nhibernate - NHibernate 中的投影

Question

假设在一个实体中有属性 id、用户名、年龄、地址。现在我只想要 id 和 username 并使用此代码。

投影可以从查询中返回实体列表以外的内容。

var proj = Projections.ProjectionList()
    .Add(Projections.Property("Id"), "Id")
    .Add(Projections.Property("Username"), "Username");

var list2 = DetachedCriteria.For<User>()
    .Add(Expression.Eq("Username", "lachlan"))
    .GetExecutableCriteria( sessionFactory.GetCurrentSession())
    .SetProjection( proj )
    .List();

我将如何检索这些值。将在哪个对象中获取这些值。

Answer 1

Unless a result transformer is used, a projection will result in a list of anonymous objects with the projected values. This would be sufficient for databinding.

For other uses, you want to set a result transformer which will create objects of a known type. The AliasToBeanTransformer will create an object of the specified type for each row, and set its properties to the row values.

If you know the type of the results, you can use the generic List<T>() method.

var proj = Projections.ProjectionList()
    .Add(Projections.Property("Id"), "Id")
    .Add(Projections.Property("Username"), "Username");

var list2 = DetachedCriteria.For<User>()
    .Add(Expression.Eq("Username", "lachlan"))
    .GetExecutableCriteria( sessionFactory.GetCurrentSession())
    .SetProjection( proj )
    .SetResultTransformer(Transformers.AliasToBean(typeof(Result)))
    .List<Result>();

Result transformers can also be used on SQL and HQL queries.

list2 = Session.CreateSQLQuery("select Id, Username from user_table")
    .SetResultTransformer(Transformers.AliasToBean(typeof(Result)))
    .List<Result>();

list2 = Session.CreateQuery("select Id, Username from User")
    .SetResultTransformer(Transformers.AliasToBean(typeof(Result)))
    .List<Result>();

In these examples the the Result class does not need to be a mapped class, and must have the selected properties.

partial class Result
{
    public int Id { get; set; }
    public string Username { get; set; }
}