Fix generation of oneOf interfaces for oneOf usage in properties (OpenAPITools#5400)

* Fix generation of oneOf interfaces for oneOf usage in properties

* Only generate oneOf interface models when oneOf is defined

Author: Slavek Kabrda

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -744,13 +744,34 @@ public void preprocessOpenAPI(OpenAPI openAPI) {
                 }
             }
 
+            // also add all properties of all schemas to be checked for oneOf
+            Map<String, Schema> propertySchemas = new HashMap<String, Schema>();
+            for (Map.Entry<String, Schema> e : schemas.entrySet()) {
+                Schema s = e.getValue();
+                Map<String, Schema> props = s.getProperties();
+                if (props == null) {
+                    props = new HashMap<String, Schema>();
+                }
+                for (Map.Entry<String, Schema> p : props.entrySet()) {
+                    propertySchemas.put(e.getKey() + "/" + p.getKey(), p.getValue());
+                }
+            }
+            schemas.putAll(propertySchemas);
+
             // go through all gathered schemas and add them as interfaces to be created
             for (Map.Entry<String, Schema> e : schemas.entrySet()) {
                 String n = toModelName(e.getKey());
                 Schema s = e.getValue();
                 String nOneOf = toModelName(n + "OneOf");
                 if (ModelUtils.isComposedSchema(s)) {
-                    addOneOfNameExtension((ComposedSchema) s, n);
+                    if (e.getKey().contains("/")) {
+                        // if this is property schema, we also need to generate the oneOf interface model
+                        addOneOfNameExtension((ComposedSchema) s, nOneOf);
+                        addOneOfInterfaceModel((ComposedSchema) s, nOneOf);
+                    } else {
+                        // else this is a component schema, so we will just use that as the oneOf interface model
+                        addOneOfNameExtension((ComposedSchema) s, n);
+                    }
                 } else if (ModelUtils.isArraySchema(s)) {
                     Schema items = ((ArraySchema) s).getItems();
                     if (ModelUtils.isComposedSchema(items)) {
@@ -5726,15 +5747,19 @@ public void addOneOfNameExtension(ComposedSchema s, String name) {
     }
 
     /**
-     * Add a given ComposedSchema as an interface model to be generated
+     * Add a given ComposedSchema as an interface model to be generated, assuming it has `oneOf` defined
      * @param cs ComposedSchema object to create as interface model
      * @param type name to use for the generated interface model
      */
     public void addOneOfInterfaceModel(ComposedSchema cs, String type) {
+        if (cs.getOneOf() == null) {
+            return;
+        }
         CodegenModel cm = new CodegenModel();
 
         cm.discriminator = createDiscriminator("", (Schema) cs);
-        for (Schema o : cs.getOneOf()) {
+
+        for (Schema o : Optional.ofNullable(cs.getOneOf()).orElse(Collections.emptyList())) {
             if (o.get$ref() == null) {
                 if (cm.discriminator != null && o.get$ref() == null) {
                     // OpenAPI spec states that inline objects should not be considered when discriminator is used