Why Am I Getting \"ObjectIDs must be exactly 12 bytes long (got 24)\" When Retrieving Documents with mgo?

Troubleshooting Mongo ID Retrieval with mgo in Go

In using the mgo library for MongoDB operations in Go, you may encounter difficulties in finding a document by its ID. This article aims to address one such issue, providing a solution to the error "ObjectIDs must be exactly 12 bytes long (got 24)."

Problem:

When attempting to retrieve a document by its ID using the FindId method, you receive an error indicating that the specified ID is 24 bytes long instead of the expected 12 bytes. Despite verifying the existence of the document in MongoDB, you fail to retrieve it.

Answer:

The error message provides a crucial hint: "ObjectIDs must be exactly 12 bytes long (got 24)." This means that the ID you are using is not a valid ObjectId value.

In Go, MongoDB's object ID is represented as a 12-byte value. However, the ID you are using is 24 characters long. The discrepancy arises from the hexadecimal representation of the ID, where each byte is represented as two hexadecimal digits.

To resolve the issue, you need to utilize the bson.ObjectIdHex function to convert the 24-character hexadecimal ID into a valid bson.ObjectId value. The corrected code should look like this:

<code class="go">err = coll.FindId(bson.ObjectIdHex(message.ID)).One(&result)</code>

Alternatively, you can use the ObjectId.Hex method to obtain the hexadecimal representation of the ObjectId value. This will allow you to work with the ID in string format when needed.

By following these steps, you can effectively retrieve documents by their ID using the mgo library, ensuring that the specified ID is in the correct format and size.

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