Validation for fallback languages.

backend/src/Squidex.Domain.Apps.Entities/Apps/Commands/UpdateLanguage.cs

@@ -18,6 +18,6 @@ namespace Squidex.Domain.Apps.Entities.Apps.Commands
 
         public bool IsMaster { get; set; }
 
-        public List<Language> Fallback { get; set; }
+        public List<Language>? Fallback { get; set; }
     }
 }

backend/src/Squidex.Domain.Apps.Entities/Apps/Guards/GuardAppLanguages.cs

@@ -69,6 +69,11 @@ namespace Squidex.Domain.Apps.Entities.Apps.Guards
                     e("Master language cannot be made optional.", nameof(command.IsMaster));
                 }
 
+                if ((languages.Master == config || command.IsMaster) && command.Fallback?.Count > 0)
+                {
+                    e("Master language cannot have fallback languages.", nameof(command.Fallback));
+                }
+
                 if (command.Fallback == null)
                 {
                     return;

backend/src/Squidex.Domain.Apps.Events/Apps/AppLanguageUpdated.cs

@@ -20,6 +20,6 @@ namespace Squidex.Domain.Apps.Events.Apps
 
         public bool IsMaster { get; set; }
 
-        public List<Language> Fallback { get; set; }
+        public List<Language>? Fallback { get; set; }
     }
 }

backend/src/Squidex.Infrastructure/Commands/DomainObjectBase.cs

@@ -149,7 +149,7 @@ namespace Squidex.Infrastructure.Commands
                 throw new DomainObjectVersionException(id.ToString(), GetType(), Version, command.ExpectedVersion);
             }
 
-            if (isUpdate == true && Version < 0)
+            if (isUpdate && Version < 0)
             {
                 throw new DomainObjectNotFoundException(id.ToString(), GetType());
             }

backend/tests/Squidex.Domain.Apps.Entities.Tests/Apps/Guards/GuardAppLanguagesTests.cs

@@ -105,10 +105,21 @@ namespace Squidex.Domain.Apps.Entities.Apps.Guards
                 new ValidationError("Master language cannot be made optional.", "IsMaster"));
         }
 
+        [Fact]
+        public void CanUpdateLanguage_should_throw_exception_if_fallback_language_defined_and_master()
+        {
+            var command = new UpdateLanguage { Language = Language.DE, Fallback = new List<Language> { Language.EN } };
+
+            var languages_1 = languages_0.Set(new LanguageConfig(Language.EN));
+
+            ValidationAssert.Throws(() => GuardAppLanguages.CanUpdate(languages_1, command),
+                new ValidationError("Master language cannot have fallback languages.", "Fallback"));
+        }
+
         [Fact]
         public void CanUpdateLanguage_should_throw_exception_if_language_has_invalid_fallback()
         {
-            var command = new UpdateLanguage { Language = Language.DE, Fallback = new List<Language> { Language.IT } };
+            var command = new UpdateLanguage { Language = Language.EN, Fallback = new List<Language> { Language.IT } };
 
             var languages_1 = languages_0.Set(new LanguageConfig(Language.EN));
 
@@ -129,7 +140,7 @@ namespace Squidex.Domain.Apps.Entities.Apps.Guards
         [Fact]
         public void CanUpdateLanguage_should_not_throw_exception_if_language_is_valid()
         {
-            var command = new UpdateLanguage { Language = Language.DE, Fallback = new List<Language> { Language.EN } };
+            var command = new UpdateLanguage { Language = Language.EN, Fallback = new List<Language> { Language.EN } };
 
             var languages_1 = languages_0.Set(new LanguageConfig(Language.EN));