c# - How to post-process IQueryable fields in code without losing the expression sent to DB?

Question

My asp.net core program exposes a GraphQL endpoint that is handled by HotChocolate. It naturally supports filtering/sorting that is translated to DB calls via IQueryable mechanism.

UI --(graph ql)--> HotChocolate --(IQueryable)--> Logic --(IQueryable)--> DB

Now in the logic layer I want to format some string in a property of the class. I can do that only in C# code. But if I materialize the IQueryable I have into an enumerable, I lose that translation. So I will always query for all items and will return an IEnumerable instead which will be manually filtered in-memory by HotChocolate:

UI --(graph ql)--> HotChocolate ==(IEnumerable)==> Logic ==(all items)==> DB

So how to "inject" the C# processing into IQueryable so that the filtering/sorting expression is preserved? I think I can do a visitor of sorts, but wouldn't that get translated into the DB call anyway? I assume there should be a straight-forward solution but I don't know who it's called?

question from:https://stackoverflow.com/questions/65952428/how-to-post-process-iqueryable-fields-in-code-without-losing-the-expression-sent

Answer 1

Figured out that one way to achieve this would be to use AutoMapper's expression translation with OnEnumerated(): https://docs.automapper.org/en/stable/Expression-Translation-(UseAsDataSource).html

_dbSet.UseAsDataSource().For<DTO>().OnEnumerated((dtos) => /* mutate */);

AutoMapper does the basic translation when mapping, but this is available in a separate package and isn't discoverable.