JPA-为什么我的一对多关系没有出现在数据库中

Question

I am learning JPA and I have 2 entities (Customer and CheckingAccount) where the first entity has a one to many relationship with the 2nd entity. CheckingAccount subclasses a BankAccount class. I am using MySQL.

I create 2 checking accounts in a test program and assign them both to one customer that I have created.

After I persist everything and check my database I expected to see 2 rows in the CheckingAccount table which I do and 2 rows in the Customer table indicating this customer has 2 checking accounts but I only see one row and the value of the Accounts column is "blob" where I was expecting to see an account number.

Shouldn't I see 2 rows in my Customer table to indicate that the customer has 2 checking accounts?

Here is my Customer entity...

package com.gmail.gmjord;

import java.io.Serializable;
import java.lang.String;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.*;

/**
 * Entity implementation class for Entity: Customer
 * 
 */
@Entity
public class Customer implements Serializable {

    @Id
    @GeneratedValue
    private String id;
    @Column(nullable = false)
    private String name;
    @Column(nullable = false)
    private List<BankAccount> accounts;
    @Column(nullable = false)
    private String infoId;
    @Column(nullable = false)
    private String pin;
    private static final long serialVersionUID = 1L;

    public Customer() {
        super();
        accounts = new ArrayList<BankAccount>();
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAccountsLength() {
        return this.accounts.size();
    }

    public boolean isAccountsNull() {
        if (accounts == null) {
            return true;
        } else {
            return false;
        }
    }

    public List<BankAccount> getAccounts() {
        return accounts;
    }

    public void setAccounts(List<BankAccount> accounts) {
        this.accounts = accounts;
    }

    public String getInfoId() {
        return this.infoId;
    }

    public void setInfoId(String infoId) {
        this.infoId = infoId;
    }

    public String getPin() {
        return this.pin;
    }

    public void setPin(String pin) {
        this.pin = pin;
    }

}

Here is my CheckingAccount entity...

package com.gmail.gmjord;

import com.gmail.gmjord.BankAccount;
import java.io.Serializable;
import javax.persistence.*;

/**
 * Entity implementation class for Entity: CheckingAccount
 *
 */
@Entity

public class CheckingAccount extends BankAccount implements Serializable {


    private static final long serialVersionUID = 1L;

    public CheckingAccount() {
        super();
    }

}

Here is my BankAccount superclass of CheckingAccount...

package com.gmail.gmjord;

import java.io.Serializable;
import java.lang.String;
import javax.persistence.*;

/**
 * Entity implementation class for Entity: BankAccount
 *
 */
@MappedSuperclass

public abstract class BankAccount implements Serializable {


    @Id
    @Column(nullable=false)
    private String accountNum;
    @Column(nullable=false)
    private String balance;
    @Column(nullable=false)
    private String accountType;
    private static final long serialVersionUID = 1L;

    public BankAccount() {
        super();
    }   
    public String getAccountNum() {
        return this.accountNum;
    }

    public void setAccountNum(String accountNum) {
        this.accountNum = accountNum;
    }   
    public String getBalance() {
        return this.balance;
    }

    public void setBalance(String balance) {
        this.balance = balance;
    }   
    public String getAccountType() {
        return this.accountType;
    }

    public void setAccountType(String accountType) {
        this.accountType = accountType;
    }

}

Answer 1

When you use "one to many" with 2 checking accounts and one customer, you have the following structure in your database (only an arbitrary example - I am using json format just to explain better but imagine its attributes as columns):

Customer = [{id: 1, name: Mr. Anderson}]

CheckingAccount = [{id:1, value: $100, id_customer: 1}, {id: 2, value: $250, id_customer: 1}]

You don't need to have two rows in customer table. When you have one-to-many relationship, you have the foreign key in the many-relationship-side table.

Got it.. does it make sense?