I have 5 tables:
User:
username (PK)
Role:
role_id(PK)
role_name
Permissions:
perm_id(PK)
perm_name
User_role_rels:
ur_id(PK)
username(FK) -> user.username
role_id(FK) -> role.role_id
Role_perm_rels:
rp_id(PK)
role_id(FK) -> role.role_id
perm_id(FK) -> Permissions.perm_id
When I create JPA entities for these five tables I get List of UserRoleRels in User entity, but I would need list of permissions for this user. So, there should be a
List permList in User entity.
I am new to JPA and not sure how I can achieve this using annotations?
Since there is no direct relationship between User and Permission, it's correct that User doesn't have a permList collection property.
Assuming that your IDE has created correctly the entities according to your tables, you don't have ManyToMany annotations in your classes, but only OneToMany and ManyToOne. This makes a little cumbersome to fetch the permissions collection:
String userName = "";
User myUser = entityManager.find(User.class, userName);
for (UserRoleRels urr : myUser.getUserRoleRelsList()) {
Role r = urr.getRole();
for(RolePermRels rpr : r.getRolePermRelsList()) {
Permission p = rpr.getPermission();
}
}
If you don't have an extra property in the ManyToMany relation tables (in your case User_role_rels and Role_perm_rels), you can drop the PK field for both. This will let your IDE: (1) create simpler Entity classes, (2) avoid the creation of the Entity classes for the relation tables, (3) use the ManyToMany annotations. With that change, you'll be able to access the permissions more easily and in a more readable way:
String userName = "";
User myUser = entityManager.find(User.class, userName);
for (Role r : myUser.getRoleList()) {
List<Permission> pl = r.getPermissionList();
}
Useful links: EclipseLink UserGuide, JPA Wikibooks.
Related
Apparently, EF6 doesn't like objects that have multiple foreign key properties that use the same key value, but do not share the same reference. For example:
var user1 = new AppUser { Id = 1 };
var user2 = new AppUser { Id = 1 };
var address = new Address
{
CreatedBy = user1, //different reference
ModifiedBy = user2 //different reference
};
When I attempt to insert this record, EF throws this exception:
Saving or accepting changes failed because more than one entity of type
'AppUser' have the same primary key value. [blah blah blah]
I've discovered that doing this resolves the issue:
var user1 = new AppUser { Id = 1 };
var user2 = user1; //same reference
I could write some helper code to normalize the references, but I'd rather EF just know they're the same object based on the ID alone.
As for why EF does this, one explanation could be that its trying to avoid doing multipe CRUD operations on the same object since separate instances of the same entity could contain different data. I'd like to be able to tell EF not to worry about that.
Update
So it's as I suspected per my last paragraph above. In absense of a means to tell EF not to do CRUD on either instance, I will just do this for now:
if (address.ModifiedBy.Id == address.CreatedBy.Id)
{
address.ModifiedBy = address.CreatedBy;
}
Works well enough so long as I am not trying to do CRUD on either.
Update2
I've previously resorted to doing this to prevent EF from validating otherwise-required null properties when all I need is the child entity's ID. However, it doesn't keep EF from going into a tizzy over separate instances with the same ID. If it's not going to do CRUD on either AppUser object, why does it care if the instances are different?
foreach (var o in new object[] { address.ModifiedBy, address.CreatedBy })
{
db.Entry(o).State = EntityState.Unchanged;
}
If you get AppUser from context, then you will not need to do anything, because Entity Framework will track entities:
var user1 = context.AppUsers.Find(1);
var user2 = context.AppUsers.Find(1);
var address = new Address
{
CreatedBy = user1, //different reference
ModifiedBy = user2 //different reference
};
Now, they both will point to same objects and will not cause to conflict.
You can add two extra properties to have the Id for the main objects which is the AppUser, then you can use only one AppUser object and reference it for both the created and modified by properties.
CreatedById = user1.Id,
ModifiedById = user1.Id
Otherwise, your code will end up by saving two instances of AppUser with the same primary key.
