nhibernate - 在 Concat 投影中使用 Cast Projection

Question

我有以下查询:

var result = _session.QueryOver<Entity>()
    .Where(e => e.Property == value)
    .SelectList(list => list
        .Select(f => Projections.Concat("prefix-", e.BigIntProperty)).WithAlias(() => alias.Whatever)
        ...
    )
    .TransformUsing(Transformers.AliasToBean<Model>())
    .Future<Model>();

问题是Projections.Concat()只接受字符串，因为 e.BigIntProperty不是，上面的不能编译。有没有办法投e.BigIntProperty串？

我尝试了以下类似的方法，但也不起作用:

.Select(f => Projections.Concat("prefix-", Projection.Cast(NHibernateUtil.String, e.BigIntProperty))).WithAlias(() => alias.Whatever)

, 自 Projections.Cast返回 IProjection而不是字符串。

Answer 1

Projections.Cast似乎非常有限，因为它不能随意使用 Projection s。幸运的是，您可以轻松创建自己的自定义投影，使您能够做到这一点:

public static class CustomProjections
{
    public static IProjection Concat(params IProjection[] projections)
    {
        return Projections.SqlFunction(
            "concat",
            NHibernateUtil.String,
            projections);
    }
}

然后，您就可以使用您的 CustomProjections像这样的类:

var result = _session.QueryOver<Entity>()
    .Where(e => e.Property == value)
    .SelectList(list => list
        .Select(CustomProjections.Concat(
            Projections.Constant("prefix-"),
            Projections.Cast(
                NHibernateUtil.String,
                Projections.Property<Entity>(e => e.BigIntProperty))))
            .WithAlias(() => alias.Whatever)
        ...
    )
    .TransformUsing(Transformers.AliasToBean<Model>())
    .Future<Model>();