Fix for IEnumerable<int?> property mapping (opensearch-project#503)

Fixes opensearch-project#380

Signed-off-by: Kostas <ghost0002001@hotmail.com>

CHANGELOG.md

@@ -13,6 +13,11 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Fixed
 - Fix `HttpConnection.ConvertHttpMethod` to support `Patch` method ([#489](https://github.com/opensearch-project/opensearch-net/pull/489))
 
+
+### Fixed
+- Fixed `IEnumerable<int?>` property mapping. ([#503](https://github.com/opensearch-project/opensearch-net/pull/503))
+
+
 ### Dependencies
 - Bumps `Microsoft.CodeAnalysis.CSharp` from 4.2.0 to 4.6.0
 - Bumps `NSwag.Core.Yaml` from 13.19.0 to 13.20.0

src/OpenSearch.Client/Mapping/Visitor/PropertyWalker.cs

@@ -205,11 +205,25 @@ private static Type GetUnderlyingType(Type type)
 			if (type.IsArray)
 				return type.GetElementType();
 
-			if (type.IsGenericType && type.GetGenericArguments().Length == 1
-				&& (type.GetInterfaces().HasAny(t => t == typeof(IEnumerable)) || Nullable.GetUnderlyingType(type) != null))
-				return type.GetGenericArguments()[0];
+			if (ShouldUnwrapType(type))
+			{
+				var returnType = type.GetGenericArguments()[0];
+				if (ShouldUnwrapType(returnType)) // This is needed for types like IEnumerable<int?>.
+				{
+					return returnType.GetGenericArguments()[0];
+				}
+				return returnType;
+			}
 
 			return type;
 		}
+
+		private static bool ShouldUnwrapType(Type ty) =>
+					ty.IsGenericType &&
+					ty.GetGenericArguments().Length == 1 &&
+					(
+						ty.GetInterfaces().HasAny(t => t == typeof(IEnumerable)) ||
+						Nullable.GetUnderlyingType(ty) != null
+					);
 	}
 }

tests/Tests/Mapping/PropertyWalkerTests.cs

@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: Apache-2.0
+*
+* The OpenSearch Contributors require contributions made to
+* this file be licensed under the Apache-2.0 license or a
+* compatible open source license.
+*/
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using FluentAssertions;
+using OpenSearch.Client;
+using OpenSearch.OpenSearch.Xunit.XunitPlumbing;
+
+namespace Tests.Mapping
+{
+	public class PropertyWalkerTests
+	{
+		[U]
+		public void BoolGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<bool>();
+			result.Should().Be("boolean");
+		}
+
+		[U]
+		public void StringGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<string>();
+			result.Should().Be("text");
+		}
+
+
+		[U]
+		public void DateTimeGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<DateTime>();
+			result.Should().Be("date");
+		}
+
+		[U]
+		public void IEnumerableOfNullableGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<IEnumerable<int?>>();
+			result.Should().Be("integer");
+		}
+
+		[U]
+		public void IListOfNullableGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<IList<int?>>();
+			result.Should().Be("integer");
+		}
+
+		[U]
+		public void IEnumerableOfValueTypesGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<IEnumerable<int>>();
+			result.Should().Be("integer");
+		}
+
+		[U]
+		public void IListOfValueTypesGetsMappedCorrectly()
+		{
+			var result = TestPropertyType<IList<int>>();
+			result.Should().Be("integer");
+		}
+
+		private static string TestPropertyType<T>()
+		{
+			var walker = new PropertyWalker(typeof(Test<T>), null, 0);
+			var properties = walker.GetProperties();
+			var result = properties.SingleOrDefault();
+			return result.Value?.Type;
+		}
+
+
+		private class Test<T>
+		{
+			public T Values { get; set; }
+		}
+
+	}
+}