Add validation warning for invalid Set<T> in @MappedCollection (GH-2061)

- Add validation in RelationalMappingContext.createPersistentEntity
- Add unit tests for validation scenarios

Signed-off-by: euiyoung <euiyoung1403@gmail.com>

spring-data-relational/src/main/java/org/springframework/data/relational/core/mapping/RelationalMappingContext.java

@@ -16,10 +16,12 @@
 package org.springframework.data.relational.core.mapping;
 
 import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.jspecify.annotations.Nullable;
-
 import org.springframework.beans.BeansException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.env.Environment;
@@ -45,6 +47,8 @@ import org.springframework.util.Assert;
 public class RelationalMappingContext
 		extends AbstractMappingContext<RelationalPersistentEntity<?>, RelationalPersistentProperty> {
 
+	private static final Logger logger = LoggerFactory.getLogger(RelationalMappingContext.class);
+
 	private final NamingStrategy namingStrategy;
 	private final Map<AggregatePathCacheKey, AggregatePath> aggregatePathCache = new ConcurrentHashMap<>();
 
@@ -142,6 +146,9 @@ public class RelationalMappingContext
 				this.namingStrategy, this.sqlIdentifierExpressionEvaluator);
 		entity.setForceQuote(isForceQuote());
 
+		// Validate Set<T> properties in @MappedCollection context
+		validateSetMappedCollectionProperties(entity);
+
 		return entity;
 	}
 
@@ -219,6 +226,78 @@ public class RelationalMappingContext
 		return aggregatePath;
 	}
 
+	/**
+	 * Validates Set<T> properties in nested @MappedCollection scenarios.
+	 * 
+	 * @param entity the entity to validate
+	 */
+	private <T> void validateSetMappedCollectionProperties(RelationalPersistentEntity<T> entity) {
+		for (RelationalPersistentProperty property : entity) {
+			if (isSetMappedCollection(property)) {
+				validateSetMappedCollectionProperty(property);
+			}
+		}
+	}
+
+	/**
+	 * Checks if a property is a Set with @MappedCollection annotation.
+	 */
+	private boolean isSetMappedCollection(RelationalPersistentProperty property) {
+		return property.isCollectionLike() 
+			&& Set.class.isAssignableFrom(property.getType())
+			&& property.isAnnotationPresent(MappedCollection.class);
+	}
+
+	/**
+	 * Validates a Set<T> property in @MappedCollection context.
+	 * 
+	 * @param property the Set property to validate
+	 */
+	private void validateSetMappedCollectionProperty(RelationalPersistentProperty property) {
+		Class<?> elementType = property.getComponentType();
+		if (elementType == null) {
+			return;
+		}
+		
+		RelationalPersistentEntity<?> elementEntity = getPersistentEntity(elementType);
+		if (elementEntity == null) {
+			return;
+		}
+		
+		boolean hasId = elementEntity.hasIdProperty();
+		boolean hasEntityOrCollectionReferences = hasEntityOrCollectionReferences(elementEntity);
+		
+		if (!hasId && hasEntityOrCollectionReferences) {
+			String message = String.format(
+				"Invalid @MappedCollection usage: Set<%s> in %s.%s. " +
+				"Set elements without @Id must not contain entity or collection references. " +
+				"Consider using List instead or add @Id to %s.",
+				elementType.getSimpleName(),
+				property.getOwner().getType().getSimpleName(),
+				property.getName(),
+				elementType.getSimpleName()
+			);
+			
+			logger.warn(message);
+		}
+	}
+
+	/**
+	 * Checks if an entity has any properties that are entities or collections.
+	 */
+	private boolean hasEntityOrCollectionReferences(RelationalPersistentEntity<?> entity) {
+		for (RelationalPersistentProperty prop : entity) {
+			if (prop.isIdProperty() || prop.isVersionProperty()) {
+				continue;
+			}
+			
+			if (prop.isEntity() || prop.isCollectionLike()) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 	private record AggregatePathCacheKey(RelationalPersistentEntity<?> root,
 			@Nullable PersistentPropertyPath<? extends RelationalPersistentProperty> path) {
 

spring-data-relational/src/test/java/org/springframework/data/relational/core/mapping/RelationalMappingContextUnitTests.java

@@ -19,6 +19,7 @@ import static org.assertj.core.api.Assertions.*;
 
 import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.UUID;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -152,4 +153,74 @@ public class RelationalMappingContextUnitTests {
 	static class Inherit1 extends Base {}
 
 	static class Inherit2 extends Base {}
+
+	// GH-2061 - Tests for Set<T> validation in @MappedCollection context
+
+	@Test // GH-2061
+	void doesNotThrowExceptionForInvalidSetUsage() {
+		context = new RelationalMappingContext();
+		context.setSimpleTypeHolder(holder);
+
+		// Should not throw exception, just log warning
+		assertThatCode(() -> context.getPersistentEntity(AggregateWithInvalidSet.class))
+			.doesNotThrowAnyException();
+	}
+
+	@Test // GH-2061
+	void doesNotThrowExceptionWhenSetElementHasId() {
+		context = new RelationalMappingContext();
+		context.setSimpleTypeHolder(holder);
+
+		assertThatCode(() -> context.getPersistentEntity(AggregateWithValidSetHavingId.class))
+			.doesNotThrowAnyException();
+	}
+
+	@Test // GH-2061
+	void doesNotThrowExceptionWhenSetElementWithoutIdHasNoReferences() {
+		context = new RelationalMappingContext();
+		context.setSimpleTypeHolder(holder);
+
+		assertThatCode(() -> context.getPersistentEntity(AggregateWithValidSetWithoutReferences.class))
+			.doesNotThrowAnyException();
+	}
+
+	// Test entities for GH-2061
+	static class AggregateWithInvalidSet {
+		@Id Long id;
+		@MappedCollection(idColumn = "aggregate_id", keyColumn = "idx")
+		Set<InvalidElement> elements;
+	}
+
+	static class InvalidElement {
+		String name;
+		OtherEntity reference;
+	}
+
+	static class OtherEntity {
+		@Id Long id;
+		String value;
+	}
+
+	static class AggregateWithValidSetHavingId {
+		@Id Long id;
+		@MappedCollection(idColumn = "aggregate_id")
+		Set<ElementWithId> elements;
+	}
+
+	static class ElementWithId {
+		@Id Long id;
+		String name;
+		OtherEntity reference;
+	}
+
+	static class AggregateWithValidSetWithoutReferences {
+		@Id Long id;
+		@MappedCollection(idColumn = "aggregate_id", keyColumn = "idx")
+		Set<SimpleElement> elements;
+	}
+
+	static class SimpleElement {
+		String name;
+		int value;
+	}
 }