How to select entity with join using EntityManager and CriteriaBuilder having @ManyToOne relationship?

Question

It is just an exercise to understand how to use CriteriaBuilder and EntityManager with join. I want to join class Product with Customer and select each product that have the requested customer_id.

I have a class Product and Customer as follow:

@Entity
public class Product {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;

    private String name;

    @ManyToOne(optional = false)
    @JoinColumn(name = "customer_id", nullable = false)
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Customer customer;
...
}

And Customer:

@Entity
public class Customer {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;
    private String firstName;
    private String lastName;
...
}

Basically I want what I could easiily achieve with this interface:

public interface ProductRepository extends JpaRepository<Product, String>{
    List<Product> findByCustomerId(Long custsomerId);
}

I tried as follow with no luck:

public class ProductRepositoryCustomImpl implements ProductRepositoryCustom {
    @PersistenceContext
    private EntityManager em;

    @Override
    public List<Product> join(Long customerId) {
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<Product> cq = cb.createQuery(Product.class);
        Root<Product> from = cq.from(Product.class);
        Metamodel m = em.getMetamodel();
        EntityType<Customer> CustomerMeta = m.entity(Customer.class);
        Join<Product, Customer> join = from.join(CustomerMeta.getId(Customer.class));

        ParameterExpression<Long> p = cb.parameter(Long.class);
        CriteriaQuery<Product> query = cq.multiselect(join).where(cb.equal(join.get("customer_id"), p));

        TypedQuery<Product> typedQuery = em.createQuery(query);
        typedQuery.setParameter(p, customerId);
        List<Product> results = typedQuery.getResultList();
        return results;
    }
}

I get this error:

java.lang.IllegalStateException: Failed to execute CommandLineRunner
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:821) [spring-boot-2.0.5.RELEASE.jar:2.0.5.RELEASE]
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:802) [spring-boot-2.0.5.RELEASE.jar:2.0.5.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:341) [spring-boot-2.0.5.RELEASE.jar:2.0.5.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1277) [spring-boot-2.0.5.RELEASE.jar:2.0.5.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1265) [spring-boot-2.0.5.RELEASE.jar:2.0.5.RELEASE]
    at com.jpa.jpaExample.JpaExampleApplication.main(JpaExampleApplication.java:19) [classes/:na]
Caused by: org.springframework.dao.InvalidDataAccessApiUsageException: Attribute [com.jpa.jpaExample.Customer#id : java.lang.Long] not castable to requested type [com.jpa.jpaExample.Customer]; nested exception is java.lang.IllegalArgumentException: Attribute [com.jpa.jpaExample.Customer#id : java.lang.Long] not castable to requested type [com.jpa.jpaExample.Customer]
    at org.springframework.orm.jpa.EntityManagerFactoryUtils.convertJpaAccessExceptionIfPossible(EntityManagerFactoryUtils.java:367) ~[spring-orm-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.orm.jpa.vendor.HibernateJpaDialect.translateExceptionIfPossible(HibernateJpaDialect.java:225) ~[spring-orm-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.translateExceptionIfPossible(AbstractEntityManagerFactoryBean.java:527) ~[spring-orm-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.dao.support.ChainedPersistenceExceptionTranslator.translateExceptionIfPossible(ChainedPersistenceExceptionTranslator.java:61) ~[spring-tx-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.dao.support.DataAccessUtils.translateIfNecessary(DataAccessUtils.java:242) ~[spring-tx-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:153) ~[spring-tx-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:135) ~[spring-data-jpa-2.0.10.RELEASE.jar:2.0.10.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92) ~[spring-aop-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(SurroundingTransactionDetectorMethodInterceptor.java:61) ~[spring-data-commons-2.0.10.RELEASE.jar:2.0.10.RELEASE]
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185) ~[spring-aop-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212) ~[spring-aop-5.0.9.RELEASE.jar:5.0.9.RELEASE]
    at com.sun.proxy.$Proxy75.join(Unknown Source) ~[na:na]

Answer 1

try this:

Join<Product, Customer> join = from.join(from.get("customer"));

instead of:

Join<Product, Customer> join = from.join(CustomerMeta.getId(Customer.class));

from refers to the Product class, so when creating the extended relationship it will be based on a parameter of this class.

It can be accessed like this or from the metamodel (for my taste, the recommended one)