In MongoDB, each document stored in a collection requires a unique _id field that acts as a primary key. If an inserted document omits the _id field, the MongoDB driver automatically generates an ObjectId for the _id field.
The _id field has the following behavior and constraints:
- By default, MongoDB creates a unique index on the
_id field during the creation of a collection. - The
_id field is always the first field in the documents. If the server receives a document that does not have the _id field first, then the server will move the field to the beginning. - The
_id field may contain values of any BSON data type, other than an array.
While storing values for _id, we can follow these options: Use an ObjectId, Use a natural unique identifier, Generate an auto-incrementing number, Generate a UUID in our application code.
Most MongoDB driver clients will include the _id field and generate an ObjectId before sending the insert operation to MongoDB; however, if the client sends a document without an _id field, the mongod will add the _id field and generate the ObjectId.
