fix

src/mixin.ts

@@ -22,14 +22,14 @@ function applyMixin(
   }
   visited.add(sourceClass);
   Object.getOwnPropertyNames(sourceClass.prototype).forEach((name) => {
-    if (mixedClass.prototype[name] != null) {
+    if (mixedClass.prototype[name] == null) {
       mixedClass.prototype[name] = sourceClass.prototype[name];
     }
   });
   const schemaKeys = reflector.getArray('SchemaMetaKey', sourceClass);
   for (const name of schemaKeys) {
     const schemaMeta = reflector.get('SchemaMeta', sourceClass, name);
-    if (!schemaMeta == null) {
+    if (schemaMeta == null) {
       continue;
     }
     const existingMetadata = reflector.get('SchemaMeta', mixedClass, name);

tests/mixin.spec.ts

@@ -11,11 +11,13 @@ export class Sleeve {
   }
 }
 
-@RegisterSchema()
-class Dress {
+class WearingWithSleeves {
   @SchemaProperty({ type: Sleeve })
   sleeves: Sleeve[];
+}
 
+@RegisterSchema()
+class Dress extends WearingWithSleeves {
   @SchemaProperty()
   color: string;