檢查數據庫中是否存在沒有用NHibernate加載對象的數據庫

Question

有沒有在NHibernate中檢查數據庫中是否存在對象而不必獲取/加載對象的方法？

Answer 1

總是可以做一個計數。

我傾向於使用DetachedCriteria之後，所以我有這樣的：

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

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

return result > 0; 

Answer 2

我認爲你正在尋找這...

var fooExists = session.Query<Foo>().Any(f => /*condition*/); 

Answer 3

var exists = 1 == session.CreateQuery("select 1 from MyEntity where Property = :value") 
    .SetValue("value", xxx) 
    .UniqueResult<Int32?>(); 

Answer 4

您可以使用以下3個查詢之一（或者你可以從使用標準API Projections.RowCountInt64（）大衛回答）：

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; 

請記住，任何是這三個中最糟糕的，因爲它提取實體。下面是每個生成的SQL查詢：

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 5

要擴大大流士Kucinskas'優秀的答案，你能避免使用選擇獲取實體：

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

如前所述，查詢性能應該是相同的但我希望這可以減少網絡流量。

Answer 6

根據Ricardo答案，這似乎是使用HQL檢查對象是否存在的最有效方法。它不計算，並不一定會加載對象：

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

它生成此SQL（注意，這是SQLite的，所以限制，而不是TOP）

select 
    1 as col_0_0_ 
from 
    Widgets 
where 
    Color='green' limit 1; 

Answer 7

你可以試試：

public virtual T FindById<T>(int id) 
{ 
    return session.Get(typeof(T), id)); 
} 

Answer 8

所以我讓自己做一些測試與你的例子@Jamie Ide @Darius Kucinskas @Dmitry

所以：

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

在我的情況

爲18％快於

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

14％比

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，在我看來，即使硬編碼的查詢是最快的

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

是最好的選擇，因爲良好的生活習慣和代碼清晰