Handle rusqlite Error

help
1

Hello, I want to handle a QueryReturnedNoRows error in rusqlite crate.
My code is:

let preference_iter = conn.query_row(
        "SELECT * FROM preferences where key='server'",
        params![],
        |row| {
            Ok(Preference {
                id: row.get(0)?,
                key: row.get(1)?,
                value: row.get(2)?,
            })
        }).expect("select failed");

In case of 0 result thread 'main' panicked at 'select failed: QueryReturnedNoRows'

How can I solve this problem?

Thanks
Gianluca

2

Please use code blocks for code and errors:

```
// your code here
```

As for your question, you can inspect the error instead of using expect.

match conn.query_row(...) {
    Ok(res) => {
        // got a row
    }
    Err(rusqlite::Error::QueryReturnedNoRows) => {
        // no rows
    }
    Err(err) => {
        // something else went wrong
        panic!("Select failed: {}", err);
    }
}
3

Thanks for your reply Alice but I am a beginner in Rust. I write this:

match conn.query_row("SELECT * FROM preferences where key='server'", params![],|row| {
            Ok(Preference {
                id: row.get(0)?,
                key: row.get(1)?,
                value: row.get(2)?,
            })}){
        Ok(server_preference) => Ok(Some(server_preference)),
        Err(Error::QueryReturnedNoRows) => Ok(None),
        Err(err) => Err(err),
    };

but my variable server_preference is not valorise.

Thanks again

4

Please edit your post to use code blocks for code and errors:

```
// your code here
```
5

ok done

6

So why doesn't it work? I don't know what valorise means.

7

I edited my code like this:

let server_preference = match conn.query_row("SELECT * FROM preferences where key='server'", params![],|row| {
            Ok(Preference {
                id: row.get(0)?,
                key: row.get(1)?,
                value: row.get(2)?,
            })}){
        Ok(x) => Ok(Some(x)),
        Err(Error::QueryReturnedNoRows) => Ok(None),
        Err(err) => Err(err),
    };

How can I retrieve only Preference.value?

Thansk

8

You only want the value field? One option would be this:

let query = conn.query_row("SELECT * FROM preferences where key='server'", params![], |row| {
    Ok(Preference {
        id: row.get(0)?,
        key: row.get(1)?,
        value: row.get(2)?,
    })
});
let server_preference = match query {
    Ok(x) => Ok(Some(x.value)), // <- take out only the value
    Err(Error::QueryReturnedNoRows) => Ok(None),
    Err(err) => Err(err),
};
9

ok it works but I retrieve Ok(Some(xxxxxx)).
How can I remove Ok(Some ?

10

You can match or use an if:

match server_preference {
    Ok(Some(x)) => {
        // use x
    }
    Ok(None) => {
        // handle none case
    }
    Err(err) => {
        // handle error
    }
}

If you are inside a function that returns a result, you can use the question mark to propogate the error if it happens:

match server_preference? { // question mark returns on error
    Some(x) => {
        // use x
    }
    None => {
        // handle none case
    }
}

You can also use if instead of match:

if let Some(x) = server_preference {
} else {
}

Finally you can use unwrap to remove errors instead of question mark.

11

My code is inside a function that return a string
My function now end with:

Ok(format!("{:?}", server_preference))

If I put it into match.... I receive error value borrowed here after move
Where should I put it?

12

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