public EfuQueryable(IQueryable source, Expression e)
 {
     if (e == null)
     {
         throw new ArgumentNullException(nameof(e));
     }
     _expression = e;
     _provider   = new EfuQueryProvider <T>(source);
 }
 public EfuQueryable(IQueryable source)
 {
     _expression = Expression.Constant(this);
     _provider   = new EfuQueryProvider <T>(source);
 }