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Changing a OneToMany Bidirectional Relationship to ManyToMany Bidirectional

I want to convert the following mapping on courseDetails to manyToMany.
This is because I get an exception Found shared references to a collection: com.xyz.courseDetails and I assume this happens because the relation is not actually one to many in the database, since there are some course_detail tuples that has multiple courses.
#Entity
#Table(name = "courses")
public class Course
{
#Column(name = "course_detail_id")
private Long extendedCourseDetailId;
...
#OneToMany(fetch = FetchType.LAZY, targetEntity = CourseDetail.class, cascade = CascadeType.ALL)
#JoinColumn(name="id", referencedColumnName="course_detail_id")
private List<CourseDetail> courseDetails = new ArrayList<>();
}
Simply changing the annotation to ManyToMany does not work, JPA somehow couldn't find the related columns. Why? How can I do this?

What do you think of this :
Let's assume the entity CourseDetail has as ID :
public class CourseDetail
{
#Id
#Column(name = "cd_id")
private Long courseDetailId;
So this non tested code might help you.
where the table "course__course_detail" will be automatically created to hold the relationship with 2 columns : "course_id" and "coursedetail_id".
#Entity
#Table(name = "courses")
public class Course
{
#Id
#Column(name = "c_id")
private Long courseId;
// #Column(name = "course_detail_id") // I comment because I dont understand the purpose
// private Long extendedCourseDetailId;
...
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(name = "course__course_detail",
joinColumns = #JoinColumn(name = "course_id", referencedColumnName="c_id"),
inverseJoinColumns = #JoinColumn(name = "coursedetail_id", referencedColumnName="cd_id"),
)
private List<CourseDetail> courseDetails = new ArrayList<>();
}
PS: NOT TESTED
Feel free to tell me more in comments.

Is it possible to implement (0..n) many to many (using #ManyToMany annotation) relation in JPA?

I'm looking for a way to implement (0..n) many to many relation in JPA, much possibly using #ManyToMany annotation. All examples that I found were about (1..n) relations. What I need to accomplish:
- I've got two entities: Contact and Tag. Each Contact can have 0..n Tags. Each Tag can have 0..n Contacts. From SQL point of view it would look like
this: Contact (0..n) --- (1) Contact_has_Tag (1) --- (0..n) Tag.
Code below is not working for me because JPA is linking columns with INNER JOIN.
OFC I could do this using intermediate entity and #OneToMany and #ManyToOne annotations, but I want a simpler sollution.
#Data
#Entity
public class Contact {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#JsonIgnore
private long id;
#Column(unique = true)
private String email;
// ...
#ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
#JsonIgnore
#JoinTable(
name = "contact_has_tag",
joinColumns = #JoinColumn(name = "contact_id", referencedColumnName = "id", updatable = false, nullable = true),
inverseJoinColumns = #JoinColumn(name = "tag_id", referencedColumnName = "id", updatable = false, nullable = true))
private List<ContactTag> contactTags = new ArrayList<ContactTag>();
}
#Entity
#Data
public class ContactTag {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
#Column
private String name;
#ManyToMany(mappedBy="contactTags", fetch = FetchType.LAZY)
#JsonIgnore
private List<Contact> contacts = new ArrayList<Contact>();
}
Any ideas how it should be done?

