Skip reference lookup when DocumentReference resolves to an empty collection.

Fixes #4612
Original pull request: #4613

Author: stefanbildl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceLookupDelegate.java

@@ -60,6 +60,7 @@
  *
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Stefan Bildl
  * @since 3.3
  */
 public final class ReferenceLookupDelegate {
@@ -106,6 +107,11 @@ public Object readReference(MongoPersistentProperty property, Object source, Loo
 		Object value = source instanceof DocumentReferenceSource documentReferenceSource ? documentReferenceSource.getTargetSource()
 				: source;
 
+		// GH-4612 no need to query database if target collection is empty
+		if (value != null && property.isCollectionLike() && (value instanceof Collection<?> c) && c.isEmpty()) {
+			return new ArrayList<>();
+		}
+
 		DocumentReferenceQuery filter = computeFilter(property, source, spELContext);
 		ReferenceCollection referenceCollection = computeReferenceContext(property, value, spELContext);
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/ReferenceLookupDelegateUnitTests.java

@@ -15,25 +15,23 @@
  */
 package org.springframework.data.mongodb.core.convert;
 
-import static org.assertj.core.api.Assertions.*;
-import static org.mockito.Mockito.*;
-
-import java.util.Collections;
-
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
-
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mapping.model.SpELContext;
 import org.springframework.data.mongodb.core.convert.ReferenceResolver.MongoEntityReader;
 import org.springframework.data.mongodb.core.mapping.DocumentReference;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.expression.EvaluationContext;
-import org.springframework.expression.spel.standard.SpelExpressionParser;
+
+import java.util.Collections;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.*;
 
 /**
  * Unit tests for {@link ReferenceLookupDelegate}.
@@ -52,9 +50,17 @@ class ReferenceLookupDelegateUnitTests {
 
 	@BeforeEach
 	void beforeEach() {
-
 		lookupDelegate = new ReferenceLookupDelegate(mappingContext, spELContext);
-		when(spELContext.getParser()).thenReturn(new SpelExpressionParser());
+	}
+
+	@Test // GH-4612
+	void shouldResolveEmptyListOnEmptyTargetCollection() {
+		MongoPersistentProperty property = mock(MongoPersistentProperty.class);
+		ReferenceLookupDelegate.LookupFunction lookupFunction = mock(ReferenceLookupDelegate.LookupFunction.class);
+
+		when(property.isCollectionLike()).thenReturn(true);
+		lookupDelegate.readReference(property, Collections.emptyList(), lookupFunction, entityReader);
+		verify(lookupFunction, never()).apply(any(), any());
 	}
 
 	@Test // GH-3842