This crate allows the user to represent state in the database using Rust enums. This is achieved through a proc macro. First the macro looks at your chosen sql_type, and then it devises a corresponding Rust type. The mapping is as follows:

SQL	Rust
SmallInt	i16
Integer	i32
Int	i32
BigInt	i64
VarChar	String
Text	String

The macro then generates three impls: a FromSql impl, an ToSql impl and a TryFrom impl, which allow conversion between the Sql type and the enum (FromSql and ToSql), and from the Rust type into the enum (TryInto).

Usage

use diesel_enum::DbEnum;
use diesel::{sql_types::SmallInt, expression::AsExpression, deserialize::FromSqlRow};

#[derive(Debug)]
pub struct CustomError {
    msg: String,
    status: u16,
}

impl CustomError {
    fn not_found(msg: String) -> Self {
        Self {
            msg,
            status: 404,
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, AsExpression, FromSqlRow, DbEnum)]
#[diesel(sql_type = SmallInt)]
#[diesel_enum(error_fn = CustomError::not_found)]
#[diesel_enum(error_type = CustomError)
pub enum Status {
    /// Will be represented as 0.
    Ready,
    /// Will be represented as 1.
    Pending,
}

Alternatively you can use strings, with will be cast to lowercase. (e.g. Status::Ready will be stored as "ready" in the database):

#[derive(Debug, Clone, Copy, PartialEq, Eq, AsExpression, FromSqlRow, DbEnum)]
#[diesel(sql_type = VarChar)]
#[diesel_enum(error_fn = CustomError::not_found)]
#[diesel_enum(error_type = CustomError)]
pub enum Status {
    /// Will be represented as `"ready"`.
    Ready,
    /// Will be represented as `"pending"`.
    Pending,
}

Another option is to manually override the values set for each or some of the variants. This is done using the val attribute:

#[derive(Debug, Clone, Copy, PartialEq, Eq, AsExpression, FromSqlRow, DbEnum)]
#[diesel(sql_type = VarChar)]
#[diesel_enum(error_fn = CustomError::not_found)]
#[diesel_enum(error_type = CustomError)]
pub enum Status {
    /// Will be represented as `"reddy"`.
    #[val = "reddy"]
    Ready,
    /// Will be represented as `"pending"`.
    Pending,
}