From eb73e622ddd1f08adce1996293dd295838d2c43d Mon Sep 17 00:00:00 2001
From: Ryan McCue <me@ryanmccue.info>
Date: Thu, 6 Oct 2016 17:49:45 -0400
Subject: [PATCH] Handle multiple duplicate values correctly

---
 lib/fields/class-wp-rest-meta-fields.php | 31 +++++++++++++++++++-----
 tests/test-rest-post-meta-fields.php     | 28 +++++++++++++++++++++
 2 files changed, 53 insertions(+), 6 deletions(-)

diff --git a/lib/fields/class-wp-rest-meta-fields.php b/lib/fields/class-wp-rest-meta-fields.php
index eabf5bef70..ab674a42e3 100644
--- a/lib/fields/class-wp-rest-meta-fields.php
+++ b/lib/fields/class-wp-rest-meta-fields.php
@@ -162,11 +162,30 @@ protected function update_multi_meta_value( $object, $name, $values ) {
 		}
 
 		$current = get_metadata( $this->get_meta_type(), $object, $name, false );
-		$to_add = array_diff( $values, $current );
-		$to_remove = array_diff( $current, $values );
 
-		foreach ( $to_add as $value ) {
-			if ( ! add_metadata( $this->get_meta_type(), $object, wp_slash( $name ), wp_slash( $value ) ) ) {
+		$to_remove = $current;
+		$to_add = $values;
+		foreach ( $to_add as $add_key => $value ) {
+			$remove_keys = array_keys( $to_remove, $value, true );
+			if ( empty( $remove_keys ) ) {
+				continue;
+			}
+
+			if ( count( $remove_keys ) > 1 ) {
+				// To remove, we need to remove first, then add, so don't touch.
+				continue;
+			}
+
+			$remove_key = $remove_keys[0];
+			unset( $to_remove[ $remove_key ] );
+			unset( $to_add[ $add_key ] );
+		}
+
+		// `delete_metadata` removes _all_ instances of the value, so only call
+		// once.
+		$to_remove = array_unique( $to_remove );
+		foreach ( $to_remove as $value ) {
+			if ( ! delete_metadata( $this->get_meta_type(), $object, wp_slash( $name ), wp_slash( $value ) ) ) {
 				return new WP_Error(
 					'rest_meta_database_error',
 					__( 'Could not update meta value in database.' ),
@@ -174,8 +193,8 @@ protected function update_multi_meta_value( $object, $name, $values ) {
 				);
 			}
 		}
-		foreach ( $to_remove as $value ) {
-			if ( ! delete_metadata( $this->get_meta_type(), $object, wp_slash( $name ), wp_slash( $value ) ) ) {
+		foreach ( $to_add as $value ) {
+			if ( ! add_metadata( $this->get_meta_type(), $object, wp_slash( $name ), wp_slash( $value ) ) ) {
 				return new WP_Error(
 					'rest_meta_database_error',
 					__( 'Could not update meta value in database.' ),
diff --git a/tests/test-rest-post-meta-fields.php b/tests/test-rest-post-meta-fields.php
index 9de32dfb0f..b4c3472914 100644
--- a/tests/test-rest-post-meta-fields.php
+++ b/tests/test-rest-post-meta-fields.php
@@ -340,6 +340,34 @@ public function test_set_value_multiple() {
 		$this->assertContains( 'val2', $meta );
 	}
 
+	/**
+	 * Test removing only one item with duplicate items.
+	 */
+	public function test_set_value_remove_one() {
+		add_post_meta( $this->post_id, 'test_multi', 'c' );
+		add_post_meta( $this->post_id, 'test_multi', 'n' );
+		add_post_meta( $this->post_id, 'test_multi', 'n' );
+
+		$this->grant_write_permission();
+
+		$data = array(
+			'meta' => array(
+				'test_multi' => array( 'c', 'n' ),
+			),
+		);
+		$request = new WP_REST_Request( 'POST', sprintf( '/wp/v2/posts/%d', $this->post_id ) );
+		$request->set_body_params( $data );
+
+		$response = $this->server->dispatch( $request );
+		$this->assertEquals( 200, $response->get_status() );
+
+		$meta = get_post_meta( $this->post_id, 'test_multi', false );
+		$this->assertNotEmpty( $meta );
+		$this->assertCount( 2, $meta );
+		$this->assertContains( 'c', $meta );
+		$this->assertContains( 'n', $meta );
+	}
+
 	/**
 	 * @depends test_set_value_multiple
 	 */