How to Implement Custom Marshalling and Unmarshalling to Access MongoDB Data as Go Time in Go?

Accessing MongoDB from Go: Working with Custom Data Types

When interacting with MongoDB using Go, you may encounter situations where you need to modify or convert data values during marshalling and unmarshalling. This can occur when data is stored in a specific format in MongoDB but requires a different format in your Go struct.

Let's consider an example where EndDate is stored as a string in MongoDB, but you wish to access it as a Go Time in your clientConfigData struct.

type clientConfigData struct {
    SMTPAssoc      int       `bson:"smtp_assoc"`
    PlanType       string    `bson:"plan_type"`
    EndDate        string    `bson:"end_date"`
}

To implement custom marshalling and unmarshalling, define a bson.Getter and bson.Setter interface.

import (
    "context"
    "time"

    "github.com/mongodb/mongo-go-driver/bson"
)

type clientConfigData struct {
    SMTPAssoc  int       `bson:"smtp_assoc"`
    PlanType   string    `bson:"plan_type"`
    EndDateStr string    `bson:"end_date"`
    EndDate    time.Time `bson:"-"` // Excluded from MongoDB
}

const endDateLayout = "2006-01-02 15:04:05"

// bson.Setter implementation
func (c *clientConfigData) SetBSON(raw bson.Raw) (err error) {
    type my clientConfigData
    if err = raw.Unmarshal((*my)(c)); err != nil {
        return
    }
    c.EndDate, err = time.Parse(endDateLayout, c.EndDateStr)
    return
}

// bson.Getter implementation
func (c *clientConfigData) GetBSON() (interface{}, error) {
    c.EndDateStr = c.EndDate.Format(endDateLayout)
    type my *clientConfigData
    return my(c), nil
}

// Custom code to query MongoDB
func FindConfig(ctx context.Context, client *mongo.Client) (*clientConfigData, error) {
    var configRes *clientConfigData
    err := client.Database("test").Collection("clientconfig").FindOne(ctx, bson.M{}).Decode(&configRes)
    if err != nil {
        return nil, errors.Wrap(err, "finding config collection")
    }
    return configRes, nil
}

In the SetBSON method, we first unmarshal the raw value and then parse the EndDateStr field to populate the EndDate field. In the GetBSON method, we format the EndDate field to a string before returning it.

Using this custom logic, you can now access the EndDate as a Go Time from MongoDB.

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