如何使用JPA通过外键搜索mySql数据库表？

Question

I have 2 classes (BusinessAccount and Projects (shown below) that are mapped to a MySql database) where a 1:M relationship exists between BusinessAccounts and Projects. I am successfully inserting data to the database but am having a problem when it comes to querying the database. The problem that I am having is that I have no getter or setter for the foreign key, 'contractor_id' in the Projects class. The query that I want to carry out is to return the list of the names of all projects for a given BusinessAccount, by searching by the foreign key reference in the Projects table. I can do this no problem in mySQL but as there is no reference to the contractor_id as a java entity in the Projects class, I'm not sure how to do this search from within my java class. (Note: I tried to declare the foreign key along with getters and setters in the Projects class but as I have these mapped by the 1:Many relationship in the class already, it wouldn't compile as they were flagged as duplicate entities.) I'm sure it's something obvious that I'm missing but any help is much appreciated!

public List<Projects> getProjectList() {
    factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    EntityManager em = factory.createEntityManager();   
    List<Projects> projectList = new ArrayList<Projects>();

        em.getTransaction().begin();

        String sessionEmail=Util.getEmail();
        Query myQuery = em.createQuery("SELECT u FROM BusinessAccount u WHERE u.email=:email");
        myQuery.setParameter("email", sessionEmail);

        List<BusinessAccount> userList=myQuery.getResultList();
        BusinessAccount account =userList.get(0);

        Query myQuery2 = em.createQuery("SELECT distinct p.* FROM BusinessAccount u "
                + "INNER JOIN Projects p ON p.contractor_id=:userID");

/*Note p.contractor_id above refers to the entity in the 
mysql database (and won't work obviously), I want to refer 
to it's java equivalent but am not sure how to do that*/

        myQuery2.setParameter("userID", account.getId());
        projectList=myQuery2.getResultList();
        em.getTransaction().commit();
        em.close();

        return projectList;

    } 


@Entity
@Table(name = "business_accounts")
public class BusinessAccount {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "surname")
    private String surname;

        @OneToMany(mappedBy = "businessAccount", fetch = FetchType.EAGER, cascade = { CascadeType.ALL })
    private List<Projects> projects;

    public int getId() {
    return id;
    }

    public void setId(int id) {
    this.id = id;
    }

    public String getFirstName() {
    return firstName;
    }

    public void setFirstName(String firstName) {
    this.firstName = firstName;
    }

    public String getSurname() {
    return surname;
    }

       public List<Projects> getProjects()
    {
    if (projects == null)
    {
        projects = new ArrayList<Projects>();
    }

    return projects;
    }

   public void setProjects(List<Projects> projects)
    {
    this.projects = projects;
    }

    }


@Entity
@Table(name = "projects")
public class Projects {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int project_id;

@Column(name = "project_name")
private String projectName;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumns({ @JoinColumn(name = "contractor_id", referencedColumnName="id") })
private BusinessAccount businessAccount;


public BusinessAccount getBusinessAccount() {
    if (businessAccount == null) {
        businessAccount = new BusinessAccount();
    }
    return businessAccount;
}

public void setBusinessAccount(BusinessAccount businessAccount) {
    this.businessAccount = businessAccount;
}

public int getProject_id() {
    return project_id;
}

public void setProject_id(int project_id) {
    this.project_id = project_id;
}

public String getProjectName() {
    return projectName;
}

public void setProjectName(String projectName) {
    this.projectName = projectName;
}

}

Answer 1

The JPA query would be something like (you need to use the relation property, but no need for the foreign key itself - please try, it may need some tweaking):

SELECT p FROM BusinessAccount u, IN(u.projects) p WHERE u.id=:userId

But do you really need the query? You can get the related projects from the property:

BusinessAccount account = ...
List<Projects> projectList = account.getProjects();

Answer 2

Try this:

Query myQuery2 = em.createQuery("SELECT distinct p.* FROM BusinessAccount u "
            + "INNER JOIN Projects p ON p.businessAccount=:businessAccount");

myQuery2.setParameter("businessAccount", account);