public function getFields()
	{
		// Fetch fields when the method is called for the first time.
		if ( $this->fields === null ) {
			$fields = array();
			$result = $this->api(self::REQUEST_GET, '/rest/api/2/field', array(), true);

			/* set hash key as custom field id */
			foreach ( $result as $field ) {
				$fields[$field['id']] = $field;
			}

			$this->fields = $fields;
		}

		return $this->fields;
	}