In Mattson v. City of Costa Mesa (1980) 106 Cal. App. 3d 441, the court held, "Once it is known that the federal court will not exercise pendent jurisdiction over the state claim, plaintiff's proceeding to trial in the federal court on the federal claim alone will necessarily result in splitting the plaintiff's cause of action, and that fact should be apparent to the plaintiff.In such circumstances the rule that would best accommodate the rights of the plaintiff to fully litigate his claim and to invoke the jurisdiction of the federal court and the right of the defendant, the courts and the public to be free of multiple litigation of the same cause of action, is that once the federal court has declined to exercise pendent jurisdiction over the state claim, if the plaintiff then elects to proceed to trial and judgment in the federal court, his entire cause of action is either merged in or barred by the federal court judgment so that he may not thereafter maintain a second suit on the same cause of action in a state court." ( Id. at p. 455.)