Added BeforeSave

models/participant.go

@@ -2,6 +2,8 @@ package models
 
 import (
 	"time"
+
+	"gorm.io/gorm"
 )
 
 type DBParticipant struct {
@@ -30,3 +32,11 @@ type DBParticipant struct {
 func (DBParticipant) TableName() string {
 	return "gc_participants"
 }
+
+func (p *DBParticipant) BeforeSave(tx *gorm.DB) error {
+	// Convert empty string to nil
+	if p.UserId != nil && *p.UserId == "" {
+		p.UserId = nil
+	}
+	return nil
+}