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* HasValueRef, HasArguments, HasStatement -> Database GATs replace the associated types from the generic traits `HasValueRef<'r>`, `HasArguments<'q>` and `HasStatement<'q>` with generic associated types in `Database` * fixup after rebase --------- Co-authored-by: Austin Bonander <austin.bonander@gmail.com>
80 lines
2.6 KiB
Rust
80 lines
2.6 KiB
Rust
//! Provides [`Decode`] for decoding values from the database.
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use crate::database::Database;
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use crate::error::BoxDynError;
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use crate::value::ValueRef;
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/// A type that can be decoded from the database.
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///
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/// ## How can I implement `Decode`?
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///
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/// A manual implementation of `Decode` can be useful when adding support for
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/// types externally to SQLx.
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///
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/// The following showcases how to implement `Decode` to be generic over [`Database`]. The
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/// implementation can be marginally simpler if you remove the `DB` type parameter and explicitly
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/// use the concrete [`ValueRef`](Database::ValueRef) and [`TypeInfo`](Database::TypeInfo) types.
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///
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/// ```rust
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/// # use sqlx_core::database::{Database};
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/// # use sqlx_core::decode::Decode;
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/// # use sqlx_core::types::Type;
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/// # use std::error::Error;
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/// #
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/// struct MyType;
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///
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/// # impl<DB: Database> Type<DB> for MyType {
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/// # fn type_info() -> DB::TypeInfo { todo!() }
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/// # }
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/// #
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/// # impl std::str::FromStr for MyType {
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/// # type Err = sqlx_core::error::Error;
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/// # fn from_str(s: &str) -> Result<Self, Self::Err> { todo!() }
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/// # }
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/// #
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/// // DB is the database driver
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/// // `'r` is the lifetime of the `Row` being decoded
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/// impl<'r, DB: Database> Decode<'r, DB> for MyType
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/// where
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/// // we want to delegate some of the work to string decoding so let's make sure strings
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/// // are supported by the database
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/// &'r str: Decode<'r, DB>
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/// {
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/// fn decode(
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/// value: <DB as Database>::ValueRef<'r>,
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/// ) -> Result<MyType, Box<dyn Error + 'static + Send + Sync>> {
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/// // the interface of ValueRef is largely unstable at the moment
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/// // so this is not directly implementable
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///
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/// // however, you can delegate to a type that matches the format of the type you want
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/// // to decode (such as a UTF-8 string)
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///
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/// let value = <&str as Decode<DB>>::decode(value)?;
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///
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/// // now you can parse this into your type (assuming there is a `FromStr`)
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///
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/// Ok(value.parse()?)
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/// }
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/// }
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/// ```
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pub trait Decode<'r, DB: Database>: Sized {
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/// Decode a new value of this type using a raw value from the database.
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fn decode(value: <DB as Database>::ValueRef<'r>) -> Result<Self, BoxDynError>;
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}
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// implement `Decode` for Option<T> for all SQL types
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impl<'r, DB, T> Decode<'r, DB> for Option<T>
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where
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DB: Database,
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T: Decode<'r, DB>,
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{
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fn decode(value: <DB as Database>::ValueRef<'r>) -> Result<Self, BoxDynError> {
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if value.is_null() {
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Ok(None)
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} else {
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Ok(Some(T::decode(value)?))
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}
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}
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}
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