Error projecting child entity fields to specific class

[meekys]

Steps to reproduce

    public class Model
    {
        public Guid Id { get; set; }
        public ChildModel Child { get; set; }
    }
    public class ChildModel
    {
        public Guid Id { get; set; }
        public string Name { get; set; }
    }
    public class DTO {
        public Guid Id { get; set; }
        public Guid ChildId { get; set; }
    }

    var query = Db.Models.Select(x => new DTO {
        Id = x.Id,
        ChildId = x.Child.Id
    });

    var results = query.ToList(); // Fails with below exception

The issue

When projecting into a specific class, a expression type mismatch occurs.

      System.ArgumentException : Argument types do not match
      Stack Trace:
           at System.Linq.Expressions.Expression.Bind(MemberInfo member, Expression expression)
           at System.Linq.Expressions.MemberAssignment.Update(Expression expression)
           at System.Linq.Expressions.ExpressionVisitor.VisitMemberAssignment(MemberAssignment node)
           at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
           at System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor)
           at System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node)
           at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
           at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
           at Remotion.Linq.Clauses.SelectClause.TransformExpressions(Func`2 transformation)
           at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
           at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.Rewrite(QueryModel queryModel)
           at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.OptimizeQueryModel(QueryModel queryModel)
           at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel)
        --- End of stack trace from previous location where exception was thrown ---
           at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
           at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass19_0`1.<CompileQuery>b__0()
           at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
           at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
           at Remotion.Linq.QueryableBase`1.GetEnumerator()
           at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
           at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)

Additional Information

Projecting to an anonymous type works as expected.
ChildId is of the expected type, Guid.

    var query = Db.Models.Select(x => new {
        Id = x.Id,
        ChildId = x.Child.Id
    });

    Assert.Equal(typeof(Guid), query.ElementType.GetProperty("ChildId").PropertyType);

---

Projecting to a DTO where ChildId is a Nullable Guid also works.

    public class DTO_With_Nullable {
        public Guid Id { get; set; }
        public Guid? ChildId { get; set; }
    }

    var query = Db.Models.Select(x => new DTO_With_Nullable {
        Id = x.Id,
        ChildId = x.Child.Id
    });

Further technical details

EF Core version: 1.0.0
Operating system: Windows 10 x64, Version 1511
Visual Studio version: VS Code

EF_Projection.zip

[rowanmiller]

This is because the relationship is optional, so if the Model does not have an associated ChildModel (i.e. Model.Child would be null) then the value to be stored in DTO.ChildId would be null. For that reason, the value produced by x.Child.Id is Nullable<Guid>.

Interestingly, if this were a LINQ to Objects query, then you'd get a NullReferenceException if Model didn't have a related Child, but EF is a little more forgiving.

You could make the relationship required, if Model will always have a Child:

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Model>()
                .HasOne(p => p.Child)
                .WithMany()
                .IsRequired();
        }

Or, you can defend against nulls in the query.

    var query = Db.Models.Select(x => new DTO {
        Id = x.Id,
        ChildId = (x.Child == null ? Guid.Empty : x.Child.Id)
    });

[meekys]

Thanks. Figures I was doing something silly like that.

It's odd that this scenario works for an anonymous type, where the compiler generates a Guid member , but it doesn't work for a defined class with the same member type, or am I missing something here?

[rowanmiller]

Closing as a duplicate of #5522. We want to change this so that we only throw if an actual null value comes back from the database.