Custom Diesel wrapper type example

coordinate.rs

/// This defines a custom Diesel type for storing a 6-bit integer and a 58-bit
/// integer in the same 64-bit SQL column

use std::error::Error;
use std::fmt;
use std::io::Write;
use diesel::backend::Backend;
use diesel::types::{FromSqlRow,FromSql,ToSql,HasSqlType,IsNull,BigInt,Nullable};
use diesel::expression::AsExpression;
use diesel::row::Row;

// store depth in 6 bits + store index in MAX_DEPTH bits = 64
const COORDINATE_MAX_DEPTH: usize = 58;
const COORDINATE_INDEX_MASK: u64 = (1u64<<COORDINATE_MAX_DEPTH)-1;

#[derive(Debug)]
enum CoordinateError {
    From(i64),
    To(Coordinate),
}

impl fmt::Display for CoordinateError {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        write!(fmt,"The value {:?} is not a valid coordinate", self)
    }
}

impl Error for CoordinateError {
    fn description(&self) -> &str {
        "The value is not a valid coordinate"
    }
}

#[derive(Debug,Clone,Copy,Hash,Eq,PartialEq)]
pub struct Coordinate {
    index: u64,
    depth: u8,
}

impl Coordinate {
    fn to_i64(&self) -> Result<i64,CoordinateError> {
        if self.depth as usize > COORDINATE_MAX_DEPTH || self.index > COORDINATE_INDEX_MASK {
            Err(CoordinateError::To(*self))
        } else {
            Ok((((self.depth as u64)<<COORDINATE_MAX_DEPTH) | (self.index & COORDINATE_INDEX_MASK)) as i64)
        }
    }
    
    fn from_i64(i: i64) -> Result<Self,CoordinateError> {
        let depth=((i as u64)>>COORDINATE_MAX_DEPTH) as u8;
        let index=(i as u64)&COORDINATE_INDEX_MASK;
        if depth>58 {
            Err(CoordinateError::From(i))
        } else {
            Ok(Coordinate{depth:depth,index:index})
        }
    }
}

impl<DB> FromSqlRow<BigInt, DB> for Coordinate where DB: Backend + HasSqlType<BigInt>, i64: FromSql<BigInt, DB> {
    fn build_from_row<T: Row<DB>>(row: &mut T) -> Result<Self, Box<Error + Send + Sync>> {
        <i64 as FromSqlRow<BigInt, DB>>::build_from_row(row).and_then(|i|Coordinate::from_i64(i).map_err(|e|Box::new(e) as _))
    }
}

impl<DB> Queryable<BigInt, DB> for Coordinate where DB: Backend + HasSqlType<BigInt>, Coordinate: FromSqlRow<BigInt, DB> {
    type Row = Self;
    fn build(row: Self::Row) -> Self {
		row
	}
}

impl<DB: Backend<RawValue=[u8]>> FromSql<BigInt, DB> for Coordinate {
    fn from_sql(bytes: Option<&[u8]>) -> Result<Self, Box<Error + Send + Sync>> {
        <i64 as FromSql<_,DB>>::from_sql(bytes).and_then(|i|Coordinate::from_i64(i).map_err(|e|Box::new(e) as _))
    }
}

impl<DB: Backend> ToSql<BigInt, DB> for Coordinate {
    fn to_sql<W: Write>(&self, out: &mut W) -> Result<IsNull, Box<Error + Send + Sync>> {
        let i=try!(self.to_i64().map_err(Box::new));
        <i64 as ToSql<BigInt,DB>>::to_sql(&i, out)
    }
}

impl AsExpression<BigInt> for Coordinate {
    type Expression = <i64 as AsExpression<BigInt>>::Expression;
    fn as_expression(self) -> Self::Expression {
        <i64 as AsExpression<BigInt>>::as_expression(self.to_i64().expect("The value is not a valid coordinate"))
    }
}

impl<'a> AsExpression<BigInt> for &'a Coordinate {
    type Expression = <i64 as AsExpression<BigInt>>::Expression;
    fn as_expression(self) -> Self::Expression {
        <i64 as AsExpression<BigInt>>::as_expression(self.to_i64().expect("The value is not a valid coordinate"))
    }
}

impl AsExpression<Nullable<BigInt>> for Coordinate {
    type Expression = <i64 as AsExpression<Nullable<BigInt>>>::Expression;
    fn as_expression(self) -> Self::Expression {
        <i64 as AsExpression<Nullable<BigInt>>>::as_expression(self.to_i64().expect("The value is not a valid coordinate"))
    }
}

impl<'a> AsExpression<Nullable<BigInt>> for &'a Coordinate {
    type Expression = <i64 as AsExpression<Nullable<BigInt>>>::Expression;
    fn as_expression(self) -> Self::Expression {
        <i64 as AsExpression<Nullable<BigInt>>>::as_expression(self.to_i64().expect("The value is not a valid coordinate"))
    }
}