Add ability to override the default field set when filling a struct

app_config.go

@@ -452,6 +452,10 @@ func (c *AppConfig) FillStruct(configStruct any) (err error) {
 
 	baseFieldSet := configStructFieldType.Tag.Get("bconf")
 
+	if overrideValue := configStructField.FieldByName("FieldSet"); overrideValue.String() != "" {
+		baseFieldSet = overrideValue.String()
+	}
+
 	for i := 0; i < configStructValue.NumField(); i++ {
 		field := configStructType.Field(i)
 

app_config_test.go

@@ -1621,6 +1621,20 @@ func TestAppConfigFillStruct(t *testing.T) {
 		t.Fatalf("problem adding field set to AppConfig: %v\n", errs)
 	}
 
+	errs = appConfig.AddFieldSet(
+		bconf.FSB().Key("ext_api").Fields(
+			bconf.FB().Key("host").Type(bconf.String).Default("0.0.0.0").Create(),
+			bconf.FB().Key("port").Type(bconf.Int).Default(8085).Create(),
+			bconf.FB().Key("read_timeout").Type(bconf.Duration).Default(10*time.Second).Create(),
+			bconf.FB().Key("db_switch_time").Type(bconf.Time).Default(dbSwitchTime).Create(),
+			bconf.FB().Key("debug_mode").Type(bconf.Bool).Default(true).Create(),
+		).Create(),
+	)
+
+	if len(errs) > 0 {
+		t.Fatalf("problem adding field set to AppConfig: %v\n", errs)
+	}
+
 	unfillableStruct := TestAPIConfig{}
 	if err := appConfig.FillStruct(unfillableStruct); err == nil {
 		t.Fatalf("expected error passing concrete struct\n")
@@ -1659,6 +1673,20 @@ func TestAppConfigFillStruct(t *testing.T) {
 	if configStruct.DebugMode != true {
 		t.Errorf("unexpected value for configStruct.Host ('%v'), expected: %v\n", configStruct.DebugMode, true)
 	}
+
+	overrideConfigStruct := &TestAPIConfig{ConfigStruct: bconf.ConfigStruct{FieldSet: "ext_api"}}
+
+	if err := appConfig.FillStruct(overrideConfigStruct); err != nil {
+		t.Fatalf("problem setting override struct values from AppConfig: %s\n", err)
+	}
+
+	if overrideConfigStruct.Host != "0.0.0.0" {
+		t.Errorf("unexpected value for overrideConfigStruct.Host ('%s'), expected: %s\n", configStruct.Host, "0.0.0.0")
+	}
+
+	if overrideConfigStruct.Port != 8085 {
+		t.Errorf("unexpected value for overrideConfigStruct.Port ('%d'), expected: %d\n", configStruct.Port, 8085)
+	}
 }
 
 func createBaseAppConfig() *bconf.AppConfig {

config_struct.go

@@ -1,3 +1,5 @@
 package bconf
 
-type ConfigStruct struct{}
+type ConfigStruct struct {
+	FieldSet string
+}