Adjusting code to changes in deserialization API introduced in 0.15¶

Old traits¶

The legacy API worked by deserializing rows in the query response to a sequence of Rows. The Row is just a Vec<Option<CqlValue>>, where CqlValue is an enum that is able to represent any CQL value.

The user could request this type-erased representation to be converted into something useful. There were two traits that powered this:

FromRow

pub trait FromRow: Sized {
    fn from_row(row: Row) -> Result<Self, FromRowError>;
}

FromCqlVal

// The `T` parameter is supposed to be either `CqlValue` or `Option<CqlValue>`
pub trait FromCqlVal<T>: Sized {
    fn from_cql(cql_val: T) -> Result<Self, FromCqlValError>;
}

These traits were implemented for some common types:

• FromRow was implemented for tuples up to 16 elements,

• FromCqlVal was implemented for a bunch of types, and each CQL type could be converted to one of them.

While it was possible to implement those manually, the driver provided procedural macros for automatic derivation in some cases:

• FromRow - implemented FromRow for a struct.

• FromUserType - generated an implementation of FromCqlVal for the struct, trying to parse the CQL value as a UDT.

Note: the macros above had a default behavior that is different than what FromRow and FromUserType do.

New traits¶

The new API introduces two analogous traits that, instead of consuming pre-parsed Vec<Option<CqlValue>>, are given raw, serialized data with full information about its type. This leads to better performance and allows for better type safety.

The new traits are:

DeserializeRow<'frame, 'metadata>

pub trait DeserializeRow<'frame, 'metadata>
where
    Self: Sized,
{
    fn type_check(specs: &[ColumnSpec]) -> Result<(), TypeCheckError>;
    fn deserialize(row: ColumnIterator<'frame, 'metadata>) -> Result<Self, DeserializationError>;
}

DeserializeValue<'frame, 'metadata>

pub trait DeserializeValue<'frame, 'metadata>
where
    Self: Sized,
{
    fn type_check(typ: &ColumnType) -> Result<(), TypeCheckError>;
    fn deserialize(
        typ: &'metadata ColumnType<'metadata>,
        v: Option<FrameSlice<'frame>>,
    ) -> Result<Self, DeserializationError>;
}

The above traits have been implemented for the same set of types as FromRow and FromCqlVal, respectively. Notably, DeserializeRow is implemented for Row, and DeserializeValue is implemented for CqlValue.

There are also DeserializeRow and DeserializeValue derive macros, analogous to FromRow and FromUserType, respectively - but with slightly different defaults (explained later in this doc page).

Basic queries¶

Sending queries with the single page API should work similarly as before. The Session::query_{unpaged,single_page}, Session::execute_{unpaged,single_page} and Session::batch functions have the same interface as before, the only exception being that they return a new, updated QueryResult.

Consuming rows from a result will require only minimal changes if you are using helper methods of the QueryResult. Now, there is no distinction between “typed” and “non-typed” methods; all methods that return rows need to have the type specified. For example, previously there used to be both rows(self) and rows_typed<RowT: FromRow>(self), now there is only a single rows<R: DeserializeRow<'frame, 'metadata>>(&self). Another thing worth mentioning is that the returned iterator now borrows from the QueryResult instead of consuming it.

Note that the QueryResult::rows field is not available anymore. If you used to access it directly, you need to change your code to use the helper methods instead.

Before:

let iter = session
    .query_unpaged("SELECT name, age FROM my_keyspace.people", &[])
    .await?
    .rows_typed::<(String, i32)>()?;
for row in iter {
    let (name, age) = row?;
    println!("{} has age {}", name, age);
}

After:

// 1. Note that the result must be converted to a rows result, and only then
// an iterator created.
let result = session
    .query_unpaged("SELECT name, age FROM my_keyspace.people", &[])
    .await?
    .into_rows_result()?;

// 2. Note that `rows` is used here, not `rows_typed`.
// 3. Note that the new deserialization framework support deserializing types
//    that borrow directly from the result frame; let's use them to avoid
//    needless allocations.
for row in result.rows::<(&str, i32)>()? {
    let (name, age) = row?;
    println!("{} has age {}", name, age);
}

Iterator queries¶

The Session::query_iter and Session::execute_iter have been adjusted, too. They now return a QueryPager - an intermediate object which needs to be converted into TypedRowStream first before being actually iterated over.

Before:

let mut rows_stream = session
    .query_iter("SELECT name, age FROM my_keyspace.people", &[])
    .await?
    .into_typed::<(String, i32)>();

while let Some(next_row_res) = rows_stream.next().await {
    let (a, b): (String, i32) = next_row_res?;
    println!("a, b: {}, {}", a, b);
}

After:

let mut rows_stream = session
    .query_iter("SELECT name, age FROM my_keyspace.people", &[])
    .await?
    // The type of the TypedRowStream is inferred from further use of it.
    // Alternatively, it can be specified using turbofish syntax:
    // .rows_stream::<(String, i32)>()?;
    .rows_stream()?;

while let Some(next_row_res) = rows_stream.next().await {
    let (a, b): (String, i32) = next_row_res?;
    println!("a, b: {}, {}", a, b);
}

Currently, QueryPager/TypedRowStream do not support deserialization of borrowed types due to limitations of Rust with regard to lending streams. If you want to deserialize borrowed types not to incur additional allocations, use manual paging ({query/execute}_single_page) API.

Procedural macros¶

As mentioned in the Introduction section, the driver provides new procedural macros for the DeserializeRow and DeserializeValue traits that are meant to replace FromRow and FromUserType, respectively. The new macros are designed to be slightly more type-safe by matching column/UDT field names to rust field names dynamically. This is a different behavior to what the old macros used to do, but the new macros can be configured with #[attributes] to simulate the old behavior.

FromRow vs. DeserializeRow

The impl generated by FromRow expected columns to be in the same order as the struct fields. The FromRow trait did not have information about column names, so it could not match them with the struct field names. You can use enforce_order and skip_name_checks attributes to achieve such behavior via DeserializeRow trait.

FromUserType vs. DeserializeValue

The impl generated by FromUserType expected UDT fields to be in the same order as the struct fields. Field names should be the same both in the UDT and in the struct. You can use the enforce_order attribute to achieve such behavior via the DeserializeValue trait.

Adjusting custom impls of deserialization traits¶

If you have a custom type with a hand-written impl FromRow or impl FromCqlVal, the best thing to do is to just write a new impl for DeserializeRow or DeserializeValue manually.