probably reproduced the issue in test

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/JdbcRepositoryCrossAggregateHsqlIntegrationTests.java

@@ -15,26 +15,38 @@
  */
 package org.springframework.data.jdbc.repository;
 
+import static java.util.Arrays.*;
 import static org.assertj.core.api.Assertions.*;
 
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.FilterType;
 import org.springframework.context.annotation.Import;
+import org.springframework.core.convert.converter.Converter;
 import org.springframework.data.annotation.Id;
+import org.springframework.data.convert.CustomConversions;
+import org.springframework.data.convert.ReadingConverter;
+import org.springframework.data.convert.WritingConverter;
+import org.springframework.data.jdbc.core.convert.JdbcCustomConversions;
 import org.springframework.data.jdbc.core.mapping.AggregateReference;
+import org.springframework.data.jdbc.core.mapping.JdbcSimpleTypes;
 import org.springframework.data.jdbc.repository.config.EnableJdbcRepositories;
 import org.springframework.data.jdbc.testing.DatabaseType;
 import org.springframework.data.jdbc.testing.EnabledOnDatabase;
 import org.springframework.data.jdbc.testing.IntegrationTest;
 import org.springframework.data.jdbc.testing.TestConfiguration;
+import org.springframework.data.mapping.model.SimpleTypeHolder;
+import org.springframework.data.relational.core.dialect.Dialect;
 import org.springframework.data.relational.core.mapping.RelationalMappingContext;
 import org.springframework.data.repository.CrudRepository;
 import org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate;
 import org.springframework.test.jdbc.JdbcTestUtils;
 
+import java.util.Collections;
+
 /**
  * Very simple use cases for creation and usage of JdbcRepositories.
  *
@@ -52,13 +64,18 @@ public class JdbcRepositoryCrossAggregateHsqlIntegrationTests {
 	@Configuration
 	@Import(TestConfiguration.class)
 	@EnableJdbcRepositories(considerNestedRepositories = true,
-			includeFilters = @ComponentScan.Filter(value = Ones.class, type = FilterType.ASSIGNABLE_TYPE))
+			includeFilters = @ComponentScan.Filter(value = {Ones.class, ReferencingAggregateRepository.class}, type = FilterType.ASSIGNABLE_TYPE))
 	static class Config {
 
+		@Bean
+		JdbcCustomConversions jdbcCustomConversions() {
+			return new JdbcCustomConversions(asList( AggregateIdToLong.INSTANCE, LongToAggregateId.INSTANCE ));
+		}
 	}
 
 	@Autowired NamedParameterJdbcTemplate template;
 	@Autowired Ones ones;
+	@Autowired ReferencingAggregateRepository referencingAggregates;
 	@Autowired RelationalMappingContext context;
 
 	@SuppressWarnings("ConstantConditions")
@@ -95,6 +112,18 @@ public class JdbcRepositoryCrossAggregateHsqlIntegrationTests {
 		).isEqualTo(1);
 	}
 
+	@Test // DATAJDBC-221
+	public void savesAndReadWithConvertableId() {
+
+
+		AggregateReference<AggregateWithConvertableId, AggregateId> idReference = AggregateReference.to(new AggregateId(TWO_ID));
+		ReferencingAggregate reference = referencingAggregates.save(new ReferencingAggregate(null, "Reference", idReference));
+
+
+		ReferencingAggregate reloaded = referencingAggregates.findById(reference.id).get();
+		assertThat(reloaded.id()).isEqualTo(idReference);
+	}
+
 	interface Ones extends CrudRepository<AggregateOne, Long> {}
 
 	static class AggregateOne {
@@ -109,4 +138,40 @@ public class JdbcRepositoryCrossAggregateHsqlIntegrationTests {
 		@Id Long id;
 		String name;
 	}
+
+	interface ReferencingAggregateRepository extends CrudRepository<ReferencingAggregate, Long> {
+
+	}
+
+	record AggregateWithConvertableId(@Id AggregateId id, String name) {
+
+	}
+
+	record AggregateId(Long value) {
+
+	}
+
+	record ReferencingAggregate(@Id Long id, String name,
+								AggregateReference<AggregateWithConvertableId, AggregateId> ref) {
+	}
+
+	@WritingConverter
+	enum AggregateIdToLong implements Converter<AggregateId, Long> {
+		INSTANCE;
+
+		@Override
+		public Long convert(AggregateId source) {
+			return source.value;
+		}
+	}
+
+	@ReadingConverter
+	enum LongToAggregateId implements Converter< Long,AggregateId> {
+		INSTANCE;
+
+		@Override
+		public AggregateId convert(Long source) {
+			return new AggregateId(source);
+		}
+	}
 }

spring-data-jdbc/src/test/resources/org.springframework.data.jdbc.repository/JdbcRepositoryCrossAggregateHsqlIntegrationTests-hsql.sql

@@ -1 +1,8 @@
 CREATE TABLE aggregate_one ( id BIGINT GENERATED BY DEFAULT AS IDENTITY ( START WITH 1 ) PRIMARY KEY, NAME VARCHAR(100), two INTEGER);
+
+CREATE TABLE REFERENCING_AGGREGATE
+(
+    ID   BIGINT GENERATED BY DEFAULT AS IDENTITY ( START WITH 1 ) PRIMARY KEY,
+    NAME VARCHAR(100),
+    REF  INTEGER
+);