java QueryDSL 中的多列
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Multiple columns in QueryDSL
提问by Gon?alo Cardoso
I'm trying to get a list of multiple columns from my table using QueryDSL, and automatically fill my DB object, like this example in an older manual:
我正在尝试使用 QueryDSL 从我的表中获取多列的列表，并自动填充我的 DB 对象，就像旧手册中的这个例子：
List<CatDTO> catDTOs = query.from(cat)
.list(EConstructor.create(CatDTO.class, cat.id, cat.name));
The problem is that it looks like the EConstructor class was removed in version 2.2.0, and all the examples I find now are like this:
问题是2.2.0版本好像去掉了EConstructor类，我现在找到的所有例子都是这样的：
List<Object[]> rows = query.from(cat)
.list(cat.id, cat.name);
Which forces me to manually cast all the objects into my CatDTO class.
这迫使我手动将所有对象转换为 CatDTO 类。
Is there any alternative to this? Any EConstructor alternative?
有什么替代方法吗？任何 EConstructor 替代方案？
采纳答案by Timo Westk?mper
EConstructor has been replaced with ConstructorExpression in Querydsl 2.0. So your example would become
EConstructor 已被 Querydsl 2.0 中的 ConstructorExpression 取代。所以你的例子会变成
List<CatDTO> catDTOs = query.from(cat)
.list(ConstructorExpression.create(CatDTO.class, cat.id, cat.name));
You can also annotate the CatDTO constructor and query like this
您还可以像这样注释 CatDTO 构造函数和查询
List<CatDTO> catDTOs = query.from(cat)
.list(new QCatDTO(cat.id, cat.name));
Alternatively you can use the QTuple projection which provides a more generic access option
或者，您可以使用 QTuple 投影，它提供了更通用的访问选项
List<Tuple> rows = query.from(cat)
.list(new QTuple(cat.id, cat.name));
The actual values can be accessed via their path like this
可以像这样通过它们的路径访问实际值
tuple.get(cat.id)
and
和
tuple.get(cat.name)
Tuple projection will probably be used in Querydsl 3.0 for multiple columns projections instead of Object arrays.
元组投影可能会在 Querydsl 3.0 中用于多列投影而不是对象数组。
回答by Rafael Zeffa
using queryDSL 4 and Java 8 stream:
使用 queryDSL 4 和 Java 8 流：
List<CatDTO> cats = new JPAQueryFactory(entityManager)
.select(cat.id, cat.name)
.from(cat)
.fetch()
.stream()
.map(c -> new CatDTO(c.get(cat.id), c.get(cat.name)))
.collect(Collectors.toList());
回答by Daniel Higueras
Another alternative is to use the class Projections. It will construct the object using the fields you pass as parameters, like the EConstructor. Example:
另一种选择是使用 class Projections。它将使用您作为参数传递的字段来构造对象，例如EConstructor. 例子：
List<CatDTO> catDTOs = query.from(cat)
.list(Projections.bean(CatDTO.class, cat.id, cat.name));
Reference: http://www.querydsl.com/static/querydsl/4.0.5/apidocs/com/querydsl/core/types/Projections.html
参考：http: //www.querydsl.com/static/querydsl/4.0.5/apidocs/com/querydsl/core/types/Projections.html
