Field Transformers

Field transformers are an adapter between MongoDB field and Go struct field. They help in transforming field types in both directions i.e. from entity model to mongo doc and vice versa while building intermediate BSON document.

note

A field transformer is defined by the tag mgoType.

mgod supports the following field transformers -

ID

• Tag Value: id

It is a transformer that converts a field of type string in Go struct to primitive.ObjectID for MongoDB document and vice versa.

Example

Type with id transformer.

type User struct {
	ID   string `bson:"_id" mgoType:"id"`
	Name string
}

// id = "65697705d4cbed00e8aba717"
id := primitive.NewObjectID().Hex()
userDoc := User{
	ID: id,
	Name: "Gopher",
}

user, _ := userModel.InsertOne(context.TODO(), userDoc)

Output:

{
	"_id": ObjectId("65697705d4cbed00e8aba717"),
	"name": "Gopher"
}

_id_ field will be of type ObjectId instead of String in MongoDB.

Invalid user doc -

userDoc := User{
	ID: "randomId"
	Name: "Gopher",
}

Inserting this doc will throw error as ID field cannot be converted to primitive.ObjectID.

---

Type without id transformer.

type User struct {
	ID   string
	Name string
}

userDoc := User{
	ID: "randomId",
	Name: "Gopher",
}

This is a valid doc now because there is no transformer applied on ID field. Also, note that ID field will be converted to id instead of _id because BSON tag is not present.

Date

• Tag Value: date

It is a transformer that converts a field of type string in ISO 8601 format to primitive.DateTime for MongoDB document and vice versa.

Example

Type with date transformer.

type User struct {
	Name     string
	JoinedOn string `bson:"joinedOn" mgoType:"date"`
}

// joinedOn = "2023-12-01T11:32:19.290Z"
joinedOn, _ := dateformatter.New(time.Now()).GetISOString()
userDoc := User{
	Name: "Gopher",
	JoinedOn: joinedOn,
}

user, _ := userModel.InsertOne(context.TODO(), userDoc)

Output:

{
	"_id": ObjectId("65697705d4cbed00e8aba717"),
	"name": "Gopher",
	"joinedOn": ISODate("2023-12-01T11:32:19.290Z")
}

joinedOn field will be of type Date instead of String in MongoDB.

Invalid user doc -

userDoc := User{
	Name: "Gopher",
	JoinedOn: "2023-12-01",
}

Inserting this doc will throw error as JoinedOn field is not in expected ISO 8601 format.

---

Type without date transformer.

type User struct {
	Name     string
	JoinedOn string `bson:"joinedOn"`
}

userDoc := User{
	Name: "Gopher",
	JoinedOn: "2023-12-01",
}

This is a valid doc now because there is no transformer applied on JoinedOn field.