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Using maps as changesets
You are looking at the docs for an older version (v0.17.42). The latest version is v0.17.49.

Occasionally you need to distinguish presence from nil (undefined vs null). In gqlgen we do this using maps:

type Mutation {
	updateUser(id: ID!, changes: UserChanges!): User

input UserChanges {
	name: String
	email: String

Then in config set the backing type to map[string]interface{}

    model: "map[string]interface{}"

After running go generate you should end up with a resolver that looks like this:

func (r *mutationResolver) UpdateUser(ctx context.Context, id int, changes map[string]interface{}) (*User, error) {
	u := fetchFromDb(id)
	/// apply the changes
	return u, nil

Please note that map values are automatically coerced to the types defined in the schema. This means that optional, nested inputs or scalars will conform to their expected types.

We often use the mapstructure library to directly apply these changesets directly to the object using reflection:

func ApplyChanges(changes map[string]interface{}, to interface{}) error {
	dec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
		ErrorUnused: true,
		TagName:     "json",
		Result:      to,
		ZeroFields:  true,
		// This is needed to get mapstructure to call the gqlgen unmarshaler func for custom scalars (eg Date)
		DecodeHook: func(a reflect.Type, b reflect.Type, v interface{}) (interface{}, error) {
			if reflect.PtrTo(b).Implements(reflect.TypeOf((*graphql.Unmarshaler)(nil)).Elem()) {
				resultType := reflect.New(b)
				result := resultType.MethodByName("UnmarshalGQL").Call([]reflect.Value{reflect.ValueOf(v)})
				err, _ := result[0].Interface().(error)
				return resultType.Elem().Interface(), err

			return v, nil

	if err != nil {
		return err

	return dec.Decode(changes)