How Can I Set a Default Current Timestamp for a Date Field in Mongoose Using Go?

Custom Date Default with Mgo

In Mongoose, it's possible to define a schema with a default Date.now value. However, in Go, default values aren't permitted for fields, so a different approach is needed.

Constructor Function

One solution is to create a constructor-like function, such as NewUser(), which sets the CreatedAt field to the current time:

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

However, this method won't enforce the creation of new instances through this function or timestamp the instance when it's saved.

Custom Marshaling

A more robust solution involves implementing custom marshaling logic via the bson.Getter interface. This allows for modifying the value stored before it's saved:

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

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

In the GetBSON() method, the CreatedAt field is updated with the current time. To avoid stack overflow, a new type, my, is introduced. This new type doesn't inherit the bson.Getter implementation, breaking the recursive loop.

An additional check can be added to ensure the CreatedAt field is only set if it's initially empty:

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

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