Add H2 Interval to Duration converters to H2Dialect

[Bittuw]

In R2DBC as example of obscured types.

Is it normally that MappingR2dbcConverter doesn't call Row::get(String, Class)? I thought that MappingR2dbcConverter provides all target field types transformations to r2dbc-spi implementation. But instead it calls Row::get(String) and SPI implementation doesn't know about the target field type.

[mp911de]

Yes, that's by intent. There's a bit of background to understand why we don't use the target type.

Spring Data is a library that ships with extensive mapping and conversion abstractions. Spring Data can map various types to the domain model applying lenient conversion. Using Row.get(…) with a Class parameter requires the target type to be a R2DBC primitive and it requires that a driver can convert the actual type into the desired target type.

Drivers typically don't convert timestamps or numeric values to e.g. String. Going forward, Spring Data allows using converters to convert the actual database value into something else (e.g. for JSON String to YourPersonObject). While Spring Data is aware of how to convert a JSON String into a complex object, the R2DBC is not.

You seem to have an issue with the current approach and we'd like to understand your use-case, what you want to achieve and which obstacles you encounter.

[Bittuw]

@mp911de, Thanks for explaning. My problem:
As I mentioned in linked issue, I use Duration field type in my DTOs and H2 as default DB. If I don't override any converters in Spring Data R2dbc, Duration class will be use the next mapping chain:

1. Reading: H2 will be use IntervalCodec because its doesn't know about field type and Interval is assignable to Object class (Which is default). So there will be convertion error, because there is no any converter from Interval to Duration;
2. Writing: The first, spring will convert Duration into String (I don't know why Spring Data R2dbc provide this converter by default), then there will be invalid SQL bindings error, because we cannot assign String to Interval in SQL.

To avoid this behavior I use special writing Converter<Duration,Duration> to override Converter<Duration,String> and reading Converter<Interval, Duration>.

I think that its very strange behavior, because H2 r2dbc driver can convert Duration itself and it will be useful to "ask" R2DBC driver implementation, that it can convert this type itself before using any converters from ConverterService.

[mp911de]

Converter<Duration,String> comes from Spring Data's early days JSR-310 support where Duration were converted to String because JPA, MongoDB, and other stores didn't support Duration.

Generally speaking, Intervall is an H2 native type. Writing-wise, it could make sense to register a converter Duration to Interval.ofDaysHoursMinutesNanos(…) and Interval to Duration.

Out of interest, how would that work with JDBC?

[Bittuw]

But what about the layered transformations I suggested? It is more flexible way to support already implemented convertions in H2/MySQL/Postgres and not implemented JPA/MongoDB. Actually Row::get(String) can be replaced by:

try { return row.get("table_name", field_type); }
catch (CannotConvert e) { /*logs*/ }
return convertionService.convert(row.get("table_name"), field_type);

What to do with Converter<Duration,String> i don't know, because I don't know who provide this converter? R2DBC SPI implementation or Spring Data R2DBC itself?

[mp911de]

Exception-driven conditionals cause a lot of performance drag for an edge-case that could be solved in a better style.

Regarding converters, we can ship these with Spring Data R2DBC.

[Bittuw]

@mp911de , not necessarily as I wrote. I think it's enough to be able to ask R2DBC SPI implementation about possability of such convertion. But how it must be done I don't know.