Combining server-generated values with changsets

Sounds like you want what the read_after_writes option to Ecto.Schema.field does.

This still doesn’t explain how to set the value to a fragment combined with a changeset.

read_after_writes is pretty much the same as always listing the field in the returning option.

Let’s say I have a field called voided and if it’s set there I want to also set voided_at to NOW(). Forget all the ways I could do this and just presume I want the application to say, “hey, during this update, set voided_at to NOW().” read_after_writes or returning will get the value back to me, but how do I set it?

Ecto.Changeset.prepare_changes/2 possibly?

Before trying to give you a working solution I’ll ask you this:

Why do you need voided at all? It’s basically voided_at IS NOT NULL. It seems superfluous.

Dang. I thought that was going to be it. Nope.

After looking through a bunch of docs last night, I’m probably going to have to write my own Repo function to handle this.

You’re focusing too much on a contrived example. Here’s another one (also contrived). I want to be able to do something like this:

%ExampleSchema{id: 1, name: "Jane Smith", changed_name_at: nil}
|> change(name: "Jane J Smith")
|> put_sql_change(:changed_name_at, sql_fragment("NOW()"))
|> Repo.update(returning: [:changed_name_at])

If I wanted the date generated by the server, not the application, is there a way to do this either in a changeset or as an option in update (and similarly, with insert)?

Ah, so you need something similar to how updated_at behaves but not pertaining to the entire record, more like for individual fields.

:autogenerate the functionality used for timestamps does compute the datetimes in elixir as well, not in the db.

Yep I’m aware, simply trying to understand the problem that OP is trying to solve.

I do not know how to to this as you wish on the Elixir side.. but I would probably go for a trigger and :read_after_write if I absolutely had to do it this way