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Oct 12, 2019

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.

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