Expose category ID

I am new to web app development. I am trying to expose the category ID:s so that I can implement "search by category" in my app.

But even after writing the below code I am not able to expose the ID:s.

I have seen some solutions, but they were of very little use to me because of the incompetence.

I am using Eclipse IDE.

import java.util.ArrayList;
import java.util.List;
import java.util.Set;

import javax.persistence.EntityManager;
import javax.persistence.metamodel.EntityType;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
import org.springframework.data.rest.webmvc.config.RepositoryRestConfigurer;
import org.springframework.http.HttpMethod;

import com.kausar.ecommerce.entity.Product;
import com.kausar.ecommerce.entity.ProductCategory;

@Configuration
public class MyDataRestConfig implements RepositoryRestConfigurer {
    private EntityManager entityManager;

    @Autowired
    public MyDataRestConfig(EntityManager theEntityManager) {
        entityManager = theEntityManager;
    }

    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
        HttpMethod[] theUnsupportedActions = {HttpMethod.PUT, HttpMethod.POST, HttpMethod.DELETE};
        // disable HTTP methods for Product: PUT, POST and DELETE
        config.getExposureConfiguration()
                .forDomainType(Product.class)
                .withItemExposure((metdata, httpMethods) -> httpMethods.disable(theUnsupportedActions))
                .withCollectionExposure((metdata, httpMethods) -> httpMethods.disable(theUnsupportedActions));

        // disable HTTP methods for ProductCategory: PUT, POST and DELETE
        config.getExposureConfiguration()
                .forDomainType(ProductCategory.class)
                .withItemExposure((metdata, httpMethods) -> httpMethods.disable(theUnsupportedActions))
                .withCollectionExposure((metdata, httpMethods) -> httpMethods.disable(theUnsupportedActions));

        // call an internal helper method
        exposeIds(config);
    }

    private void exposeIds(RepositoryRestConfiguration config) {
        // expose entity ids

        //get a list of all entity classes from the entity manager
        Set<EntityType<?>> entities = entityManager.getMetamodel().getEntities();

        //create an array of the entity types
        List<Class> entityClasses = new ArrayList<>();

        //get the entity types for the entities
        for (EntityType tempEntityType : entities) {
            entityClasses.add(tempEntityType.getJavaType());
        }

        //expose entity ids for the array of entity/domain types
        Class[] domainTypes = entityClasses.toArray(new Class[0]);
        config.exposeIdsFor(domainTypes);
    }
}