JPA traps for the unwary

There are some JPA traps worthy of special examination. We will start by creating the following test case:

package com.gieman.tttracker.dao;

import com.gieman.tttracker.domain.Company;
import com.gieman.tttracker.domain.Project;
import com.gieman.tttracker.domain.User;
import static org.junit.Assert.assertTrue;
import org.junit.Test;

public class JpaTrapTest extends AbstractDaoForTesting {

    @Test
    public void testManyToOne() throws Exception {

        logger.debug("\nSTARTED testManyToOne()\n");

        Company c = companyDao.findAll().get(0);
        Company c2 = companyDao.findAll().get(1);

        Project p = c.getProjects().get(0);

        p.setCompany(c2);
        p = projectDao.merge(p);

        assertTrue("Original company still has project in its collection!",
                !c.getProjects().contains(p));
        assertTrue("Newly assigned company does not have project in its collection",
                c2.getProjects().contains(p));

    ...