How Can I Set a Default Date and Time for a Field in a Go Mongoose Document?

Set Default Date When Inserting Document with Time.Time Field

In mongoose, setting a default value for a Date field is straightforward, but in Go using mgo, things are a bit different.

Why Can't You Set Default Values for Fields in Go?

Go does not allow default values for fields. Upon creation, they will have the zero-value of their type.

Solution 1: Constructor-Like Function

One approach is to create a constructor-like function that initializes the CreatedAt field. However, it doesn't handle cases where the timestamp needs to match the database's timestamp.

Solution 2: Custom Marshaling Logic

A better solution involves implementing custom bson.Getter logic.

GetBSON() Function

The GetBSON() function in the Go implementation provides a value to be saved. In this case, we want to set the CreatedAt field to the current time before saving.

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

Creating a New Type

Note that a new type, my, is created and returned to prevent endless recursion.

Handling Existing Records

Adding a check ensures that the CreatedAt field is only set if it's the zero value, preventing overwrites on existing records:

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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