Scylla Rust Driver Making queries Query result

Query result¶

Session::query and Session::execute return a QueryResult with rows represented as Option<Vec<Row>>.

Basic representation¶

Row is a basic representation of a received row. It can be used by itself, but it’s a bit awkward to use:

# extern crate scylla;
# use scylla::Session;
# use std::error::Error;
# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
if let Some(rows) = session.query("SELECT a from ks.tab", &[]).await?.rows {
    for row in rows {
        let int_value: i32 = row.columns[0].as_ref().unwrap().as_int().unwrap();
    }
}
# Ok(())
# }

Parsing using `into_typed`¶

The driver provides a way to parse a row as a tuple of Rust types:

# extern crate scylla;
# use scylla::Session;
# use std::error::Error;
# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
use scylla::IntoTypedRows;

// Parse row as a single column containing an int value
if let Some(rows) = session.query("SELECT a from ks.tab", &[]).await?.rows {
    for row in rows {
        let (int_value,): (i32,) = row.into_typed::<(i32,)>()?;
    }
}

// rows.into_typed() converts a Vec of Rows to an iterator of parsing results
if let Some(rows) = session.query("SELECT a from ks.tab", &[]).await?.rows {
    for row in rows.into_typed::<(i32,)>() {
        let (int_value,): (i32,) = row?;
    }
}

// Parse row as two columns containing an int and text columns
if let Some(rows) = session.query("SELECT a, b from ks.tab", &[]).await?.rows {
    for row in rows.into_typed::<(i32, String)>() {
        let (int_value, text_value): (i32, String) = row?;
    }
}
# Ok(())
# }

Parsing using convenience methods¶

QueryResult provides convenience methods for parsing rows. Here are a few of them:

rows_typed::<RowT>() - returns the rows parsed as the given type
maybe_first_row_typed::<RowT> - returns Option<RowT> containing first row from the result
first_row_typed::<RowT> - same as maybe_first_row, but fails without the first row
single_row_typed::<RowT> - same as first_row, but fails when there is more than one row
result_not_rows() - ensures that query response was not rows, helps avoid bugs

# extern crate scylla;
# use scylla::Session;
# use std::error::Error;
# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
// Parse row as a single column containing an int value
let rows = session
    .query("SELECT a from ks.tab", &[])
    .await?
    .rows_typed::<(i32,)>()?; // Same as .rows()?.into_typed()
for row in rows {
    let (int_value,): (i32,) = row?;
}

// maybe_first_row_typed gets the first row and parses it as the given type
let first_int_val: Option<(i32,)> = session
    .query("SELECT a from ks.tab", &[])
    .await?
    .maybe_first_row_typed::<(i32,)>()?;

// no_rows fails when the response is rows
session.query("INSERT INTO ks.tab (a) VALUES (0)", &[]).await?.result_not_rows()?;
# Ok(())
# }

For more see QueryResult

`NULL` values¶

NULL values will return an error when parsed as a Rust type. To properly handle NULL values parse column as an Option<>:

# extern crate scylla;
# use scylla::Session;
# use std::error::Error;
# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
use scylla::IntoTypedRows;

// Parse row as two columns containing an int and text which might be null
if let Some(rows) = session.query("SELECT a, b from ks.tab", &[]).await?.rows {
    for row in rows.into_typed::<(i32, Option<String>)>() {
        let (int_value, str_or_null): (i32, Option<String>) = row?;
    }
}
# Ok(())
# }

Parsing row as a custom struct¶

It is possible to receive row as a struct with fields matching the columns.
The struct must:

have the same number of fields as the number of queried columns
have field types matching the columns being received
derive FromRow

Field names don’t need to match column names.

# extern crate scylla;
# use scylla::Session;
# use std::error::Error;
# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
use scylla::IntoTypedRows;
use scylla::macros::FromRow;
use scylla::frame::response::cql_to_rust::FromRow;

#[derive(FromRow)]
struct MyRow {
    age: i32,
    name: Option<String>
}

// Parse row as two columns containing an int and text which might be null
if let Some(rows) = session.query("SELECT a, b from ks.tab", &[]).await?.rows {
    for row in rows.into_typed::<MyRow>() {
        let my_row: MyRow = row?;
    }
}
# Ok(())
# }

Other data types¶

For parsing other data types see Data Types