ON DUPLICATE KEY: Prevent adding a comma if key isn't in the first position (#113)

[WordPress Playground observed an
issu](WordPress/wordpress-playground#731
with insert queries that use `ON DUPLICATE KEY` and don't have the KEY
as the first value of the insert.

After investigating it it turns out that the condition for adding commas
didn't work well in case the KEY value is in a "random" place.

This PR ensures the comma is added for all items except for the last
one.

## Testing instructions

- Ensure tests pass

Author: bgrgicak

tests/WP_SQLite_Query_Tests.php

@@ -505,6 +505,21 @@ public function testRecoverSerialized() {
 		$this->assertEquals( $obj, $unserialized );
 	}
 
+	public function testOnDuplicateKey() {
+		$this->assertQuery(
+			'CREATE TABLE `test` (
+				`id` INT PRIMARY KEY,
+				`text` VARCHAR(255),
+			);'
+		);
+		// The order is deliberate to test that the query works with the keys in any order.
+		$this->assertQuery(
+			'INSERT INTO test (`text`, `id`)
+			VALUES ("test", 1)
+			ON DUPLICATE KEY UPDATE `text` = "test1"'
+		);
+	}
+
 	public function testShowColumns() {
 
 		$query = 'SHOW COLUMNS FROM wp_posts';

wp-includes/sqlite/class-wp-sqlite-translator.php

@@ -2803,9 +2803,10 @@ private function translate_on_duplicate_key( $table_name ) {
 		$this->rewriter->add( new WP_SQLite_Token( '(', WP_SQLite_Token::TYPE_OPERATOR ) );
 
 		$max = count( $conflict_columns );
-		foreach ( $conflict_columns as $i => $conflict_column ) {
+		$i   = 0;
+		foreach ( $conflict_columns as $conflict_column ) {
 			$this->rewriter->add( new WP_SQLite_Token( '"' . $conflict_column . '"', WP_SQLite_Token::TYPE_KEYWORD, WP_SQLite_Token::FLAG_KEYWORD_KEY ) );
-			if ( $i !== $max - 1 ) {
+			if ( ++$i < $max ) {
 				$this->rewriter->add( new WP_SQLite_Token( ',', WP_SQLite_Token::TYPE_OPERATOR ) );
 				$this->rewriter->add( new WP_SQLite_Token( ' ', WP_SQLite_Token::TYPE_WHITESPACE ) );
 			}