Check if object exists in database without loading object with NHibernate - nhibernate

Question

Is there a way in NHibernate to check if an object exists in the database without having to get/load the object?

Answer 1

You could use one of the following 3 queries (or you could use Criteria API Projections.RowCountInt64() from David answer):
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;
Just keep in mind that Any is worst of those three because it fetches entity. Here is sql query generated for each:
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

So I let myself to do some tests with your examples #Jamie Ide #Darius Kucinskas #Dmitry
So:
var exists = session
.CreateQuery("select 1 from Widget where _color = 'green'")
.SetMaxResults(1)
.UniqueResult<Int32?>()
.HasValue;
in my case was 18% faster than
bool exist = session.Query<Employee>()
.Any(x => x.EmployeeID == 1);
14% than
bool exist = session.Query<Employee>()
.Count(x => x.EmployeeID == 1) > 0;
and 8%
bool exist = session.QueryOver<Employee>()
.Where(x => x.EmployeeID == 1)
.RowCount() > 0;
So in my opinion even if hard coded query is fastest the
bool exist = session.QueryOver<Employee>()
.Where(x => x.EmployeeID == 1)
.RowCount() > 0;
is best option because of good habits and code clearness

Answer 3

Could always do a count.
I tend to use DetachedCriteria, so I'd have something like:
var criteria = // some criteria that will identify your object
var result = criteria
.GetExecutableCriteria(Session)
.SetProjection(Projections.RowCountInt64())
.UniqueResult();
return result > 0;

Answer 4

To expand on Darius Kucinskas' excellent answer, you can avoid fetching the entity using Select:
bool exist = session.Query<Employee>()
.Where(x => x.EmployeeID == 1)
.Select(x => x.EmployeeID)
.Any();
As mentioned, the query performance should be the same however I would expect this to reduce network traffic.

Answer 5

I think you are looking for this...
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

Based on Ricardo answer, this seems like the most efficient way to check if object exists using HQL. It doesn't COUNT and doesn't load the object unnecessarily:
var exists = session
.CreateQuery("select 1 from Widget where _color = 'green'")
.SetMaxResults(1)
.UniqueResult<Int32?>()
.HasValue;
It generates this SQL (note that this is SQLite, so LIMIT instead of TOP)
select
1 as col_0_0_
from
Widgets
where
Color='green' limit 1;

Answer 8

You can have a try:
public virtual T FindById<T>(int id)
{
return session.Get(typeof(T), id));
}

Answer 9

I created an extension method from the answers above. I choose to use the Any version but with a Select on the primary key the same amount of data will be returned depend on the primary key type.
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);
}