fix: stop encoding synthetic fields

encoding/protobq/schema.go

@@ -55,6 +55,11 @@ func (o SchemaOptions) InferMessageSchema(msg protoreflect.MessageDescriptor) bi
 	if o.UseOneofFields {
 		for i := 0; i < msg.Oneofs().Len(); i++ {
 			oneof := msg.Oneofs().Get(i)
+			// The `optional` keyword creates a synthetic field treated as an oneof
+			// which creates a new field with _ as prefix missmatching the schema creation
+			if oneof.IsSynthetic() {
+				continue
+			}
 			schema = append(schema, o.inferOneofFieldSchema(oneof))
 		}
 	}
@@ -220,7 +225,7 @@ func (o SchemaOptions) inferFieldSchemaType(field protoreflect.FieldDescriptor)
 	return bigquery.RecordFieldType
 }
 
-func (o SchemaOptions) inferEnumFieldType(field protoreflect.FieldDescriptor) bigquery.FieldType {
+func (o SchemaOptions) inferEnumFieldType(_ protoreflect.FieldDescriptor) bigquery.FieldType {
 	if o.UseEnumNumbers {
 		return bigquery.IntegerFieldType
 	}

encoding/protobq/schema_test.go

@@ -386,6 +386,20 @@ func TestSchemaOptions_InferSchema(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "examplev1.ExampleOptional (with enable `UseOneofFields`)",
+			opt: SchemaOptions{
+				UseOneofFields: true,
+			},
+			msg: &examplev1.ExampleOptional{},
+			expected: bigquery.Schema{
+				{
+					Name:     "opt",
+					Type:     bigquery.FloatFieldType,
+					Required: false,
+				},
+			},
+		},
 	} {
 		tt := tt
 		t.Run(tt.name, func(t *testing.T) {

internal/examples/proto/einride/bigquery/example/v1/example_optional.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+package einride.bigquery.example.v1;
+
+message ExampleOptional {
+  optional double opt = 1;
+}

internal/examples/proto/gen/json/einride_bigquery_example_v1_exampleoptional.json

@@ -0,0 +1,7 @@
+[
+  {
+    "name": "opt",
+    "type": "FLOAT",
+    "mode": "NULLABLE"
+  }
+]