How to Set Default Date Values for Time.Time Fields in Mgo Documents?

Default Date Insertion for Mgo Documents with Time.Time Field

Introduction

In MongoDB, it is possible to set default values for document fields. This can be useful for automatically populating fields with specific values during document creation, such as a timestamp field.

Issue

In Mgo, the Go driver for MongoDB, there is no direct way to define default values for fields as in the Mongoose JavaScript model. Developers are looking for a solution to automatically insert a date or time value into a document's field upon creation.

Solution

There are two main approaches to setting default date values in Mgo:

1. Constructor Function:

   • Create a constructor-like function that sets the desired field value, such as CreatedAt.
   • Always use this function to create new document instances.
   • This approach provides control over when the default value is set, but it requires manual intervention.

2. Custom Marshaling:

   • Implement the bson.Getter interface in the custom struct to provide a value before it is actually saved.
   • Use GetBSON() to return a copy of the struct with the desired field value set (if it's not already set).
   • This approach is more automatic, but it requires careful consideration to avoid potential stack overflows or unintended overwriting of existing values.

Implementation Example

For the custom marshaling approach, here is an example implementation:

import (
    "context"
    "time"

    "github.com/globalsign/mgo/bson"
)

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
}

func main() {
    session, err := mgo.Dial("localhost:27017")
    if err != nil {
        panic(err)
    }
    defer session.Close()

    c := session.DB("mydb").C("users")

    // Create a new user with a default CreatedAt timestamp
    user := User{}
    err = c.Insert(user)
    if err != nil {
        panic(err)
    }
}

In this example, when a new User document is inserted into the database, the CreatedAt field will be automatically set to the current time. This is achieved by overriding the GetBSON() method to provide a modified struct value when the database driver attempts to serialize the object for insertion.

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