From d4912b424d8ec46953e479379dd8c435dc3118ef Mon Sep 17 00:00:00 2001
From: K Zhang <kandazhang@gmail.com>
Date: Mon, 17 Jan 2022 11:08:15 -0500
Subject: [PATCH] if discriminator is not defined with
 useOneOfDiscriminatorLookup set to true, we generated as
 useOneOfDiscriminatorLookup false (#11178)

---
 .../main/resources/go/model_oneof.mustache    | 32 ++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/modules/openapi-generator/src/main/resources/go/model_oneof.mustache b/modules/openapi-generator/src/main/resources/go/model_oneof.mustache
index 45562dcc7c30..6938308d763e 100644
--- a/modules/openapi-generator/src/main/resources/go/model_oneof.mustache
+++ b/modules/openapi-generator/src/main/resources/go/model_oneof.mustache
@@ -48,8 +48,38 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	}
 
 	{{/mappedModels}}
-	{{/discriminator}}
 	return nil
+	{{/discriminator}}
+	{{^discriminator}}
+        match := 0
+        {{#oneOf}}
+        // try to unmarshal data into {{{.}}}
+        err = json.Unmarshal(data, &dst.{{{.}}})
+        if err == nil {
+                json{{{.}}}, _ := json.Marshal(dst.{{{.}}})
+                if string(json{{{.}}}) == "{}" { // empty struct
+                        dst.{{{.}}} = nil
+                } else {
+                        match++
+                }
+        } else {
+                dst.{{{.}}} = nil
+        }
+
+        {{/oneOf}}
+        if match > 1 { // more than 1 match
+                // reset to nil
+                {{#oneOf}}
+                dst.{{{.}}} = nil
+                {{/oneOf}}
+
+                return fmt.Errorf("Data matches more than one schema in oneOf({{classname}})")
+        } else if match == 1 {
+                return nil // exactly one match
+        } else { // no match
+                return fmt.Errorf("Data failed to match schemas in oneOf({{classname}})")
+        }
+	{{/discriminator}}
 	{{/useOneOfDiscriminatorLookup}}
 	{{^useOneOfDiscriminatorLookup}}
 	match := 0