Another approach is to set both the foreign key properties to only one AppUserobject
The explanation is that EF's change tracker is an identity map. I.e. a record in the database is mapped to one, and only one, CLR object.
This can be demonstrated easily by trying to attach two objects with the same key:
context.AppUsers.Attach(new AppUser { Id = 1 });
context.AppUsers.Attach(new AppUser { Id = 1 });
The second line will throw an exception:
Attaching an entity of type 'AppUser' failed because another entity of the same type already has the same primary key value.
This also happens if you assign
CreatedBy = user1, //different reference
ModifiedBy = user2 //different reference
Somewhere in the process, user1 and user2 must be attached to the context, giving rise to the exception you get.
Apparently, you have a function that receives two Id values that can be different or identical. Admittedly, it would be very convenient if you could simply create two AppUser instances from these Ids, not having to worry about identical keys. Unfortunately, your solution ...
if (address.ModifiedBy.Id == address.CreatedBy.Id)
... is necessary. Solid enough, though.
I have a User entity generated in Netbeans from an existing database table. The table has a column lastUpdatedByUser that is a User entity. Most of the tables in this database have a lastUpdatedByUser column and queries against those entities correctly return a user object as part of the result.
Ex. Retrieve FROM ProductionTable WHERE date = 'someDate' has a lastUpdatedByUser object that shows who last updated the table row and the rest of their user attributes.
If the productionTable data is edited in the web-app and submitted I need to update the lastUpdatedByUser column.
Users userUpdating = usersService.selectUserEntityByUserId(userId);
Users userEntity = usersFacade.findSingleWithNamedQuery("Users.findByUserId", parameters);
SELECT u FROM Users u WHERE u.userId = :userId
returns a User object that contains a lastUpdatedByUser that is a User object that contains a lastUpdatedByUser that is a User object that contains a lastUpdatedByUser object.... (I have no clue how many there are, and twenty rows of these adds up)
After I persist this
productionEntity.setLastUpdatedByUser(userUpdating);
I get Json StackOverflowError in the next request for the updated entity
gson.toJson(updatedProductionEntity)
The Users entity definition:
#OneToMany(mappedBy = "lastUpdatedByUser")
private Collection<Users> usersCollection;
#JoinColumn(name = "LastUpdatedByUser", referencedColumnName = "UserId")
#ManyToOne
private Users lastUpdatedByUser;
#OneToMany(mappedBy = "lastUpdatedByUser")
private Collection<Production> productionCollection;
How can edit that such that I continue to get a user object as part of other entities like Production, but only a single lastUpdatedByUser object for a User entity?
Thanks for any insight.
I'm guessing this is my issue:
#JoinColumn(name = "LastUpdatedByUser", referencedColumnName = "UserId")
as I found a FK in the Users table to its own UserId
Love refactoring
================================
Drop that FK from the Users table and regenerate the entity in Netbeans and I get
private Integer lastUpdatedByUser;
like it should be
instead of
private Users lastUpdatedByUser;
Now I get to edit all the entities that have valid FKs into the Users table and code and...
Thanks for listening.
I have a REST interface for a datamodel that has several one-to-many and many-to-many relationships between entities. While many-to-many relationships seem easy to manage statelessly, I'm having trouble with one-to-many. Consider the following one-to-many relationship:
Employee:
#ManyToOne
#JoinColumn(name = "Company_id")
private Company company;
Company:
#OneToMany(mappedBy = "company", cascade = CascadeType.ALL, orphanRemoval=true)
public Set<Employee> employees = new HashSet<Employee>();
When a company is updated, its employee collection may have been updated as well (employees removed or added) but since the REST interface only allows updating the company as a whole, I cannot explicitly delete or add employees.
Simply replacing the collection does not work, but I found that this seems to work:
public void setEmployees(Set<Employee> employee) {
this.employees.clear(); // magic happens here?
this.employees.addAll(employees);
for (Iterator<Employee> iterator = employees.iterator(); iterator.hasNext();) {
Employee employee = (Employee) iterator.next();
employee.setCompany(this);
}
}
Is this the way it should be done, or is there a better way?
