使用左联接生成泛型查询DSL orderBy动态路径

Question

我在将JPA与Querydsl和Hibernate一起用于数据存储管理时遇到了一个问题。示例模型如下：

@Entity
public class User {
    ....

    @ManyToOne
    @JoinColumn(name = "CATEGORY_ID")
    private Category category;
}

@Entity
public class Category {
    ..
    private String acronym;

    @OneToMany(mappedBy = "category")    
    List<User> userList;    
}

在我的Spring MVC webapp中，我有一个包含用户参数和orderBy选择的搜索表单。orderBy选择可以是User属性或Category属性。orderBy参数存储为Map（f.e.{“login”：“adm”,{“firstname”：“john”}。搜索函数接收搜索参数（以字符串形式）和上面带有订单规格的Map。订购的简化代码如下：

Map<String, String> orderByMap = new HashMap<String, String>();
orderByMap.put("firstName", "asc");
orderByMap.put("unit.acronym", "desc");

PathBuilder<User> pbu = new PathBuilder<User>(User.class, "user");

....

for (Map.Entry<String, String> order : orderByMap.entrySet())
{
    // for simplicity I've omitted asc/desc chooser
    query.orderBy(pbu.getString(order.getKey()).asc());
}

当我想要通过category的参数（如{“category.acronym”,“desc”}）进行排序时，问题就开始了。正如这里所解释的，上面的代码将使querydsl使用cross join with Category表，并省略没有Categories的用户，这不是预期的行为。

我知道，我必须引入左联接与类别，并使用别名排序使其工作，如果我正在寻找一种高效的方法来动态地做它。剥离每个字符串以查找类别或任何其他实体（如“user.category.subcategory.propetry”）将引入大量难看的代码，我不希望这样做。

如果你能帮我找到更好的解决方案我会很感激的。

Answer 1

我现在向Querydsl https://github.com/mysema/Querydsl/issues/582的测试端添加了实现的原型

如果这是一个常见的用例，我将考虑直接集成到Querydsl中

public class OrderHelper {

private static final Pattern DOT = Pattern.compile("\\.");

public static PathBuilder<?> join(JPACommonQuery<?> query, PathBuilder<?> builder, Map<String, PathBuilder<?>> joins, String path) {
    PathBuilder<?> rv = joins.get(path);
    if (rv == null) {
        if (path.contains(".")) {
            String[] tokens = DOT.split(path);
            String[] parent = new String[tokens.length - 1];
            System.arraycopy(tokens, 0, parent, 0, tokens.length - 1);
            String parentKey = StringUtils.join(parent, ".");
            builder = join(query, builder, joins, parentKey);
            rv = new PathBuilder(Object.class, StringUtils.join(tokens, "_"));
            query.leftJoin((EntityPath)builder.get(tokens[tokens.length - 1]), rv);
        } else {
            rv = new PathBuilder(Object.class, path);
            query.leftJoin((EntityPath)builder.get(path), rv);
        }
        joins.put(path, rv);
    }
    return rv;
}

public static void orderBy(JPACommonQuery<?> query, EntityPath<?> entity, List<String> order) {
    PathBuilder<?> builder = new PathBuilder(entity.getType(), entity.getMetadata());
    Map<String, PathBuilder<?>> joins = Maps.newHashMap();

    for (String entry : order) {
        String[] tokens = DOT.split(entry);
        if (tokens.length > 1) {
            String[] parent = new String[tokens.length - 1];
            System.arraycopy(tokens, 0, parent, 0, tokens.length - 1);
            PathBuilder<?> parentAlias = join(query, builder, joins, StringUtils.join(parent, "."));
            query.orderBy(parentAlias.getString(tokens[tokens.length - 1]).asc());
        } else {
            query.orderBy(builder.getString(tokens[0]).asc());
        }
    }
}

}