Get id's of a ManyToMany mapping table

I'm writing an API using Spring Boot and Hibernate where my persisted entity objects are also used as DTOs sent to and from the client. This is a simplified version of a typical entity I use:
#Entity
#Table(name = "STUDENT")
public class Student {
#Id
#GeneratedValue
#Column(name = "ID")
private Long id;
#ElementCollection
#CollectionTable(name = "GROUP_STUDENT",
joinColumns = #JoinColumn(name = "GROUP_ID"))
#Column(name="STUDENT_ID")
private Set<Long> groupIds;
#JsonIgnore
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(name="GROUP_STUDENT",
joinColumns = #JoinColumn(name="GROUP_ID"),
inverseJoinColumns = #JoinColumn(name="STUDENT_ID")
)
private Set<Group> groups = new HashSet<>();
// getters and setters
}
and this is the associated class:
#Entity
#Table(name = "GROUP")
public class Group {
#Id
#GeneratedValue
#Column(name = "ID")
private Long id;
#JsonIgnore
#ManyToMany(fetch = FetchType.LAZY, mappedBy = "groups")
private Set<Student> students = new HashSet<>();
// getters and setters
}
As you can see, there is a #ManyToMany association between Student and Group.
Since I send objects like these to the client, I choose to send only the id's of the associations and not the associations themselves. I've solved this using this answer and it works as expected.
The problem is this. When hibernate tries to persist a Student object, it inserts the groups as expected, but it also tries to insert the groupIds into the mapping table GROUP_STUDENT. This will of course fail because of the unique constraint of the mapping table composite id. And it isn't possible to mark the groupIds as insertable = false since it is an #ElementCollection. And I don't think I can use #Formula since I require a Set and not a reduced value.
This can of course be solved by always emptying either the groups of the groupIds before saving or persisting such an entity, but this is extremely risky and easy to forget.
So what I want is basically a read only groupIds in the Student class that loads the data from the GROUP_STUDENT mapping table. Is this possible? I'm grateful for any suggestions and glad to ellaborate on the question if it seems unclear.

I've managed to solve this by making the id-collection #Transient and populating it using #PostLoad:
#Entity
#Table(name = "STUDENT")
public class Student {
#PostLoad
private void postLoad() {
groupIds = groups.stream().map(Group::getId).collect(Collectors.toSet());
}
#Id
#GeneratedValue
#Column(name = "ID")
private Long id;
#Transient
private Set<Long> groupIds;
#JsonIgnore
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(name="GROUP_STUDENT",
joinColumns = #JoinColumn(name="GROUP_ID"),
inverseJoinColumns = #JoinColumn(name="STUDENT_ID")
)
private Set<Group> groups = new HashSet<>();
// getters and setters
}

Hibernate: Join table with extra columns, remove children from one side

The scenario is the following
I have 2 tables, Company and Activity. A company can have one or more activities. One of these activities is a "primary" activity, and all others become secondary.
To handle this, I created 2 entities (Activity, Company) and a third entity for the join table, which is CompanyActivity
I used this tutorial as a starting point
Below my code (getters and setters omitted)
Company.java
#Entity
#Table(name = "T_COMPANY")
public class Company {
#Id
#Column(name = "COM_ID")
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "company")
private List<CompanyActivity> activities = new ArrayList<>();
}
Activity.java
#Entity
#Table(name = "T_ACTIVITY")
public class Activity {
#Id
#Column(name = "ACT_ID")
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String code;
private String description;
private boolean availableOnline;
}
CompanyActivity.java
#Entity
#Table(name = "T_COMPANY_ACTIVITY")
public class CompanyActivity {
#Id
#Column(name = "COM_ACT_ID")
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#ManyToOne(cascade = CascadeType.ALL)
#JoinColumn(name = "COM_ID")
private Company company;
#ManyToOne(cascade = CascadeType.ALL)
#JoinColumn(name = "ACT_ID")
private Activity activity;
private boolean primary;
}
Adding activities for a company works without a problem. The children collection contains the newly added activities, and there is always one marked as primary as expected.
The problem happens when updating a company.
When I add a new activity, all previous existing activities are persisted again.
When I remove an activity, it is not removed from the table.
I'm using this code to update a company' activities
company.getActivities().clear();
company.getActivities().addAll(newActivities);
company = repository.save(company);
In this code, newActivities have the new activities that should be considered (this collection does not have the previous ones, I just replace them all)
I tried adding orphanRemoval=true to the #OneToMany(cascade = CascadeType.ALL, mappedBy = "company") on Company, but this deletes the activity type when no other company is using it, which is wrong as they should be available always.
Can you please help me sync the activities collection on Company without removing elements from Activity table ?
Thanks a lot!

