Include more detail for implementing custom types for diesel::MultiCo…

diesel/src/pg/types/date_and_time/chrono.rs

@@ -280,8 +280,8 @@ mod tests {
         let query = select(sql::<Date>("'J0'::date").eq(julian_epoch));
         assert!(query.get_result::<bool>(connection).unwrap());
 
-        let max_date = NaiveDate::MAX;
-        let query = select(sql::<Date>("'262143-12-31'::date").eq(max_date));
+        let max_date = NaiveDate::from_ymd_opt(262142, 12, 31).unwrap();
+        let query = select(sql::<Date>("'262142-12-31'::date").eq(max_date));
         assert!(query.get_result::<bool>(connection).unwrap());
 
         let january_first_2018 = NaiveDate::from_ymd_opt(2018, 1, 1).unwrap();
@@ -311,8 +311,8 @@ mod tests {
         let query = select(sql::<Date>("'J0'::date"));
         assert_eq!(Ok(julian_epoch), query.get_result::<NaiveDate>(connection));
 
-        let max_date = NaiveDate::MAX;
-        let query = select(sql::<Date>("'262143-12-31'::date"));
+        let max_date = NaiveDate::from_ymd_opt(262142, 12, 31).unwrap();
+        let query = select(sql::<Date>("'262142-12-31'::date"));
         assert_eq!(Ok(max_date), query.get_result::<NaiveDate>(connection));
 
         let january_first_2018 = NaiveDate::from_ymd_opt(2018, 1, 1).unwrap();

diesel_derives/src/lib.rs

@@ -1630,8 +1630,99 @@ pub fn table_proc(input: TokenStream) -> TokenStream {
 /// * `diesel::sql_types::Timestamp`
 ///
 /// Support for additional types can be added by providing manual implementations of
-/// `HasSqlType`, `FromSql` and `ToSql` for the corresponding type + the generated
-/// database backend.
+/// `HasSqlType`, `FromSql` and `ToSql` for the corresponding type, all databases included
+/// in your enum, and the backend generated by this derive called `MultiBackend`.
+/// For example to support a custom enum `MyEnum` with the custom SQL type `MyInteger`:
+/// ```
+/// extern crate diesel;
+/// use diesel::backend::Backend;
+/// use diesel::deserialize::{self, FromSql, FromSqlRow};
+/// use diesel::serialize::{self, IsNull, ToSql};
+/// use diesel::AsExpression;
+/// use diesel::sql_types::{HasSqlType, SqlType};
+/// use diesel::prelude::*;
+///
+/// #[derive(diesel::MultiConnection)]
+/// pub enum AnyConnection {
+/// #   #[cfg(feature = "postgres")]
+///     Postgresql(diesel::PgConnection),
+/// #   #[cfg(feature = "mysql")]
+///     Mysql(diesel::MysqlConnection),
+/// #   #[cfg(feature = "sqlite")]
+///     Sqlite(diesel::SqliteConnection),
+/// }
+///
+/// // defining an custom SQL type is optional
+/// // you can also use types from `diesel::sql_types`
+/// #[derive(Copy, Clone, Debug, SqlType)]
+/// #[diesel(postgres_type(name = "Int4"))]
+/// #[diesel(mysql_type(name = "Long"))]
+/// #[diesel(sqlite_type(name = "Integer"))]
+/// struct MyInteger;
+///
+///
+/// // our custom enum
+/// #[repr(i32)]
+/// #[derive(Debug, Clone, Copy, AsExpression, FromSqlRow)]
+/// #[diesel(sql_type = MyInteger)]
+/// pub enum MyEnum {
+///     A = 1,
+///     B = 2,
+/// }
+///
+/// // The `MultiBackend` type is generated by `#[derive(diesel::MultiConnection)]`
+/// // This part is only required if you define a custom sql type
+/// impl HasSqlType<MyInteger> for MultiBackend {
+///    fn metadata(lookup: &mut Self::MetadataLookup) -> Self::TypeMetadata {
+///        // The `lookup_sql_type` function is exposed by the `MultiBackend` type
+///        MultiBackend::lookup_sql_type::<MyInteger>(lookup)
+///    }
+/// }
+///
+/// impl FromSql<MyInteger, MultiBackend> for MyEnum {
+///    fn from_sql(bytes: <MultiBackend as Backend>::RawValue<'_>) -> deserialize::Result<Self> {
+///        // The `from_sql` function is exposed by the `RawValue` type of the
+///        // `MultiBackend` type
+///        // This requires a `FromSql` impl for each backend
+///        bytes.from_sql::<MyEnum, MyInteger>()
+///    }
+/// }
+///
+/// impl ToSql<MyInteger, MultiBackend> for MyEnum {
+///    fn to_sql<'b>(&'b self, out: &mut serialize::Output<'b, '_, MultiBackend>) -> serialize::Result {
+///        /// `set_value` expects a tuple consisting of the target SQL type
+///        /// and self for `MultiBackend`
+///        /// This requires a `ToSql` impl for each backend
+///        out.set_value((MyInteger, self));
+///        Ok(IsNull::No)
+///    }
+/// }
+/// # #[cfg(feature = "postgres")]
+/// # impl ToSql<MyInteger, diesel::pg::Pg> for MyEnum {
+/// #    fn to_sql<'b>(&'b self, out: &mut serialize::Output<'b, '_, diesel::pg::Pg>) -> serialize::Result { todo!() }
+/// # }
+/// # #[cfg(feature = "mysql")]
+/// # impl ToSql<MyInteger, diesel::mysql::Mysql> for MyEnum {
+/// #    fn to_sql<'b>(&'b self, out: &mut serialize::Output<'b, '_, diesel::mysql::Mysql>) -> serialize::Result { todo!() }
+/// # }
+/// # #[cfg(feature = "sqlite")]
+/// # impl ToSql<MyInteger, diesel::sqlite::Sqlite> for MyEnum {
+/// #    fn to_sql<'b>(&'b self, out: &mut serialize::Output<'b, '_, diesel::sqlite::Sqlite>) -> serialize::Result { todo!() }
+/// # }
+/// # #[cfg(feature = "postgres")]
+/// # impl FromSql<MyInteger, diesel::pg::Pg> for MyEnum {
+/// #    fn from_sql(bytes: <diesel::pg::Pg as Backend>::RawValue<'_>) -> deserialize::Result<Self> { todo!() }
+/// # }
+/// # #[cfg(feature = "mysql")]
+/// # impl FromSql<MyInteger, diesel::mysql::Mysql> for MyEnum {
+/// #    fn from_sql(bytes: <diesel::mysql::Mysql as Backend>::RawValue<'_>) -> deserialize::Result<Self> { todo!() }
+/// # }
+/// # #[cfg(feature = "sqlite")]
+/// # impl FromSql<MyInteger, diesel::sqlite::Sqlite> for MyEnum {
+/// #    fn from_sql(bytes: <diesel::sqlite::Sqlite as Backend>::RawValue<'_>) -> deserialize::Result<Self> { todo!() }
+/// # }
+/// # fn main() {}
+/// ```
 #[proc_macro_derive(MultiConnection)]
 pub fn derive_multiconnection(input: TokenStream) -> TokenStream {
     multiconnection::derive(syn::parse_macro_input!(input)).into()