Query: Compiled Queries: Support "non-rooted" query expressions

User compiled queries allow for more expressiveness in creating query LINQ expressions. E.g.

var query = EF.CompileQuery(
                (NorthwindContext context)
                    => context.Customers.OrderBy(c => c.CustomerID).Select(c => c.CustomerID).FirstOrDefault() 
                        + context.Orders.OrderBy(o => o.CustomerID).Select(o => o.CustomerID).FirstOrDefault());

This particular query currently fails, I think because of a re-linq issue. In general, we should decide whether we want to spend time on making this work and testing it.

[maumar]

Exception:

The 'InitializeStateManager' method has been called multiple times on the current query context. This method is intended to be called only once before query enumeration starts.
  Stack Trace: 
    QueryContext.InitializeStateManager(Boolean standAlone) line 137
    Enumerator.InitializeReader(DbContext _, Boolean result) line 217
    ExecutionStrategy.ExecuteImplementation[TState,TResult](Func`3 operation, Func`3 verifySucceeded, TState state) line 173
    ExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded) line 160
    Enumerator.MoveNext()

[jnm2]

This:

EF.CompileQuery<MyDataContext, int, ContainerBase>((ctx, contId) =>
    ctx.Container.AddIncludes().FirstOrDefault(d => d.Id == contId));

Is sugar for:

(oops, I did (queryable, contId) => queryable.AddIncludes().FirstOrDefault(d => d.Id == contId) instead, but you get the point)

var queryableParameter = Expression.Parameter(typeof(IQueryable<ContainerBase>), "queryable");
var idParameter = Expression.Parameter(typeof(int), "id");
var dParameter = Expression.Parameter(typeof(ContainerBase), "d");

var expression = Expression.Lambda<Func<IQueryable<ContainerBase>, int, ContainerBase>>(
    Expression.Call(
        typeof(Queryable),
        nameof(Queryable.FirstOrDefault),
        typeArguments: new[] { typeof(ContainerBase) },
        new Expression[]
        {
            Expression.Call(typeof(YourClass).GetMethod("AddIncludes"), new[] { queryableParameter }),
            Expression.Quote(Expression.Lambda<Func<ContainerBase, bool>>(
                Expression.Equal(Expression.Property(dParameter, "Id"), idParameter),
                new[] { dParameter }))
        }),
    new[] { queryableParameter, idParameter });

EF.CompileQuery(expression);

Note how AddIncludes is referenced above. In order for the calls inside your AddIncludes method to become available, Entity Framework would have to somehow decompile your AddIncludes method and construct an expression which represents what it does.