Why Is My Mgo '_id' Field Returning an Empty String?

Retrieving "_id" Values with Mgo and Go

This question addresses an issue where the "_id" value of a MongoDB document is consistently returned as an empty string when using mgo with Go.

Problem Description:

The user defined a struct to represent MongoDB documents, where the "_id" field is annotated with both json:"id" and bson:"_id,omitempty". When fetching the documents from the database, the "_id" field is always set to an empty string.

Debugging and Solution:

After examining the code, it was found that there was a subtle error in the annotation of the "_id" field. In the bson:"_id,omitempty" annotation, a tab was used as whitespace instead of a space. This caused the mgo driver to incorrectly interpret the annotation.

The following code snippet demonstrates the correct syntax:

type Article struct {
    Id bson.ObjectId `json:"id" bson:"_id,omitempty"`
}

With this correction, the _id value is now properly retrieved from the database and returned as expected.

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