检查数据库中是否存在对象而不使用NHibernate加载对象

Question

NHibernate中是否有一种方法可以检查数据库中是否存在对象而无需获取/加载对象？

Answer 1

您可以使用以下3个查询之一(或者您可以使用David回答的Criteria API Projections.RowCountInt64()):

bool exist = session.Query<Employee>()
    .Any(x => x.EmployeeID == 1);

bool exist = session.QueryOver<Employee>()
    .Where(x => x.EmployeeID == 1)
    .RowCount() > 0;

bool exist = session.Query<Employee>()
    .Count(x => x.EmployeeID == 1) > 0;

请记住,Any是最糟糕的,因为它取实体.这是为每个生成的sql查询:

exec sp_executesql N'select TOP (1)  employee0_.EmployeeID as EmployeeID0_, employee0_.Name as Name0_ from Employee employee0_ where employee0_.EmployeeID=@p0',N'@p0 int',@p0=1

exec sp_executesql N'SELECT count(*) as y0_ FROM Employee this_ WHERE this_.EmployeeID = @p0',N'@p0 int',@p0=1

exec sp_executesql N'select cast(count(*) as INT) as col_0_0_ from Employee employee0_ where employee0_.EmployeeID=@p0',N'@p0 int',@p0=1

Answer 2

可以随时做点算.

我倾向于使用DetachedCriteria,所以我有类似的东西:

var criteria = // some criteria that will identify your object

var result = criteria
    .GetExecutableCriteria(Session)
    .SetProjection(Projections.RowCountInt64())
    .UniqueResult();

return result > 0;

Answer 3

所以我让自己用你的例子做一些测试 @Jamie Ide @Darius Kucinskas @Dmitry

所以:

var exists = session
.CreateQuery("select 1 from Widget where _color = 'green'")
.SetMaxResults(1)
.UniqueResult<Int32?>()
.HasValue;

在我的情况下比18%快

bool exist = session.Query<Employee>()
    .Any(x => x.EmployeeID == 1);

比14%

bool exist = session.Query<Employee>()
    .Count(x => x.EmployeeID == 1) > 0;

和8%

bool exist = session.QueryOver<Employee>()
    .Where(x => x.EmployeeID == 1)
    .RowCount() > 0;

所以在我看来,即使硬编码查询是最快的

bool exist = session.QueryOver<Employee>()
    .Where(x => x.EmployeeID == 1)
    .RowCount() > 0;

因为良好的习惯和代码清晰度是最好的选择

Answer 4

要扩展Darius Kucinskas的优秀答案,您可以避免使用Select获取实体:

bool exist = session.Query<Employee>()
                    .Where(x => x.EmployeeID == 1)
                    .Select(x => x.EmployeeID)
                    .Any();

如前所述,查询性能应该相同,但我希望这可以减少网络流量.