With mongodb, and mgo, you are often expected to generate the _id-value yourself using bson.NewObjectId.
The MongoDB manual states:
If the document does not specify an _id field, then MongoDB will add the _id field and assign a unique ObjectId for the document before inserting. Most drivers create an ObjectId and insert the _id field, but the mongod will create and populate the _id if the driver or application does not.
Simply, creating the _id yourself is the way to go!