HHH-11026 Test cascade merge many-to-many leading to exception

hibernate-core/src/test/java/org/hibernate/orm/test/annotations/idmanytoone/Course.java

@@ -5,7 +5,10 @@
 package org.hibernate.orm.test.annotations.idmanytoone;
 
 import java.io.Serializable;
+import java.util.HashSet;
 import java.util.Set;
+
+import jakarta.persistence.CascadeType;
 import jakarta.persistence.Entity;
 import jakarta.persistence.GeneratedValue;
 import jakarta.persistence.Id;
@@ -25,8 +28,8 @@
 
 	private String name;
 
-	@OneToMany(mappedBy = "course")
-	private Set<CourseStudent> students;
+	@OneToMany(mappedBy = "course", cascade = CascadeType.ALL)
+	private Set<CourseStudent> students = new HashSet<>();
 
 	public Course() {
 	}
@@ -47,7 +50,7 @@
 		this.name = name;
 	}
 
 	public Set<CourseStudent> getStudents() {
 		return students;
 	}
 

hibernate-core/src/test/java/org/hibernate/orm/test/annotations/idmanytoone/CourseStudent.java

@@ -12,12 +12,14 @@
 import jakarta.persistence.JoinColumn;
 import jakarta.persistence.ManyToOne;
 import jakarta.persistence.Table;
+import org.hibernate.annotations.processing.Exclude;
 
 /**
  * @author Alex Kalashnikov
  */
 @Entity
 @Table(name = "idmanytoone_course_student")
+@Exclude // Avoid generating an IdClass through the annotation processor. See https://hibernate.atlassian.net/browse/HHH-18829
 public class CourseStudent implements Serializable {
 
 	@Id

hibernate-core/src/test/java/org/hibernate/orm/test/annotations/idmanytoone/IdManyToOneTest.java

@@ -14,6 +14,7 @@
 import org.hibernate.boot.model.naming.ImplicitNamingStrategyJpaCompliantImpl;
 import org.hibernate.cfg.Configuration;
 
+import org.hibernate.testing.orm.junit.Jira;
 import org.hibernate.testing.orm.junit.JiraKey;
 import org.hibernate.testing.junit4.BaseCoreFunctionalTestCase;
 import org.junit.Test;
@@ -78,6 +79,41 @@ public void testCriteriaRestrictionOnIdManyToOne() {
 		} );
 	}
 
+	@Test
+	@Jira("https://hibernate.atlassian.net/browse/HHH-11026")
+	public void testMerge() {
+		inTransaction( s-> {
+			Student student = new Student();
+			student.setName( "s1" );
+			Course course = new Course();
+			course.setName( "c1" );
+			s.persist( student );
+			s.persist( course );
+
+			CourseStudent courseStudent = new CourseStudent();
+			courseStudent.setStudent( student );
+			courseStudent.setCourse( course );
+			student.getCourses().add( courseStudent );
+			course.getStudents().add( courseStudent );
+			s.merge( student );
+
+			// Merge will cascade Student#courses and replace the CourseStudent instance within,
+			// but the original CourseStudent is still contained in Student#courses that will be cascaded on flush,
+			// which is when the NonUniqueObjectException is thrown, because at that point,
+			// two CourseStudent objects with the same primary key exist.
+			// This can be worked around by replacing the original CourseStudent with the merged on as hinted below,
+			// but I'm not sure if copying the CourseStudent instance on merge really makes sense,
+			// since the load for the merge showed that there is no row for that key in the database.
+			// I tried avoiding the copy in org.hibernate.event.internal.DefaultMergeEventListener#copyEntity
+			// which also required updating the child-parent state in StatefulPersistenceContext to point to
+			// the new parent according to the MergeContext. This mostly worked, but required further investigation
+			// to fix a few failing tests. This copy on merge topic needs to be discussed further before continuing.
+
+//			course.getStudents().remove( courseStudent );
+//			course.getStudents().add( student.getCourses().iterator().next() );
+		} );
+	}
+
 	@Override
 	protected Class[] getAnnotatedClasses() {
 		return new Class[] {

hibernate-core/src/test/java/org/hibernate/orm/test/annotations/idmanytoone/Student.java

@@ -5,7 +5,10 @@
 package org.hibernate.orm.test.annotations.idmanytoone;
 
 import java.io.Serializable;
+import java.util.HashSet;
 import java.util.Set;
+
+import jakarta.persistence.CascadeType;
 import jakarta.persistence.Entity;
 import jakarta.persistence.GeneratedValue;
 import jakarta.persistence.Id;
@@ -25,8 +28,8 @@
 
 	private String name;
 
-	@OneToMany(mappedBy = "student")
-	private Set<CourseStudent> courses;
+	@OneToMany(mappedBy = "student", cascade = CascadeType.ALL)
+	private Set<CourseStudent> courses = new HashSet<>();
 
 	public Student() {
 	}
@@ -47,7 +50,7 @@
 		this.name = name;
 	}
 
 	public Set<CourseStudent> getCourses() {
 		return courses;
 	}