diff --git a/tests/phpunit/tests/post/query.php b/tests/phpunit/tests/post/query.php
index 2e31f4d24e..1d746fc0b7 100644
--- a/tests/phpunit/tests/post/query.php
+++ b/tests/phpunit/tests/post/query.php
@@ -677,4 +677,67 @@ class Tests_Post_Query extends WP_UnitTestCase {
 
 		return $posts;
 	}
+
+	function test_post__in_ordering() {
+		$post_id1 = $this->factory->post->create( array( 'post_type' => 'page', 'menu_order' => rand( 1, 100 ) ) );
+		$post_id2 = $this->factory->post->create( array( 'post_type' => 'page', 'menu_order' => rand( 1, 100 ) ) );
+		$post_id3 = $this->factory->post->create( array(
+			'post_type' => 'page',
+			'post_parent' => $post_id2,
+			'menu_order' => rand( 1, 100 )
+		) );
+		$post_id4 = $this->factory->post->create( array(
+			'post_type' => 'page',
+			'post_parent' => $post_id2,
+			'menu_order' => rand( 1, 100 )
+		) );
+		$post_id5 = $this->factory->post->create( array( 'post_type' => 'page', 'menu_order' => rand( 1, 100 ) ) );
+
+		$ordered = array( $post_id2, $post_id4, $post_id3, $post_id1, $post_id5 );
+
+		$q = new WP_Query( array(
+			'post_type' => 'any',
+			'post__in' => $ordered,
+			'orderby' => 'post__in'
+		) );
+		$this->assertEqualSets( $ordered, wp_list_pluck( $q->posts, 'ID' ) );
+	}
+
+	function test_post__in_attachment_ordering() {
+		$post_id = $this->factory->post->create();
+		$att_ids = array();
+		$att_ids[1] = $this->factory->attachment->create_object( 'woo1.jpg', $post_id, array(
+			'post_mime_type' => 'image/jpeg',
+			'menu_order' => rand( 1, 100 )
+		) );
+		$att_ids[2] = $this->factory->attachment->create_object( 'woo2.jpg', $post_id, array(
+			'post_mime_type' => 'image/jpeg',
+			'menu_order' => rand( 1, 100 )
+		) );
+		$att_ids[3] = $this->factory->attachment->create_object( 'woo3.jpg', $post_id, array(
+			'post_mime_type' => 'image/jpeg',
+			'menu_order' => rand( 1, 100 )
+		) );
+		$att_ids[4] = $this->factory->attachment->create_object( 'woo4.jpg', $post_id, array(
+			'post_mime_type' => 'image/jpeg',
+			'menu_order' => rand( 1, 100 )
+		) );
+		$att_ids[5] = $this->factory->attachment->create_object( 'woo5.jpg', $post_id, array(
+			'post_mime_type' => 'image/jpeg',
+			'menu_order' => rand( 1, 100 )
+		) );
+
+		$ordered = array( $att_ids[5], $att_ids[1], $att_ids[4], $att_ids[3], $att_ids[2] );
+
+		$attached = new WP_Query( array(
+			'post__in' => $ordered,
+			'post_type' => 'attachment',
+			'post_parent' => $post_id,
+			'post_mime_type' => 'image',
+			'post_status' => 'inherit',
+			'posts_per_page' => '-1',
+			'orderby' => 'post__in'
+		) );
+		$this->assertEqualSets( $ordered, wp_list_pluck( $attached->posts, 'ID' ) );
+	}
 }
\ No newline at end of file