如何从具有多对多关系的MySQL数据库中检索对象？

Question

I have 2 tables: Document and Category with a many to many relationships. In addition, the relationship has an attribute called weight . I also have the corresponding Classes with Document being represented like this:

public class Document {
    private Integer id; // Id from DB, 0 if new object
    private String name;
    private String url;
    private Date dateCreated;
    private List<Category> categories;

    // getters & setters...
    }

Now I have 2 questions - best practices concerns especially. I want to retrieve data from the database (MySQL) and and build my objects. I use a SELECT statement like that:

SELECT doc.*, GROUP_CONCAT(cat.CategoryID SEPARATOR ',') AS CategoryGrp FROM document doc
LEFT JOIN doc_category dc ON doc.Doc_ID = dc.DC_DocID
LEFT JOIN category cat ON dc.DC_CategoryID = cat.CategoryID
GROUP BY doc.Doc_ID

In the resultset, rs.getString(5) --> CategoryGrp will have the IDs of the related categories. Now how do I proceed to build my object Document by linking its Categories as the Class requires it.

I tend to do new SELECT request on table Category to get the others details (category name for instance) in a loop while still on the first ResultSet. Is that a good practice? Isn't there a "recommended" way?

My second question is about the attribute weight in junction table. In which object should I record its value? I guess it's Category but not very sure since Category object can't have a weight on its own...

Hope I have clear enough. Thanks

Answer 1

Answer to 1st question: 2 ways: one sql or like you suggest 2 sql. (retriev the docs and for every cat id perform sql to retrive cat attributes) I think this is better: Get your all docs with different cats and its attributes. Loop it and compare the doc with the previous one. If different, then you have a new doc object. Every line in the loop is a different cat object.

doc1 cat1
doc1 cat2
doc1 cat3
doc2 cat4
doc3 cat5
doc3 cat6
... 

Answer to 2nd question: I think you should consider this: is the weight more related to doc or cat? On the other hand, you cannot have multiple weights on 1 doc, so my guess would be to add the to cat (as that is a 1 on 1 relationship.

Answer 2

Many-to-many relationship in RDBMS are ALWAYS done by at least 3 tables. So you need a third table, which will contain the ID`s from the first two, all of your SELECT-s will go through this third table.