Trait diesel::deserialize::FromSql
source · pub trait FromSql<A, DB: Backend>: Sized {
// Required method
fn from_sql(bytes: RawValue<'_, DB>) -> Result<Self>;
// Provided method
fn from_nullable_sql(bytes: Option<RawValue<'_, DB>>) -> Result<Self> { ... }
}Expand description
Deserialize a single field of a given SQL type.
When possible, implementations of this trait should prefer to use an
existing implementation, rather than reading from bytes. (For example, if
you are implementing this for an enum which is represented as an integer in
the database, prefer i32::from_sql(bytes) (or the explicit form
<i32 as FromSql<Integer, DB>>::from_sql(bytes)) over reading from bytes
directly)
Types which implement this trait should also have #[derive(FromSqlRow)]
§Backend specific details
- For PostgreSQL, the bytes will be sent using the binary protocol, not text.
- For SQLite, the actual type of
DB::RawValueis private API. All implementations of this trait must be written in terms of an existing primitive. - For MySQL, the value of
byteswill depend on the return value oftype_metadatafor the given SQL type. SeeMysqlTypefor details. - For third party backends, consult that backend’s documentation.
§Examples
Most implementations of this trait will be defined in terms of an existing implementation.
#[repr(i32)]
#[derive(Debug, Clone, Copy, FromSqlRow)]
pub enum MyEnum {
A = 1,
B = 2,
}
impl<DB> FromSql<Integer, DB> for MyEnum
where
DB: Backend,
i32: FromSql<Integer, DB>,
{
fn from_sql(bytes: backend::RawValue<DB>) -> deserialize::Result<Self> {
match i32::from_sql(bytes)? {
1 => Ok(MyEnum::A),
2 => Ok(MyEnum::B),
x => Err(format!("Unrecognized variant {}", x).into()),
}
}
}Required Methods§
Provided Methods§
sourcefn from_nullable_sql(bytes: Option<RawValue<'_, DB>>) -> Result<Self>
fn from_nullable_sql(bytes: Option<RawValue<'_, DB>>) -> Result<Self>
A specialized variant of from_sql for handling null values.
The default implementation returns an UnexpectedNullError for
an encountered null value and calls Self::from_sql otherwise
If your custom type supports null values you need to provide a custom implementation.
Object Safety§
This trait is not object safe.
Implementations on Foreign Types§
source§impl FromSql<Binary, Pg> for *const [u8]
impl FromSql<Binary, Pg> for *const [u8]
The returned pointer is only valid for the lifetime to the argument of
from_sql. This impl is intended for uses where you want to write a new
impl in terms of Vec<u8>, but don’t want to allocate. We have to return a
raw pointer instead of a reference with a lifetime due to the structure of
FromSql
source§impl FromSql<Text, Pg> for *const str
impl FromSql<Text, Pg> for *const str
The returned pointer is only valid for the lifetime to the argument of
from_sql. This impl is intended for uses where you want to write a new
impl in terms of String, but don’t want to allocate. We have to return a
raw pointer instead of a reference with a lifetime due to the structure of
FromSql