Custom converters not working for collecton-like properties

[denniseffing]

Hi,
it seems that it is impossible to save an entity that includes a collection-like property using a custom type because the converting process doesn't work properly.

The following entity can not be saved, even with the supplied custom converters:

class SomeEntity {
    // ... id, persistence creator etc.
   
    List<CustomType> types;
}

record CustomType(String value) { }

class CustomTypeToStringConverter implements Converter<CustomType, String> {

	@Override
	public String convert(CustomType source) {
		return source.value;
	}
}

class StringToCustomTypeConverter implements Converter<String, CustomType> {

	@Override
	public Region convert(String source) {
		return new CustomType(source);
	}
}

The only way to make this work is to create another wrapper type like this:

record CustomTypes(List<CustomType> types) {}

// converters for CustomTypes record

I've done some digging:
The derived column type of every collection-like property that is not an entity is the Array class: spring-projects/spring-data-relational@3117465d:spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/MappingJdbcConverter.java#180

When a collection is written, every value of it is mapped and stored in an ArrayList (as expected), however, the code then checks if the mapped ArrayList is an instance of the derived column type (i.e. Array) before returning it, which can never be true.
spring-projects/spring-data-relational@3117465d:spring-data-relational/src/main/java/org/springframework/data/relational/core/conversion/MappingRelationalConverter.java#770

[schauder]

I'm not completely sure, what you want to try to achieve.

The converters you posted seem like you want to convert the elements of a collection to some other type (String in your example) on writing and back on reading. This should work. If it doesn't please provide a full reproducer.

Or do you want to convert the complete List, like a List<CustomType> gets converted to a single String.
This is indeed not supported, right now. If this is what you want we'd discuss this internally if we want to support this.

[denniseffing]

I want to convert the elements of a collection to some other type. If I understood it correctly, I should be able to convert a property of type List<CustomType> to the SQL type text[] given correctly implemented custom converters.

Here is a full reproducer: denniseffing/collection-like-property-converter. Execute the test SomeEntityRepositoryIntegrationTest to reproduce the issue.

Thank you for your help! Let me know if you need anything else.