How to Fix \"ObjectIDs must be exactly 12 bytes long\" Error When Using FindId() in Golang\'s Mgo Library?

Troubleshooting FindId() Usage in Golang's Mgo Library

When attempting to retrieve a document using the FindId() function in mgo, an error may arise due to an invalid ObjectId value. The error message "ObjectIDs must be exactly 12 bytes long" indicates that the provided ID is not in the expected format.

Understanding ObjectId Format

An ObjectId in MongoDB is a 12-byte value that uniquely identifies a document. It is typically represented as a hexadecimal string with 24 characters. The mgo library expects the ObjectId to be provided as either a bson.ObjectId object or a 12-byte binary representation.

Resolving the Issue

In the provided code, message.ID is a string containing the hexadecimal representation of the ObjectId. To use this value with FindId(), it must be converted to a bson.ObjectId object using the bson.ObjectIdHex() function:

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

By using bson.ObjectIdHex(), you ensure that the provided string is correctly interpreted and converted into a valid ObjectId value, resolving the error.

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