映射地图 <String,List<Foo> &gt;在休眠

Question

Considering the following object:

Keyword{
  int id;
  String area;
  String keyword;
  Article article;
}

What is the best way to Map the following association in Hibernate, where the map's key should be Keyword.area:

Article{
  private Map<String,List<Keyword>> keywords = new HashMap<String, List<Keyword>>();
}

It is straightforward if it was a Map, but in this situation I will mostly need to access the keywords by area (eg article.findAll("science") to return all science keywords).

The best idea I've had so far is to transform the former into

private Map<String,KeywordGroup> keywords = new HashMap<String, KeywordGroup>();

but this leaves me with some questions, like:

• Properties in KeywordGroup? Logically it would be composed of "area" and List, but how to map this?
• The simplest database design for this is to have a FK column in Keyword for article_id, won't KeywordGroup complicate this?

What approach would you suggest?

Answer 1

I think the following implementation of Article would work for you, without the need for a Map.

@Entity
public class Article {

    @OneToMany(mappedBy="article")
    private Set<Keyword> keywords;

    //..

    public Set<Keyword> findKeywordsByArea(String area) {

        Set<Keyword> matchingKeywords = new HashSet<Keyword>();
        for(Keyword keyword : this.keywords) {
            if(keyword.getArea().equals(area)) {
                matchingKeywords.add(keyword);
            }
        }
        return matchingKeywords;
    }
}