为什么我无法使用golang的mgo库找到ID？

Question

I am using mgo library for mongo operationg in golang and here is my code :

session.SetMode(mgo.Monotonic, true)
coll := session.DB("aaaw_web").C("cron_emails")
var result Result
fmt.Printf("%v", message.ID)
err = coll.FindId(bson.ObjectId(message.ID)).One(&result)
fmt.Printf("%v", result)
fmt.Println(err)

I am getting this output :

595f2c1a6edcba0619073263
{ObjectIdHex("") 0   0  0    0 {         0    false 0    } 0 0 0  0 0 0 0}
ObjectIDs must be exactly 12 bytes long (got 24)
not found

But I checked, document exists in mongo, but getting here no result, any idea what am i missing ...

Answer 1

As the error message hints, an object id is exactly 12 bytes long (12 bytes of data). The 24 char long ID you see printed is the hexadecimal representation of the 12 bytes of the ID (1 byte => 2 hexa digits).

Use the bson.ObjectIdHex() function to obtain a value of bson.ObjectId if the hex representation is available.

err = coll.FindId(bson.ObjectIdHex(message.ID)).One(&result)

For the reverse direction, you may use the ObjectId.Hex() method, detailed in this answer: Obtain ObjectIdHex value from mgo query

What you did in your code is a simple type conversion (given that message.ID is of type string), and the syntax is valid because the underlying type of bson.ObjectId is string, so that basically interprets the 24 characters as bson.ObjectId type, but it is an invalid ObjectId value because it will be 24 bytes and not 12.