HasOne Linq select query issue

[BrennanVanCoillie]

Hi, We have an issue when using a one to one(or zero) relationship in our code

For example we have a class Parent and class Child which have one to one or zero relation

This the Parent class :

public class Parent
{
    public virtual Guid Id { get; set; }
    public virtual Child Child { get; set; }
}

This is the Child class

public class Child
{
    public virtual Guid Id { get; set; }
    public virtual Parent Parent { get; set; }
}

This is the mapping we use for the parent class

public class ParentMapping : ClassMap<Parent>
{
    public ParentMapping()
    {
        Id(x => x.Id);
        HasOne(x => x.Child)
            .PropertyRef(x => x.Parent)
            .Cascade.All()
            .LazyLoad();
    }
}

This is the mapping we use for the child class

public class ChildMapping : ClassMap<Child>
{
    public ChildMapping()
    {
        Id(x => x.Id);
        References(x => x.Parent)
            .Unique()
            .Not.Nullable();
    }
}

Then we try getting all the parent records with their child records.

The problem is that when we query the database using .Query<Parent>().ToList() Nhibernate generates a query for the Parent and then a single query for every Child record that is found.

public IEnumerable<Parent> TestMethodUsingQuery()
    {
        return Session.Query<Parent>().ToList();
    }

When we use QueryOver<>().List() however Nhibernate generates a left outer join to fetch all the data, which for us is as expected.

    public IEnumerable<Parent> TestMethodUsingQueryOver()
    {
        return Session.QueryOver<Parent>().List();
    }

Can someone please help us on this issue?
This was tested using Nhibernate 5.4.2 and FLuentNHibernate 3.1.0

[gliljas]

return Session.Query<Parent>().Fetch(x=>x.Child).ToList();

[BrennanVanCoillie]

This indeed gives the expected result.
Could this be solved with mapping rather then putting fetch in the query?

If not...
We have quite a lot of issues when trying to test this when just using it as an extension in a query on a list.
We get the following error.

System.InvalidOperationException : There is no method 'Fetch' on type 'NHibernate.Linq.EagerFetchingExtensionMethods' that matches the specified arguments
   at System.Linq.EnumerableRewriter.FindMethod(Type type, String name, ReadOnlyCollection`1 args, Type[] typeArgs)
   at System.Linq.EnumerableRewriter.VisitMethodCall(MethodCallExpression m)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.Visit(ReadOnlyCollection`1 nodes)
   at System.Linq.EnumerableRewriter.VisitMethodCall(MethodCallExpression m)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.Visit(ReadOnlyCollection`1 nodes)
   at System.Linq.EnumerableRewriter.VisitMethodCall(MethodCallExpression m)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at System.Linq.EnumerableQuery`1.GetEnumerator()
   at System.Linq.EnumerableQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at

Example in how we query it :

IEnumerable<Parent> parents = new List<Parent>().AsQueryable();
        parents
            .Fetch(x => x.Child)
            .Where(x => x.Amount > 0)
            .OrderByDescending(x => x.Date)
            .ToList();

We are just testing what the query returns here, there is no session used!
Is there any way to make this work, or is this not possible?
Thanks a lot!

[Madajevas]

Maybe this will help #3292

[Madajevas]

I am having the same issue. Problem is, parent entities are queried in quite a lot of locations, project is maintained by a team of developers and new queries may pop up any day. Also, info contained in child is needed only for a fraction of scenarios.

I have tried adding .Constrained() to has one mapping and it does work (create proxies) if no .PropertyRef is there. In this case id fields of both entities must match and with amount of data we have it would be preferable not to update primary key and have a join of "parent_id" field.

Can this be achieved somehow with mapping? My effort to google this led nowhere.

UPDATE:
.Constrained() with .PropertyRef seems to working when used with QueryOver API. The very same mapping with Query produces N+1.
UPDATE#2:
Assumption that QueryOver acts differently was false.