What is the result of inserting a document without an _id field in MongoDB?

In MongoDB, every document must have a unique identifier that is stored in the _id field. If a document is inserted without explicitly specifying an _id field, MongoDB automatically generates a unique ObjectId for that document. This automatic generation ensures that each document can be uniquely identified, which is critical for operations such as updates and deletions.

The automatic generation process allows for efficient handling and management of documents, as developers do not need to manually create an _id for each document. The generated ObjectId consists of a timestamp, machine identifier, process identifier, and a counter, ensuring that it is unique within the database.

This capability enhances the usability of MongoDB, allowing developers to focus on other aspects of their document structure without worrying about the uniqueness of the identifier. Inserting a document without an _id field will not lead to rejection or failure of the operation, as MongoDB seamlessly handles this by creating the necessary identifier.