take returns an Option, when can it be None

[ds-gipe]

I am using the take api and in order to handle nulls I need to do something like this:
let qualifying: Option<u32> = if let Some(some_val) = row.take("isQualifying") { some_val } else { None };

What I don't understand, is the take api returns an Optional. At what case can it return None if not in the null case.

If I try

let qualifying: Option<u32> = row.take("isQualifying");
it crashes on null saying it can't convert null to u32. It seems like it should convert null to None?

Thanks for helping me understand

[blackbeam]

Hi.

At what case can it return None if not in the null case

In case if a cell value was already taken — the behavior here is similar to the Option::take — in other words Row can roughly be considered as a Vec<Option<T>>:

let row: Row = "SELECT 'cell_value' as `cell_name`".first(&mut conn).await?.expect("a single row will be here");
let cell_value = row.take("cell_name").expect("no panic here because the value of the cell `cell_name` is in the row");
let cell_value2 = row.take("cell").expect("this line will panic because the value of the call `cell_name` was taken at the previous line");

If I try

let qualifying: Option<u32> = row.take("isQualifying");

it crashes on null saying it can't convert null to u32. It seems like it should convert null to None?

For the same reason as described above this should be:

let qualifying: Option<Option<u32>> = row.take("isQualifying");

// or in case you can assert that the value of `isQualifying` wasn't taken
let qualifying: Option<u32> = row.take("isQualifying").expect("it is absurd if column value isn't there");

Consider also:

• from_row function — to convert a row into a tuple
• FromRow derive macro — to convert row into a struct