problem with linq to nhibernate and eager loading in mvc application

Question

Hello I have the following code to retrieve data from my db using nhibernate 3.0 with linq -

public IQueryable<myEntity> getEntityWithChild
        {
            get { return _currentSession.Query<myEntity>().Fetch(c => c.myOtherEntity); }
        }

When I try to pass that to a view, I am getting the following error which isn't very specific. This worked fine when I wasn't eager loading using the following -

public IQueryable<myEntity> getEntityWithChild
            {
                get { return _currentSession.Query<myEntity>(); }
            }

But I was creating a seperate query each time the other entity would be lazy loaded. Has anyone seen this before that may be able to point me in the right direction. Thanks for any thoughts.

System.NotSupportedException was unhandled by user code
  Message=Specified method is not supported.
  Source=NHibernate
  StackTrace:
       at NHibernate.Hql.Ast.ANTLR.PolymorphicQuerySourceDetector.GetClassName(IASTNode querySource)
       at NHibernate.Hql.Ast.ANTLR.PolymorphicQuerySourceDetector.Process(IASTNode tree)
       at NHibernate.Hql.Ast.ANTLR.AstPolymorphicProcessor.Process()
       at NHibernate.Hql.Ast.ANTLR.AstPolymorphicProcessor.Process(IASTNode ast, ISessionFactoryImplementor factory)
       at NHibernate.Hql.Ast.ANTLR.ASTQueryTranslatorFactory.CreateQueryTranslators(IASTNode ast, String queryIdentifier, String collectionRole, Boolean shallow, IDictionary`2 filters, ISessionFactoryImplementor factory)
       at NHibernate.Hql.Ast.ANTLR.ASTQueryTranslatorFactory.CreateQueryTranslators(String queryIdentifier, IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 filters, ISessionFactoryImplementor factory)
       at NHibernate.Engine.Query.HQLExpressionQueryPlan.CreateTranslators(String expressionStr, IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
       at NHibernate.Engine.Query.HQLExpressionQueryPlan..ctor(String expressionStr, IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
       at NHibernate.Engine.Query.HQLExpressionQueryPlan..ctor(String expressionStr, IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
       at NHibernate.Engine.Query.QueryPlanCache.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters)
       at NHibernate.Impl.AbstractSessionImpl.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow)
       at NHibernate.Impl.AbstractSessionImpl.CreateQuery(IQueryExpression queryExpression)
       at NHibernate.Linq.NhQueryProvider.PrepareQuery(Expression expression, IQuery& query, NhLinqExpression& nhQuery)
       at NHibernate.Linq.NhQueryProvider.Execute(Expression expression)
       at NHibernate.Linq.NhQueryProvider.Execute[TResult](Expression expression)
       at Remotion.Data.Linq.QueryableBase`1.GetEnumerator()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
       at vCalWebCalendar.Controllers.HomeController.Hearings() in C:\Users\carl.PAMB.000\Documents\Visual Studio 2010\Projects\Calendar\Calendar\Controllers\HomeController.cs:line 41
       at lambda_method(Closure , ControllerBase , Object[] )
       at System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters)
       at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters)
       at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
       at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClassd.<InvokeActionMethodWithFilters>b__a()
       at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation)
  InnerException:

Chris Shouts · Accepted Answer · 2010-11-09 19:07:16Z

5

According to Mike Hadlow's blog post NHibernate Linq Eager Fetching,

Note that if you want to mix Fetch with other clauses, Fetch must always come last.

Is the query in your question the exact query that you are executing?

answered Nov 9, 2010 at 19:07

Chris Shouts

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3 Comments

czuroski Over a year ago

yeah - that is the exact query. (the entity names have obviously been changed, but the order of the query is the same.)

Chris Shouts Over a year ago

Are you sure? Don't forget that since this is an IQueryable, execution will be deferred and anything you might do with it in the Model or the View will be treated as part of the original query. A quick way to test would be to make the getEntityWithChild method return IEnumerable<myEntity>.

czuroski Over a year ago

it does work if I change the method to return IEnumerable. I was doing a filter in my controller. I hadn't realized that by using IQueryable, that everything I did in the controller affected the original query. I removed my filter from the controller and just used the original query and it worked perfectly. Thank You.

James Kovacs · Accepted Answer · 2010-11-09 19:09:32Z

4

Looks like a bug or limitation of the current LINQ provider. The Fetch() clause needs to be the very last method in the chain - even after Select(). So this produces the above exception:

_currentSession.Query<myEntity>().Fetch(c => c.myOtherEntity).Select(x => x);

whereas this works:

_currentSession.Query<myEntity>().Select(x => x).Fetch(c => c.myOtherEntity);

Since you're returning an IQueryable, I suspect that you're filtering or selecting higher up in your application stack, which would cause the .Fetch() to throw an exception. Mike Hadlow has more information about this annoyance on his blog here:

http://mikehadlow.blogspot.com/2010/08/nhibernate-linq-eager-fetching.html

answered Nov 9, 2010 at 19:09

James Kovacs

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1 Comment

ari Over a year ago

what heppand if you nead to select y. but you still wanna fetch some property of x? _currentSession.Query<myEntity>().Select(x => y).Fetch(?????);

Daniel Schilling · Accepted Answer · 2013-10-24 16:50:06Z

0

Evidently this is fixed in NH 3.1: https://nhibernate.jira.com/browse/NH-2502

edited Oct 24, 2013 at 16:50

Daniel Schilling

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answered Mar 19, 2011 at 3:20

hcoverlambda

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