public function deleteContactIds()
    {
        $conn = $this->getConnection();
        $num = $conn->update(
            $this->getTable(Schema::EMAIL_CONTACT_TABLE),
            ['contact_id' => $this->expressionFactory->create(["expression" => 'null'])],
            $conn->quoteInto(
                'contact_id is ?',
                $this->expressionFactory->create(["expression" => 'not null'])
            )
        );

        return $num;
    }