Add error explanations when preloading assocations w/o foreign fields, close #4356

callbacks/preload.go

@@ -1,6 +1,7 @@
 package callbacks
 
 import (
+	"fmt"
 	"reflect"
 
 	"gorm.io/gorm"
@@ -144,7 +145,8 @@ func preload(db *gorm.DB, rel *schema.Relationship, conds []interface{}, preload
 			fieldValues[idx], _ = field.ValueOf(elem)
 		}
 
-		for _, data := range identityMap[utils.ToStringKey(fieldValues...)] {
+		if datas, ok := identityMap[utils.ToStringKey(fieldValues...)]; ok {
+			for _, data := range datas {
 				reflectFieldValue := rel.Field.ReflectValueOf(data)
 				if reflectFieldValue.Kind() == reflect.Ptr && reflectFieldValue.IsNil() {
 					reflectFieldValue.Set(reflect.New(rel.Field.FieldType.Elem()))
@@ -162,5 +164,8 @@ func preload(db *gorm.DB, rel *schema.Relationship, conds []interface{}, preload
 					}
 				}
 			}
+		} else {
+			db.AddError(fmt.Errorf("failed to assign association %#v, make sure foreign fields exists", elem.Interface()))
+		}
 	}
 }