DATAMONGO-2661 - Handle nullable types for KPropertyPath.

Original pull request: #894.

spring-data-mongodb/src/main/kotlin/org/springframework/data/mongodb/core/query/KPropertyPath.kt

@@ -27,7 +27,7 @@ import kotlin.reflect.KProperty1
  * @since 2.2
  */
 class KPropertyPath<T, U>(
-		internal val parent: KProperty<U>,
+		internal val parent: KProperty<U?>,
 		internal val child: KProperty1<U, T>
 ) : KProperty<T> by child
 
@@ -52,7 +52,8 @@ internal fun asString(property: KProperty<*>): String {
  * Book::author / Author::name isEqualTo "Herman Melville"
  * ```
  * @author Tjeu Kayim
+ * @author Yoann de Martino
  * @since 2.2
  */
-operator fun <T, U> KProperty<T>.div(other: KProperty1<T, U>) =
+operator fun <T, U> KProperty<T?>.div(other: KProperty1<T, U>) =
 		KPropertyPath(this, other)

spring-data-mongodb/src/test/kotlin/org/springframework/data/mongodb/core/query/KPropertyPathTests.kt

@@ -93,6 +93,15 @@ class KPropertyPathTests {
 		assertThat(property).isEqualTo("entity.book.author.name")
 	}
 
+	@Test
+	fun `Convert nullable KProperty to field name`() {
+		class Cat(val name: String)
+		class Owner(val cat: Cat?)
+
+		val property = asString(Owner::cat / Cat::name)
+		assertThat(property).isEqualTo("cat.name")
+	}
+
 	class Book(val title: String, val author: Author)
 	class Author(val name: String)
 }