onvalidation for letter

entity/Letter_entity/Letter_entity.aod

@@ -2,6 +2,7 @@
 <entity xmlns="http://www.adito.de/2018/ao/Model" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" VERSION="1.3.11" xsi:schemaLocation="http://www.adito.de/2018/ao/Model adito://models/xsd/entity/1.3.11">
   <name>Letter_entity</name>
   <majorModelMode>DISTRIBUTED</majorModelMode>
+  <onValidation>%aditoprj%/entity/Letter_entity/onValidation.js</onValidation>
   <recordContainer>jdito</recordContainer>
   <entityFields>
     <entityProvider>

entity/Letter_entity/onValidation.js

+import("system.vars");
+import("system.translate");
+import("system.result");
+
+if (!vars.get("$field.bindata") && !vars.get("$field.DOCUMENT_TEMPLATE"))
+{
+    result.string(translate.text("Please select a documenttemplate or upload a new file."))
+}
\ No newline at end of file