private static Expression <Func <T, bool> > AddDocTypePredicate(Expression <Func <T, bool> > predicate)
{
var type = typeof(T);
var docTypeField = type.GetProperty("_docType");
var memberExp = Expression.Property(predicate.Parameters[0], docTypeField);
var secondPredicate = LambdaExpression.Equal(memberExp, Expression.Constant(type.Name));
var and = LambdaExpression.AndAlso(predicate.Body, secondPredicate);
var extendedPredicate = Expression.Lambda <Func <T, bool> >(and, predicate.Parameters);
return(extendedPredicate);
}