Convert Row to type manually

[busslina]

It works for me to query_as but I want to use instead query and fetch_all, then check the row count and optionally convert the first row into the type. This is not working for me using into. Is there any builtin public way to do this?

[CommanderStorm]

Have a look at fetch_optional() if you want an Option.

[busslina]

Have a look at fetch_optional() if you want an Option.

The thing is that I need to know the match count and based on it, get the first or discard it.

I wanted to know if there is a way to manually convert a row into model value.

I want to avoid a costly conversion of may items that I won't use.

fetch_optional doesn't give me the count.

[CommanderStorm]

What do you want to achieve concretely?
Manually converting and giving up compile time checking is error prown and not something I would recommend.

=> I think your SQL should only return one row (via LIMIT 1, gouping or NOT EXISTS) and thus using fetch_optional might be a better Option

Also: share your code. Makes discussion simpler ^^

[busslina]

What do you want to achieve concretely?

My server, at startup does security checks. This one checks that there is one and only one root user. If not, it panics. If success, it loads the root user in memory.

So my easy way would be to first query the count of root users and second load it. But I wanted to do all in one: Query all root users, not mapping them for now to the user model, panic if there isn't one and only one, convert that one row to use model manually.

[CommanderStorm]

If you panic anyway the "cost" (don't think that is very significant, but would need to benchmark) of serialising all root users likely does not matter all that much.
You likely want which users exist in the error/panic anyway.

In the expected case (1 user), you are serialising only 1 user.

=> fetch_all the rows (maybe with a limit 10, but that is likely over engineering) and panic/pop the first user.

[busslina]

But I maybe need in other situation to be able to convert a row into a model. Is this not available? I'm new to Rust, but it seems that if fetch functions can do it we also should be able to do it in the same way.

[CommanderStorm]

You can conveniently convert sql -> struct. (I think that is what you are trying to do)
As an example via the macro-api (use it if you can. Compile time checked queries are awsome):

let root_users = sqlx::query_as!(User, "SELECT id,name,is_root_admin FROM users WHERE is_root_admin = true").fetch_all(&pool).await?;
// root_users is now Vec<User>

For the non-macro (and thus not compiletime checked!) api, examples exist in the readme.

Is that what you are looking for?