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Posts: 61
Posted 00:19 Feb 13, 2015 |

Hello Dr. Sun,

Could you please help me understand how hibernate works in the case of @CollectionTable types? I am not clear on how hibernate identifies the column types of the collection table specially when the collection type doesn't have any entity associated with it. I was referring to the "question_choices" table and it is only mentioned in the following code in csns also it didn't map to any entity.

    @ElementCollection(fetch = FetchType.EAGER)
    @CollectionTable(name = "question_choices",
        joinColumns = @JoinColumn(name = "question_id"))
    @Column(name = "choice")
    @OrderColumn(name = "choice_index")
    protected List<String> choices;

So how does hibernate identify question_id & choice_index as a primary key & how does it understand about the data types of choice_index & choice columns? Am I missing something here? Could we have created a QuestionChoice entity in this case?


Posts: 2935
Posted 10:58 Feb 13, 2015 |

The type of the choice column is the type of the elements in the collection, i.e. List<String>, and the default SQL type for String is varchar(255).

The default type for choice_index is integer as it is an @OrderColumn - as the name says, it's a column used to keep track of the order of the elements in the collection, so of course it's an integer.

Given a question and the index of a choice in the question, we can uniquely identify a choice. This is why the combination of (question_id, choice_index) is the primary key of the collection table.

Posts: 61
Posted 12:20 Feb 13, 2015 |

Thanks Dr. Sun,

I do understand the fact why (question_id, choice_index) is the primary key. What i am not able to understand is that how hibernate understands that it should be the primary key and generates the create statements. Why does it not think (question_id, choice) is the primary key.

Is it because we declare a @OrderColumn and as a rule it will always be a part of the primary key along with the foreign key? Also i noted for another such scenario for the "answer_selections" table where we don't have any primary key mentioned. Is it because in case of @ElementCollection the primary key is only formed if there is a @OrderColumn?


Posts: 2935
Posted 13:00 Feb 13, 2015 |

Two choices may have the same value. For example, I could create a question that says "what are the 3 most important things in real estate", and the choices would be:

  1. Location
  2. Location
  3. Location

This is why (question_id, choice_index) is the primary key but (question_id, choice) is not.

Hibernate will generate a primary key when you have an @OrderColumn. Strictly speaking it should generate one for Set as well (like ChoiceAnswer.selections), but I guess in JPA a List with @OrderBy and a Set are pretty much the same, so Hibernate doesn't generate primary key constraint for answer_selections, but you can add one if you want.

Posts: 61
Posted 13:53 Feb 13, 2015 |

Thanks Dr. Sun. That helps. I might bother you with further questions if and when i have them.