Repository query keyword IS_EMPTY not supported

[ssppooff]

The documentation for repository query keywords (link) mentions support for the IsEmpty keyword, however, it fails with "Unsupported keyword" when used.

Steps to reproduce
0) spring initializr:

• Maven & Kotlin on Java 17
• Spring Boot 3.2.1
• Dependencies: MongoDB Reactive

1. Inside file with @SpringBootApplication, add the code below
2. Start the app (no need for a running Mongo instance to get the exception)

interface CustomRepository : ReactiveMongoRepository<CustomData, String> {
	fun findByIdIsEmpty(id: String): Mono<Boolean>
}

data class CustomData(val id: String)

@Component
class Demo(val customRepository: CustomRepository) {
	@Bean
	fun accessRepo() {
		customRepository.findByIdIsEmpty("non-existent").block()
	}
}

Result
Error creating bean 'accessRepo', caused by org.springframework.beans.BeanInstantiationException: Failed to instantiate [void]: Factory method 'accessRepo' threw exception with message: Unsupported keyword

The exception is thrown inside org.springframework.data.mongodb.repository.query.from (line number 254), because the switch-statement above (line 168) has no case for IS_EMPTY, even though the keyword is defined in org.springframework.data.repository.query.parser.Type.

[christophstrobl]

True, the documentation is misleading as the repository sections lists the generic overview from data-commons which holds the entire list of supported keywords.

Supporting IS_EMPTY would be an enhancement and we'd have to define it's behaviour given the various scenarios the MongoDB data model allows.

I could imagine using an $eq comparision to build the support. Like { field : { $eq : "" } } would be sufficient for simple String. Collections could be checked with { field: { $size: 0 } } and objects with { field : { $eq : {} } }. Numeric values, should error, though one might argue a stored null may be considered empty, but what about primitive types then. Generic types in the java domain model may also be tricky and we'd need to consider @Field(targetType=...) to pick the right format.