How to Find a Document by ID Using mgo Library in Golang?

Find ID with mgo Library in Golang

In the provided code, a common issue arises when trying to find a document by its ID using the mgo library in Golang. The key to understanding this error lies in the nature of object IDs in MongoDB.

Understanding Object IDs

Object IDs in MongoDB are unique 12-byte hexadecimal values that serve as the primary key for every document. However, when working with Golang, these IDs are often represented as 24-character hexadecimal strings. This is because these strings encode the 12 bytes of the object ID using two hexadecimal digits per byte.

The Error

The error message "ObjectIDs must be exactly 12 bytes long (got 24)" indicates that the code is attempting to use a 24-character string as an object ID. However, MongoDB expects a 12-byte object ID, which is not what the 24-character string represents.

The Solution

The solution to this issue is to use the bson.ObjectIdHex() function to convert the 24-character hexadecimal string into a valid bson.ObjectId. The modified code should look like this:

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

Now, the err variable will correctly indicate whether the document with the specified ID was found.

Converting Back

In scenarios where you have a bson.ObjectId and need to obtain its 24-character hexadecimal representation, you can use the ObjectId.Hex() method.

Additional Information

It is important to note that while the error message suggests that the length discrepancy was 24 bytes, it is actually 12 bytes. This is because the size of an object ID is 12 bytes, but the error message also includes the quotes that surround the hexadecimal string.

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