With updateOne() new fields can be added without losing the fields in the old document.

For example if you have the following document:

{
   "_id" : ObjectId("0123456789abcdef01234567"),

   "my_test_key3" : 3333
}

Using:

replaceOne({"_id" : ObjectId("0123456789abcdef01234567")}, { "my_test_key4" : 4})

results in:

{
   "_id" : ObjectId("0123456789abcdef01234567"),
   "my_test_key4" : 4.0
}

Using:

updateOne({"_id" : ObjectId("0123456789abcdef01234567")}, {$set: { "my_test_key4" : 4}})

results in:

{
   "_id" : ObjectId("0123456789abcdef01234567"),
   "my_test_key3" : 3333.0,
   "my_test_key4" : 4.0
}

db.collection.replaceOne() does exactly the same thing as db.collection.updateOne().

The main difference is that db.collection.replaceOne()'s data that are being edited will have to go back and forth to the server, whereas db.collection.UpdateOne() will request only the filtered ones and not the whole document!