I solved it. Here are the steps I followed.
First, I changed my Join table entity cascade types as follows
#Entity
#Table(name = "T_COMPANY_ACTIVITY")
public class CompanyActivity {
#Id
#Column(name = "COM_ACT_ID")
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
#JoinColumn(name = "COM_ID")
private Company company;
#ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
#JoinColumn(name = "ACT_ID")
private Activity activity;
private boolean primary;
}
Then, I added again the "orphanRemoval" property to Company mapping, and changed my CascadeTypes too, as follows
#OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, mappedBy = "empresa", orphanRemoval = true)
private List<CompanyActivity> activities = new ArrayList<>();
With these changes, my mapping works as expected with the same code I used to replace the relationships.
company.getActivities().clear();
company.getActivities().addAll(newActivities);
company = repository.save(company);
Thanks :)

The way you created your entities is not correct. You don't need to create an entity for your join table (CompanyActivity/T_COMPANY_ACTIVITY). Instead you should be using the #JoinTable on your activities entity. Something like below:
#OneToMany(cascade = CascadeType.ALL, mappedBy = "company")
#JoinTable(
name = "T_COMPANY_ACTIVITY",
joinColumns = #JoinColumn(name = "COM_ID"),
inverseJoinColumns = #JoinColumn(name = "ACT_ID")
)
private List<CompanyActivity> activities = new ArrayList<>();
for more detailed explanation on how One-to-Many/Many-to-One with Join tables work here: http://www.codejava.net/frameworks/hibernate/hibernate-one-to-many-association-on-join-table-annotations-example

#ManyToMany with cascade = CascadeType.REMOVE removes associations AND entities

I have 2 entities: Group and Grouped, with 1 ManyToMany association.
In database, the Association table has a NOT NULL FK on both Group and Grouped.
I want Hibernate to delete the association but not the group when all grouped are deleted.
Code to delete a Grouped entity:
#Autowired
private final GroupedRepository groupedRepository;
public void delete(Grouped groupedToRemove) {
groupedRepository.delete(groupedToRemove);
}
If I set cascade = CascadeType.ALL or cascade = CascadeType.REMOVE, my Group entities are deleted when I delete a Grouped entity, not only the associations:
#ManyToMany(cascade = CascadeType.ALL, // same behavior with CascadeType.REMOVE
mappedBy = "grouped",
targetEntity = Group.class)
private Set<Group> groups = new HashSet<>();
If I remove the cascade, hibernate tries to set group_id=null and it throws a ModelConstraintException. I don't want to set the FK as nullable.
Group entity:
#Entity
#Table(name = "groups")
#Getter
#Setter
public class Group {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Integer id;
#ManyToMany(targetEntity = Grouped.class)
#JoinTable(
name = "association",
joinColumns = #JoinColumn(name = "group_id", nullable = false, updatable = false),
inverseJoinColumns = #JoinColumn(name = "grouped_id", nullable = false, updatable = false)
)
private Set<Grouped> grouped= new HashSet<>();
}
Grouped entity:
#Entity
#Table(name = "grouped")
#Getter
#Setter
public class Grouped {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Integer id;
#ManyToMany(mappedBy = "grouped", targetEntity = Group.class)
private Set<Group> groups= new HashSet<>();
}

That's the expected behavior. REMOVE cascading means: when removing this entity, also remove the associated entities. It makes no sense on a ManyToXxx, since obviously, other entities are still referencing the associated entity.
If you want to delete a Grouped, but leave the associated Groups there, you need to remove the association between the two entities first:
for (Group group : grouped.getGroups()) {
group.getGrouped().remove(grouped);
}
grouped.getGroups().clear();
and then remove the Grouped entity, which is not associated to any Group anymore.