How does _id field behave?

In MongoDB, the _id field is a special field that uniquely identifies each document within a collection. Here are some key behaviors of the _id field:

1. Uniqueness: Each document in a collection must have a unique _id value. If you attempt to insert a document with a duplicate _id, MongoDB will throw an error.

2. Default Value: If you do not provide an _id field when inserting a document, MongoDB automatically generates an ObjectId for it. This ObjectId is a 12-byte identifier that is unique across collections and databases.

3. Indexing: The _id field is automatically indexed, which means that queries that filter by _id are very efficient.

4. Projection: When you perform a query, if you do not explicitly include or exclude the _id field in the projection, it is included by default. If you want to exclude it, you need to specify _id: 0 in the projection.

5. Custom Values: You can assign a custom value to the _id field when inserting a document, as long as it is unique. This can be useful if you want to use a specific identifier, such as a username or a UUID.

Example of inserting a document with a custom _id:

db.collection.insertOne({ _id: "user123", name: "John Doe" });

In this case, the document will have _id set to "user123" instead of the automatically generated ObjectId.