Set default date when inserting document with time.Time field

Question

In mongoose ( node.js ) I can define a model schema with a default Date.now like so:

...
type: Date,
default: Date.now
...

How do I achieve the same without having to insert the time.Time every time I create a document with mgo ?

type User struct {
    CreatedAt   time.Time `json:"created_at" bson:"created_at"` // Make this field filled automatically with time.Now() every time a document of this `struct` is inserted
}

Answer 1

In Go you can't define default values for fields, they will always be the zero-value of their type when a new struct value is created (unless you use a composite literal where you can give a different value explicitly).

So one option would be to create a constructor-like function NewUser() which would set this field, and use always this function to create new users:

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

Of course this can't be forced, and also this will hold the timestamp of the User struct value creation and not when it is saved.

Another, better approach is to use a custom marshaling logic.

You can write custom marshaling logic by implementing bson.Getter . GetBSON() is responsible to provide a value that will actually be saved. We want the same User instance to be saved, just its CreatedAt field set prior:

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
}

Note that a new my type is created and returned. The reason for this is to avoid stack overflow. Simply returning a value of type *User is bad, because it implements bson.Getter , so GetBSON() would get called endlessly. The new my type does not have this method, so endless "recursion" does not happen (the type keyword creates a new type, and it does not "inherit" methods of the underlying type).

Note that this solution will also overwrite (re-set) the CreatedAt field) even if you just want to re-save a User . So we should add a check whether the CreatedAt field is filled, and only set it if it's the zero value:

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

Also see related / similar question: Accesing MongoDB from Go