In MongoDB, the _id field is a unique identifier assigned to each document in a collection. It serves as the primary key for the document, and is required for all documents.
The _id field can be assigned manually by the user, or it can be automatically generated by MongoDB. If the user does not provide a value for _id when inserting a document, MongoDB will create a unique ObjectId and assign it to the document as the _id field.
An ObjectId is a 12-byte value consisting of a time stamp, a machine identifier, a process id, and a counter. This provides a high degree of uniqueness even across distributed systems, making it a reliable choice for generating _id values.
Here’s an example of inserting a document into a MongoDB collection with a manually-assigned _id field:
db.users.insertOne({_id: "john_doe", name: "John Doe", age: 42})And here’s an example of inserting a document with an automatically-generated ObjectId:
db.users.insertOne({name: "Jane Smith", age: 35})In the second example, MongoDB will generate an ObjectId for the _id field of the document.