在Projection.Conditionals中为查询添加多个条件

Question

我正在尝试编写一个包含多个when子句的案例，如下所示：

...
case
    when 'starks' then 1
    when 'wildlings' then 2
    when 'lannisters' then 3
    Else 0
End
...

我以前只做过一个条件，比如

.OrderBy(Projections.Conditional(
    Restrictions.Where<House>(r => r.Name.IsLike("starks")),
    Projections.Constant(0),
    Projections.Constant(1))).Asc();

但是我不知道如何在其中添加一个额外的条件/ when子句:/我尝试过添加一个额外的外部条件、额外的限制等等，但是总是会出现语法错误。

谢谢你的帮助。

Answer 1

Projections.Conditional返回IProjection，其签名为：

/// <summary>
/// Conditionally return the true or false part, dependention on the criterion
/// </summary>
/// <param name="criterion">The criterion.</param><param name="whenTrue">The when true.
///    </param><param name="whenFalse">The when false.</param>
/// <returns/>
public static IProjection Conditional(ICriterion criterion
                                    , IProjection whenTrue
                                    , IProjection whenFalse);

这意味着，第三个参数也可以是这个条件投影：

.OrderBy
(
    Projections.Conditional(
        Restrictions.Where<House>(r => r.Name.IsLike("starks")),
        Projections.Constant(1),
        Projections.Conditional(
            Restrictions.Where<House>(r => r.Name.IsLike("wildlings")),
            Projections.Constant(2),
            Projections.Conditional(
                Restrictions.Where<House>(r => r.Name.IsLike("lannisters")),
                Projections.Constant(3),
                Projections.Constant(0)
                )
            )
        )
)
.Asc()

生成的SQL如下所示：

ORDER BY 
(case when this_.Name LIKE 'starks'     then 1 else 
(case when this_.Name LIKE 'wildlings'  then 2 else 
(case when this_.Name LIKE 'lannisters' then 3 else 0 end) end) end) asc