Allow getting the last insert id without much hassle

lib/private/db/querybuilder/querybuilder.php

@@ -1023,6 +1023,21 @@ class QueryBuilder implements IQueryBuilder {
 		return new QueryFunction($call);
 	}
 
+	/**
+	 * Used to get the id of the last inserted element
+	 * @return int
+	 * @throws \BadMethodCallException When being called before an insert query has been run.
+	 */
+	public function getLastInsertId() {
+		$from = $this->getQueryPart('from');
+
+		if ($this->getType() === \Doctrine\DBAL\Query\QueryBuilder::INSERT && !empty($from)) {
+			return (int) $this->connection->lastInsertId($from['table']);
+		}
+
+		throw new \BadMethodCallException('Invalid call to getLastInsertId without using insert() before.');
+	}
+
 	/**
 	 * @param string $table
 	 * @return string

lib/public/db/querybuilder/iquerybuilder.php

@@ -796,4 +796,12 @@ interface IQueryBuilder {
 	 * @since 8.2.0
 	 */
 	public function createFunction($call);
+
+	/**
+	 * Used to get the id of the last inserted element
+	 * @return int
+	 * @throws \BadMethodCallException When being called before an insert query has been run.
+	 * @since 9.0.0
+	 */
+	public function getLastInsertId();
 }

tests/lib/db/querybuilder/querybuildertest.php

@@ -1086,6 +1086,31 @@ class QueryBuilderTest extends \Test\TestCase {
 		);
 	}
 
+	public function testGetLastInsertId() {
+		$qB = $this->connection->getQueryBuilder();
+
+		try {
+			$qB->getLastInsertId();
+			$this->fail('getLastInsertId() should throw an exception, when being called before insert()');
+		} catch (\BadMethodCallException $e) {
+			$this->assertTrue(true);
+		}
+
+		$qB->insert('appconfig')
+			->values([
+				'appid' => $qB->expr()->literal('testFirstResult'),
+				'configkey' => $qB->expr()->literal('testing' . 50),
+				'configvalue' => $qB->expr()->literal(100 - 50),
+			])
+			->execute();
+
+		$actual = $qB->getLastInsertId();
+
+		$this->assertNotNull($actual);
+		$this->assertInternalType('int', $actual);
+		$this->assertEquals($this->connection->lastInsertId('*PREFIX*appconfig'), $actual);
+	}
+
 	public function dataGetTableName() {
 		return [
 			['*PREFIX*table', null, '`*PREFIX*table`'],