Empty collection not converted to correct type when using element-wise converters

[jurriaan]

We're using java Duration types that get converted to Interval using Reading/WritingConverters:

@WritingConverter
object DurationToIntervalConverter : Converter<Duration, Interval> {
    override fun convert(source: Duration): Interval = Interval.of(source)
}

This seems to work fine for columns of type Duration, however it does not work on List<Duration> when that list is an empty list.
When trying to persist an empty list I get the following error:

java.lang.IllegalArgumentException: Cannot encode parameter of type [Ljava.time.Duration; ([Ljava.time.Duration;@45b7445b)
	at io.r2dbc.postgresql.codec.DefaultCodecs.encodeParameterValue(DefaultCodecs.java:276)
	at io.r2dbc.postgresql.codec.DefaultCodecs.encode(DefaultCodecs.java:250)
	at io.r2dbc.postgresql.PostgresqlStatement.bind(PostgresqlStatement.java:107)

This can be tracked down to this part of spring-data-r2dbc:

spring-data-r2dbc/src/main/java/org/springframework/data/r2dbc/core/DefaultReactiveDataAccessStrategy.java

Lines 248 to 259 in fbe3bfb

	Class<?> actualType = null;
	if (value.getValue() instanceof Collection) {
	actualType = CollectionUtils.findCommonElementType((Collection<?>) value.getValue());
	} else if (!value.isEmpty() && value.getValue().getClass().isArray()) {
	actualType = value.getValue().getClass().getComponentType();
	}
	if (actualType == null) {
	actualType = property.getActualType();
	}
	actualType = converter.getTargetType(actualType);

For collections it first tries to find a common type based on the elements using CollectionUtils.findCommonElementType((Collection<?>) value.getValue()) (which are already converted if there were elements in the array).

This means that for non-empty collections it will correctly use the Interval type, but for empty collections it will actually use converter.getTargetType(actualType), which returns Duration.

Is there a way to get converters to work properly?

Especially since the docs say that per-element converters is the way to go:

Please note that converters get applied on singular properties. Collection properties (e.g. Collection) are iterated and converted element-wise.

[mp911de]

That's fixed and forward-ported now.