_id bigint(20) unsigned DEFAULT NULL,
				profile_id bigint(20) unsigned DEFAULT NULL,
				test_mode tinyint(1) NOT NULL,
				email varchar(100) DEFAULT NULL,
				name varchar(255) DEFAULT NULL,
				PRIMARY KEY  ( id ),
				UNIQUE KEY mollie_id ( mollie_id ),
				KEY organization_id ( organization_id ),
				KEY profile_id ( profile_id ),
				KEY test_mode ( test_mode ),
				KEY email ( email )
			) $table_options;
			CREATE TABLE $wpdb->pronamic_pay_mollie_customer_users (
				id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
				customer_id bigint(20) unsigned NOT NULL,
				user_id bigint(20) unsigned NOT NULL,
				PRIMARY KEY  ( id ),
				UNIQUE KEY customer_user ( customer_id, user_id )
			) $table_options;
		";

		/**
		 * Execute.
		 *
		 * @link https://developer.wordpress.org/reference/functions/dbdelta/
		 * @link https://github.com/WordPress/WordPress/blob/5.3/wp-admin/includes/upgrade.php#L2538-L2915
		 */
		\dbDelta( $queries );
	}

	/**
	 * Add foreign keys.
	 *
	 * @return void
	 */
	private function add_foreign_keys() {
		global $wpdb;

		/**
		 * Foreign keys.
		 *
		 * @link https://core.trac.wordpress.org/ticket/19207
		 * @link https://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html
		 */
		$data = array(
			(object) array(
				'table' => $wpdb->pronamic_pay_mollie_profiles,
				'name'  => 'fk_profile_organization_id',
				'query' => "
					ALTER TABLE  $wpdb->pronamic_pay_mollie_profiles
					ADD CONSTRAINT fk_profile_organization_id
					FOREIGN KEY ( organization_id )
					REFERENCES $wpdb->pronamic_pay_mollie_organizations ( id )
					ON DELETE RESTRICT
					ON UPDATE RESTRICT
					;
				",
			),
			(object) array(
				'table' => $wpdb->pronamic_pay_mollie_customers,
				'name'  => 'fk_customer_organization_id',
				'query' => "
					ALTER TABLE $wpdb->pronamic_pay_mollie_customers
					ADD CONSTRAINT fk_customer_organization_id
					FOREIGN KEY ( organization_id )
					REFERENCES $wpdb->pronamic_pay_mollie_organizations ( id )
					ON DELETE RESTRICT
					ON UPDATE RESTRICT
					;
				",
			),
			(object) array(
				'table' => $wpdb->pronamic_pay_mollie_customers,
				'name'  => 'fk_customer_profile_id',
				'query' => "
					ALTER TABLE $wpdb->pronamic_pay_mollie_customers
					ADD CONSTRAINT fk_customer_profile_id
					FOREIGN KEY ( profile_id )
					REFERENCES $wpdb->pronamic_pay_mollie_profiles ( id )
					ON DELETE RESTRICT
					ON UPDATE RESTRICT
					;
				",
			),
			(object) array(
				'table' => $wpdb->pronamic_pay_mollie_customer_users,
				'name'  => 'fk_customer_id',
				'query' => "
					ALTER TABLE $wpdb->pronamic_pay_mollie_customer_users
					ADD CONSTRAINT fk_customer_id
					FOREIGN KEY customer_id ( customer_id )
					REFERENCES $wpdb->pronamic_pay_mollie_customers ( id )
					ON DELETE RESTRICT
					ON UPDATE RESTRICT
					;
				",
			),
			(object) array(
				'table' => $wpdb->pronamic_pay_mollie_customer_users,
				'name'  => 'fk_customer_user_id',
				'query' => "
					ALTER TABLE $wpdb->pronamic_pay_mollie_customer_users
					ADD CONSTRAINT fk_customer_user_id
					FOREIGN KEY user_id ( user_id )
					REFERENCES $wpdb->users ( id )
					ON DELETE CASCADE
					ON UPDATE CASCADE
					;
				",
			),
		);

		foreach ( $data as $item ) {
			try {
				$this->add_foreign_key( $item );
			} catch ( \Exception $e ) {
				// Foreign keys are not strictly required.
				continue;
			}
		}
	}

	/**
	 * Add specified foreign key.
	 *
	 * @param object $item Foreign key data.
	 * @return void
	 * @throws \Exception Throws exception when adding foreign key fails.
	 * @throws \InvalidArgumentException Throws invalid argument exception if item misses required `table`, `name` or `query` property.
	 */
	private function add_foreign_key( $item ) {
		global $wpdb;

		if ( ! \property_exists( $item, 'table' ) ) {
			throw new \InvalidArgumentException( 'Foreign key item must contain `table` property.' );
		}

		if ( ! \property_exists( $item, 'name' ) ) {
			throw new \InvalidArgumentException( 'Foreign key item must contain `name` property.' );
		}

		if ( ! \property_exists( $item, 'query' ) ) {
			throw new \InvalidArgumentException( 'Foreign key item must contain `query` property.' );
		}

