FluxUsingWhen cannot be cast to Iterable error when using spring data rest with reactive repository

Question

I configured a ReactiveSortingRepository to see the reactive capabilities on spring data rest, in the past I've mostly used PagingAndSortingRepository to expose resources as REST endpoints so wanted to see how well reactive stuff works.

the entity/document

public class Post extends AuditableDocument implements Serializable {

    private String title;

    private String content;

}

AuditableDocument has common properties for all resources such as @Version, @LastModifiedDate etc.

and then the repository

public interface PostRepository extends ReactiveCrudRepository<Post, String> {
}

When I access the http://localhost:8080/posts on browser I get this error

{
   "cause":null,
   "message":"class reactor.core.publisher.FluxUsingWhen cannot be cast to class java.lang.Iterable (reactor.core.publisher.FluxUsingWhen is in unnamed module of loader 'app'; java.lang.Iterable is in module java.base of loader 'bootstrap')"
}

And the stack trace

2021-06-29 09:48:38.252 ERROR 32221 --- [nio-8080-exec-3] o.s.d.r.w.RepositoryRestExceptionHandler : class reactor.core.publisher.FluxUsingWhen cannot be cast to class java.lang.Iterable (reactor.core.publisher.FluxUsingWhen is in unnamed module of loader 'app'; java.lang.Iterable is in module java.base of loader 'bootstrap')

java.lang.ClassCastException: class reactor.core.publisher.FluxUsingWhen cannot be cast to class java.lang.Iterable (reactor.core.publisher.FluxUsingWhen is in unnamed module of loader 'app'; java.lang.Iterable is in module java.base of loader 'bootstrap')
    at org.springframework.data.repository.support.ReflectionRepositoryInvoker.invokeFindAllReflectively(ReflectionRepositoryInvoker.java:309) ~[spring-data-commons-2.5.1.jar:2.5.1]
    at org.springframework.data.repository.support.ReflectionRepositoryInvoker.invokeFindAll(ReflectionRepositoryInvoker.java:101) ~[spring-data-commons-2.5.1.jar:2.5.1]
    at org.springframework.data.rest.core.support.UnwrappingRepositoryInvokerFactory$UnwrappingRepositoryInvoker.invokeFindAll(UnwrappingRepositoryInvokerFactory.java:163) ~[spring-data-rest-core-3.5.1.jar:3.5.1]
    at org.springframework.data.rest.webmvc.RepositoryEntityController.getCollectionResource(RepositoryEntityController.java:202) ~[spring-data-rest-webmvc-3.5.1.jar:3.5.1]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:197) ~[spring-web-5.3.8.jar:5.3.8]
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:141) ~[spring-web-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1063) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) ~[tomcat-embed-core-9.0.46.jar:4.0.FR]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.3.8.jar:5.3.8]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) ~[tomcat-embed-core-9.0.46.jar:4.0.FR]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.8.jar:5.3.8]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.8.jar:5.3.8]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.8.jar:5.3.8]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.8.jar:5.3.8]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.8.jar:5.3.8]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.8.jar:5.3.8]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:893) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1707) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.46.jar:9.0.46]
    at java.base/java.lang.Thread.run(Thread.java:829) ~[na:na]

here's the dependencies

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-mongodb-reactive</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-rest</artifactId>
    </dependency>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
    </dependency>

Answer 1

You are using both Spring Data Rest and Spring Data MongoDB Reactive, as Anadi Misra mentioned in the comments Spring Data Rest is only for servlet stack whereas Spring Data MongoDB Reactive, as the name states, lies on the reactive stack.

The solution would depend on your application type, Imperative or Reactive. If you're going for Reactive you need to replace spring-boot-starter-data-rest with spring-boot-starter-webflux and manually configure REST endpoints and use Spring HATEOAS if required. Or if you are making an Imperative application, you neeed to replace spring-boot-starter-data-mongodb-reactive with spring-boot-starter-data-mongodb and your Spring Data Rest should work fine.