The case is Association des Éleveurs de Candards et d'Oies du Québec v. Harris (opinion here). The plaintiffs argued (and Judge Stephen Wilson agreed) that California's state-law ban on selling foie gras is preempted by federal law.
The relevant federal statute is the Poultry Products Inspection Act (21 U.S.C. §§ 451-70), which regulates poultry-product distribution and sales, including foie gras. The Act states, in part:
"Marking, labeling, packaging, or ingredient requirements . . . [that] unduly interfere with the free flow of poultry products in commerce . . . may not be imposed by any State or Territory or the District of Columbia . . . ."Federal law can supersede state law in certain instances - in this case, the federal statute concerns interstate commerce, which is generally the domain of the federal government. The judge held that the federal Act preempts California's state statute banning foie gras sales.
California's Attorney General, Kamala Harris, can appeal to the U.S. Court of Appeals for the Ninth Circuit. In 2013, the Ninth Circuit upheld California's foie gras sales ban but explicitly did not address the issue of federal preemption. (See footnote 8 on page 21 of the slip opinion).
The Ninth Circuit will likely be called upon to address the issue, as will the U.S. Supreme Court.