Trait diesel::deserialize::FromSql

source · [−]
pub trait FromSql<A, DB: Backend>: Sized {
    fn from_sql(bytes: Option<&DB::RawValue>) -> 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) 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::RawValue is private API. All implementations of this trait must be written in terms of an existing primitive.
  • For MySQL, the value of bytes will depend on the return value of type_metadata for the given SQL type. See MysqlType for 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)]
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: Option<&DB::RawValue>) -> 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

See the trait documentation.

Implementations on Foreign Types

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

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

Implementors