1-in this example , how is the phone_order done? entity manager automatically put some sort of int  value in this field on the database side (we do not have any real field on the class side to represent this column) . if yes , how the order gets its priorities , assume that we have a form that has 10 inputs to enter different phone numbers , then the first one automatically gets the highest priority and the last one the lowest?

@ElementCollection

@OrderColumn(name = “phone_order”)

List<String> phones;

2- the other thing is that , there is no extra query required for getting these phones back for one customer ? as long as we have the annotation like above , it should be able to access to it ?.  just say "return List <String> customer.phones  "  ?

my current design for my collection type " -score " -  seems correct , I even looked at some other csns classes  , and some online sources. but when I am running TestNg it gives me this error . I have taken a snapshot of the error message.  error happens when I call something like "rubricScore.getScores"   ....

adding "Transactional" to the Dao file did not help either

Failed to Lazily initialize a collection.......  No session or session was closed.

The only thing that I am not able to figure out is that it returns one extra NULL value , I even checked the table , there is no empty cell nor is NULL value.

just tested it for criterias , and as soon as it reaches the collection object (Descriptions for each criteria) it returns one extra Null value for index 0 and the rest of the real results after that .

the other thing is that because each Rubric has Criteria and each criteria has a collection object of rate description , when we make a Rubric object and try to reach the criteria property of it , the size of criteria is , the actual size of criteria (size of List<Criteria>) * (times)  size of rate Description or rubric scale (List<description>). it is like joining between Criteria and rate Description table(rate description has duplicate Criteria id / "as joincolumn" ) .which returns multiple of Criteria objects . which makes sense if we just were doing a simple join in sql , but not for an object property.

better explanation is that the number of criteria are 3 here and each criteria has 3 rate description fields . it returns the criteria size for a rubric 9 = 3*3

so whether I am doing something wrong or there should be a better way around this

So we should not use the "@ElementCollection(fetch = FetchType.EAGER  )" ?

I am going to try removing it and to see if it works . sorry professor but I am not sure what exactly Eager Fetching does . I realized that one or two classes in csns2 are using it , that was the reason I tried it and it worked .

aligh1979 wrote:

I am going to try removing it and to see if it works . sorry professor but I am not sure what exactly Eager Fetching does . I realized that one or two classes in csns2 are using it , that was the reason I tried it and it worked .

Eager loading means that whenever an object (e.g. Criterion) is loaded, the object it references (e.g. List<String> descriptions) is also loaded. Lazy loading means that the referenced object is only loaded when it is accessed, e.g. descriptions are only loaded when you call criterion.getDescriptions(). Lazy loading is a great feature of ORM because you don't waste resources loading data that is not used, so you should stick with lazy loading in general. You would want to use eager loading in two situations:

a) Performance optimization

Lazy loading involves two queries to the database while eager loading is only one query (with join). If the two objects are always used together (like a criterion and its descriptions), you should do eager loading to save one query.

b) Lazy loading requires a connection to the database (and in fact, requires the same entity manager / Hibernate session), so in an environment where the application can be disconnected from the database, you have to do eager loading.

aligh1979 wrote:

then it seems a good design to have The Eager fetching for a Criteria and its description . but one problem that occurs as I mentioned was that because the description are loaded whenever a Criteria object is loaded , it was linking each criteria to its Criteia_description collection table and was returning multiplied size  list of Criteria for a Rubric object

Like I said, that shouldn't happen. There must be something wrong in your code.

aligh1979 wrote:

I have added this in my Rubric model

    @OneToMany(mappedBy = "rubric" , cascade=CascadeType.ALL )
    @OrderColumn(name = "criteria_order")
    private List<Criteria>  criterias;

the column is added to my criteria table but unfortunately is getting filled up automatically. (empty cells)

It's supposed to be filled up automatically by Hibernate using the index of the List. Note that if you pre-create a rubric with some criteria in csns-test-insert.sql, you need to modify the INSERT statements to include the values for the criteria_order column.

It may have something to do with the way you save a Criterion. Assuming your classes look like these:

class Rubric {


    List<Criterion> criteria;


}


class Criterion {


    Rubric rubric;


}


A bidirectional association with a List is somewhat tricky because it's not exactly bidirectional - notice that the List index is kept on the Rubric side but not the Criterion side, so if you use a criterionDao.saveCriterion() to save a new criterion, the order column will be empty. It should work if you add the new criterion to the rubric, then do a rubricDao.saveRubric().