The provisions of written agreement to the contrary notwithstanding and regardless of the oral agreement of the parties, an employment shall be deemed to be regular where the employee has been engaged to perform activities which are usually necessary or desirable in the usual business or trade of the employer, except where the employment has been fixed for a specific project or undertaking the completion or termination of which has been determined at the time of the engagement of the employee or where the work or service to be performed is seasonal in nature and the employment is for the duration of the season.
An employment shall be deemed to be casual if it is not covered by the preceding paragraph: Provided, That any employee who has rendered at least one year of service, whether such service is continuous or broken, shall be considered a regular employee with respect to the activity in which he is employed and his employment shall continue while such activity exists.