How to Set a Default Date and Time on Document Insertion in Go?

Setting Default Date on Document Insertion with Time.Time Field in Go

In Go, unlike Mongoose in Node.js, default values cannot be assigned to fields within struct definitions. To achieve the same result, there are multiple approaches available.

One method is to create a constructor-like function that instantiates a new struct and sets the CreatedAt field to the current time. This function can then be utilized to create new users, ensuring the field is set consistently.

func NewUser() *User {
    return &User{
        CreatedAt: time.Now(),
    }
}

However, this approach has limitations as it requires strict adherence to using this function for creating new users and does not capture the timestamp of the document insertion.

A more versatile solution involves implementing custom marshaling logic by implementing the bson.Getter interface. The GetBSON() method allows for modification of the value stored in the database.

type User struct {
    CreatedAt time.Time `json:"created_at" bson:"created_at"`
}

func (u *User) GetBSON() (interface{}, error) {
    if u.CreatedAt.IsZero() {
        u.CreatedAt = time.Now()
    }
    type my *User
    return my(u), nil
}

This method checks if the CreatedAt field is empty and sets it to the current time if it is. It also avoids stack overflow by returning a new type that does not implement the bson.Getter interface.

This approach ensures that the CreatedAt field is automatically updated whenever a document is saved, without overwriting it if the field already contains a value. It provides greater flexibility and aligns with the expected behavior of setting default values during document insertion.

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