		/**
		 * Suppress errors.
		 *
		 * We suppress errors because adding foreign keys to for example
		 * a `$wpdb->users` MyISAM table will trigger the following error:
		 *
		 * "Error in query (1005): Can't create table '●●●●●●●●. # Sql-●●●●●●●●●●' (errno: 150)"
		 *
		 * @link https://github.com/WordPress/WordPress/blob/5.3/wp-includes/wp-db.php#L1544-L1559
		 */
		$suppress_errors = $wpdb->suppress_errors( true );

		/**
		 * Check if foreign key exists
		 *
		 * @link https://github.com/woocommerce/woocommerce/blob/3.9.0/includes/class-wc-install.php#L663-L681
		 */
		$result = $wpdb->get_var(
			$wpdb->prepare(
				"
			SELECT COUNT(*)
			FROM information_schema.TABLE_CONSTRAINTS
			WHERE CONSTRAINT_SCHEMA = %s
			AND CONSTRAINT_NAME = %s
			AND CONSTRAINT_TYPE = 'FOREIGN KEY'
			AND TABLE_NAME = %s
			",
				$wpdb->dbname,
				$item->name,
				$item->table
			)
		);

		if ( null === $result ) {
			throw new \Exception(
				\sprintf(
					'Could not count foreign keys: %s, database error: %s.',
					$item->name,
					$wpdb->last_error
				)
			);
		}

		$number_constraints = \intval( $result );

		if ( 0 === $number_constraints ) {
			// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared -- Query is prepared.
			$result = $wpdb->query( $item->query );

			$wpdb->suppress_errors( $suppress_errors );

			if ( false === $result ) {
				throw new \Exception(
					\sprintf(
						'Could not add foreign key: %s, database error: %s.',
						$item->name,
						$wpdb->last_error
					)
				);
			}
		}

		$wpdb->suppress_errors( $suppress_errors );
	}

	/**
	 * Convert user meta.
	 *
	 * @return void
	 * @throws \Exception Throws exception when database update query fails.
	 */
	private function convert_user_meta() {
		global $wpdb;

		$query = "
			INSERT IGNORE INTO $wpdb->pronamic_pay_mollie_customers (
				mollie_id,
				test_mode
			)
			SELECT
				meta_value AS mollie_id,
				'_pronamic_pay_mollie_customer_id_test' = meta_key AS test_mode
			FROM
				$wpdb->usermeta
			WHERE
				meta_key IN (
					'_pronamic_pay_mollie_customer_id',
					'_pronamic_pay_mollie_customer_id_test'
				)
					AND
				meta_value != ''
			;
		";

		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared -- Query is prepared.
		$result = $wpdb->query( $query );

		if ( false === $result ) {
			throw new \Exception(
				sprintf(
					'Could not convert user meta, database error: %s.',
					$wpdb->last_error
				)
			);
		}

		/**
		 * Collate caluse.
		 *
		 * Force a specific collate to fix:
		 * "Illegal mix of collations (utf8mb4_unicode_ci,IMPLICIT) and
		 * (utf8mb4_unicode_520_ci,IMPLICIT) for operation '='. ".
		 *
		 * @link https://dev.mysql.com/doc/refman/8.0/en/charset-collate.html
		 */
		$collate_clause = '';

		if ( ! empty( $wpdb->collate ) ) {
			$collate_clause = \sprintf(
				'COLLATE %s',
				$wpdb->collate
			);
		}

		$query = "
			INSERT IGNORE INTO $wpdb->pronamic_pay_mollie_customer_users (
				customer_id,
				user_id
			)
			SELECT
				mollie_customer.id AS mollie_customer_id,
				wp_user.ID AS wp_user_id
			FROM
				$wpdb->pronamic_pay_mollie_customers AS mollie_customer
					INNER JOIN
				$wpdb->usermeta AS wp_user_meta
						ON wp_user_meta.meta_value = mollie_customer.mollie_id $collate_clause
					INNER JOIN
				$wpdb->users AS wp_user
						ON wp_user_meta.user_id = wp_user.ID
			WHERE
				wp_user_meta.meta_key IN (
					'_pronamic_pay_mollie_customer_id',
					'_pronamic_pay_mollie_customer_id_test'
				)
					AND
				wp_user_meta.meta_value != ''
			;
		";

		// phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared -- Query is prepared.
		$result = $wpdb->query( $query );

		if ( false === $result ) {
			throw new \Exception(
				sprintf(
					'Could not convert user meta, database error: %s.',
					$wpdb->last_error
				)
			);
		}
	}
}