检查数据库中是否存在对象而不使用NHibernate加载对象

Question

在NHibernate中是否有办法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

可以随时计算。

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@Jamie Ide @Darius Kucinskas @Dmitry

做一些测试

所以：

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

在我的情况下 18％比

快

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;

所以在我看来即使硬编码查询最快

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

是最好的选择，因为良好的习惯和代码清晰度

Answer 4

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

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。

Answer 5

我认为你正在寻找这个......

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

Answer 6

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

Answer 7

基于Ricardo答案，这似乎是使用HQL检查对象是否存在的最有效方法。它没有COUNT并且不会不必要地加载对象：

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

它生成这个SQL（注意这是SQLite，所以LIMIT而不是TOP）

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

Answer 8

你可以尝试一下：

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

Answer 9

我根据以上答案创建了扩展方法。我选择使用“任意”版本，但是在主键上选择了“选择”，根据主键类型，将返回相同数量的数据。

public static bool Exists<TModel, TKey>(
    this IQueryable<TModel> query, Expression<Func<TModel, TKey>> selector, TKey id)
    where TKey : class
{
    return query
        .Select(selector)
        .Any(x => x == id);
}

public static Task<bool> ExistsAsync<TModel, TKey>(
    this IQueryable<TModel> query, Expression<Func<TModel, TKey>> selector, TKey id)
    where TKey : class
{
    return query
        .Select(selector)
        .AnyAsync(x => x == id);
}