EDIT: In fact the above does not work! It appears to work at first, but then it will break with:
Exception in thread "main" java.lang.IllegalStateException: An entity copy was already assigned to a different entity.
I assume this happens because the db already contains a set of employees and if any of the "old" employees are also part of the replacement set, they collide with the ones in the database.
So what is the right way to replace the set?
First make sure equals is implemented properly. As per hibernate spec: http://docs.jboss.org/hibernate/orm/4.1/manual/en-US/html/ch04.html#persistent-classes-equalshashcode
I had a similar problem doing a merge. Essentially I had to fetch the existing employees associated with the company. I had to merge any changes to existing employees, and then add any new employees.
Query query = em.createQuery("select e from Employee e where e.company = '" + company.getId() + "'");
Collection<Employee> existingEmployees = new LinkedList<Employee>();
try{
Iterables.addAll(existingEmployees, (Collection<Employee>) query.getResultList());
}
catch(NoResultException nre){
//No results
}
for(Employee existingEmployee : existingEmployees){
for(Employee employee : company.getEmployees()){
if(existingEmployee.name().equals(employee.name())){
employee.setId(existingEmployee.getId());
}
employee.setCompany(company);
}
}
i think you have no better choice then to replace the existing collection and simply set the new one provided by the REST response.
Say I have two entities with about 20 properties per entity and a Many-to-Many relationship like so:
User (Id int,Name string, .......)
Issue (Id int,Name string, .......)
IssueAssignment (UserId,RoleId)
I want to create a new Issue and assign it to a number of existing Users. If I have code like so:
foreach(var userId in existingUserIds)
{
int id = userId
var user = _db.Users.First(r => r.Id == id);
issue.AssignedUsers.add(user);
}
_db.Users.AddObject(user);
_db.SaveChanges();
I noticed it seems terrribly inefficient when I run it against my SQL Database. If I look at
the SQL Profiler it's doing the following:
SELECT TOP(1) * FROM User WHERE UserId = userId
SELECT * FROM IssueAssignment ON User.Id = userId
INSERT INTO User ....
INSERT INTO IssueAssignment
My questions are:
(a) why do (1) and (2) have to happen at all?
(b) Both (1) and (2) bring back all fields do I need to do a object projection to limit the
fields, seems like unnecessary work too.
Thanks for the help
I have some possible clues for you:
This is how EF behaves. _db.Users is actaully a query and calling First on the query means executing the query in database.
I guess you are using EFv4 with T4 template and lazy loading is turned on. T4 templates create 'clever' objects which are able to fixup their navigation properties so once you add a User to an Issue it internally triggers fixup and tries to add the Issue to the User as well. This in turns triggers lazy loading of all issues related to the user.
So the trick is using dummy objects instead of real user. You know the id and you only want to create realtion between new issue and existing user. Try this (works with EFv4+ and POCOs):
foreach(var userId in existingUserIds)
{
var user = new User { Id = userId };
var _db.Users.Attach(user); // User with this Id mustn't be already loaded
issue.AssignedUsers.Add(user);
}
context.Issues.AddObject(issue);
context.SaveChanges();
I m getting this Exception-"The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects."
I ve user table and country table. The countryid is referred in user table.
I am getting the above Exception when I am trying to add entry in user table.
This is my code-
using (MyContext _db = new MyContext ())
{
User user = User .CreateUser(0, Name, address, city, 0, 0, email, zip);
Country country = _db.Country.Where("it.Id=#Id", new ObjectParameter("Id",countryId)).First();
user.Country = country;
State state = _db.State.Where("it.Id=#Id", new ObjectParameter("Id", stateId)).First();
user.State = state;
_db.AddToUser(user );//Here I am getting that Exception
_db.SaveChanges();
}
Try adding the user first, then adding the relationships.
See http://www.code-magazine.com/article.aspx?quickid=0907071&page=4
Or, don't use User.CreateUser where you are explicitly setting an Id = 0, instead use User user = new User() {Name = Name, Address = ...}
BTW, with Entity Framework 4 you can set the foreign key IDs directly removing the need to load the related object if you know its ID.