A.java

package test.testjpa.domain;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;

@Entity
public class A {

    private Long id;
    private String name;
    private List<B> bs = new ArrayList<>();

    @Id
    @GeneratedValue
    public Long getId() {
        return id;
    }

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

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    public List<B> getBs() {
        return bs;
    }

    public void setBs(List<B> bs) {
        this.bs = bs;
    }

    public String getName() {
        return name;
    }

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

B.java

package test.testjpa.domain;

import javax.persistence.*;
import java.util.List;

@Entity
public class B {

    private Long id;
    private List<A> as;

    private long value;

    public long getValue() {
        return value;
    }

    public void setValue(long value) {
        this.value = value;
    }

    @Id
    @GeneratedValue
    public Long getId() {
        return id;
    }

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


    @ManyToMany(mappedBy = "bs")
    public List<A> getAs() {
        return as;
    }

    public void setAs(List<A> as) {
        this.as = as;
    }
}

JpaTest.java

package jpa;

import test.testjpa.domain.A;
import test.testjpa.domain.B;
import test.testjpa.domain.Department;
import test.testjpa.domain.Employee;

import javax.persistence.EntityManager;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import javax.persistence.Query;
import java.util.List;

public class JpaTest {


    private EntityManager manager;

    public JpaTest(EntityManager manager) {
        this.manager = manager;
    }

    public static void main(String[] args) {
        final var factory = Persistence.createEntityManagerFactory("dev");
        final var manager = factory.createEntityManager();

        //final var test = new JpaTest(manager);

        //final var tx = manager.getTransaction();

        //tx.begin();
        //try {
        //test.createEmployees();
        //} catch (Exception e) {
        //   e.printStackTrace();
        //}

        //tx.commit();
        //test.listEmployees();

        EntityTransaction tx = manager.getTransaction();
        tx.begin();
        for (int j = 0; j < 10; j++) {
            final A a = new A();
            a.setName(String.valueOf(j));
            for (int i = 0; i < j; i++) {
                final B b = new B();
                b.setValue(i);
                manager.persist(b);
                a.getBs().add(b);
            }
            manager.persist(a);
			/*Department d = new Department("dep"+j);
			manager.persist(d);
			for (int i = 0; i < 2; i++) {
				Employee e = new Employee("emp_"+j + "_"+i);
				manager.persist(e);
				e.setDepartment(d);
				d.getEmployees().add(e);
			}*/

        }
        tx.commit();



        manager.close();
        System.out.println(".. done");

    }

    private void listEmployees() {
        var resultList = manager.createQuery("select a from Employee a", Employee.class).getResultList();
        System.out.println("Number of employees: " + resultList.size());
        for (var next : resultList) {
            System.out.println("next employee: " + next);
        }
    }

    private void createEmployees() {
        final var numOfEmployees = manager.createQuery("select a from Employee a", Employee.class).getResultList().size();

        if (numOfEmployees == 0) {
            final var department = new Department("java");
            manager.persist(department);

            manager.persist(new Employee("Employee A", department));
            manager.persist(new Employee("Employee B", department));
        }
    }
}

JpaTestRM.java

package jpa;

import test.testjpa.domain.A;
import test.testjpa.domain.Department;
import test.testjpa.domain.Employee;

import javax.persistence.EntityManager;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import javax.persistence.Query;
import java.util.List;

public class JpaTestRM {


    private EntityManager manager;

    public JpaTestRM(EntityManager manager) {
        this.manager = manager;
    }

    public static void main(String[] args) {
        final var factory = Persistence.createEntityManagerFactory("dev");
        final var manager = factory.createEntityManager();

        //final var test = new JpaTest(manager);

        //final var tx = manager.getTransaction();

        //tx.begin();
        //try {
        //test.createEmployees();
        //} catch (Exception e) {
        //   e.printStackTrace();
        //}

        //tx.commit();
        //test.listEmployees();

        EntityTransaction tx = manager.getTransaction();
        tx.begin();
        Query q = manager.createQuery("SELECT a FROM A a where a.id=120");

        A a = (A) q.getSingleResult();
        a.getBs().clear();
        manager.remove(a);
        tx.commit();

        tx.begin();
        Query q2 = manager.createQuery("SELECT a FROM A a");
        List<A> as = q2.getResultList();
        for (A aa : as) {
            System.out.println(aa + "  =  " + aa.getBs().size());
        }
        tx.commit();


        manager.close();
        System.out.println(".. done");

    }

    private void listEmployees() {
        var resultList = manager.createQuery("select a from Employee a", Employee.class).getResultList();
        System.out.println("Number of employees: " + resultList.size());
        for (var next : resultList) {
            System.out.println("next employee: " + next);
        }
    }

    private void createEmployees() {
        final var numOfEmployees = manager.createQuery("select a from Employee a", Employee.class).getResultList().size();

        if (numOfEmployees == 0) {
            final var department = new Department("java");
            manager.persist(department);

            manager.persist(new Employee("Employee A", department));
            manager.persist(new Employee("Employee B", department));
        }
    }
}