California Code, Labor Code - LAB § 1154.5

It shall be an unfair labor practice for any labor organization which represents the employees of the employer and such employer to enter into any contract or agreement, express or implied, whereby such employer ceases or refrains, or agrees to cease or refrain, from handling, using, selling, transporting, or otherwise dealing in any of the products of any other employer, or to cease doing business with any other person, and any contract or agreement entered into heretofore or hereafter containing such an agreement shall be, to such extent, unenforceable and void. Nothing in this section shall apply to an agreement between a labor organization and an employer relating to a supplier of an ingredient or ingredients which are integrated into a product produced or distributed by such employer where the labor organization is certified as the representative of the employees of such supplier, but no collective-bargaining agreement between such supplier and such labor organization is in effect. Further, nothing in this section shall apply to an agreement between a labor organization and an agricultural employer relating to the contracting or subcontracting of work to be done at the site of the farm and related operations. Nothing in this part shall prohibit the enforcement of any agreement which is within the foregoing exceptions.

Nor shall anything in this section be construed to apply or be applicable to any labor organization in its representation of workers who are not agricultural employees. Any such labor organization shall continue to be governed in its intrastate activities for nonagricultural workers by Section 923 and applicable judicial precedents.