Fix SpEL evaluation in document reference lookup.

Closes #3842
Original pull request: #3844.

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

@@ -43,6 +43,7 @@ import org.springframework.data.mongodb.util.BsonUtils;
 import org.springframework.data.mongodb.util.json.ParameterBindingContext;
 import org.springframework.data.mongodb.util.json.ParameterBindingDocumentCodec;
 import org.springframework.data.mongodb.util.json.ValueProvider;
+import org.springframework.data.mongodb.util.spel.ExpressionUtils;
 import org.springframework.data.util.Streamable;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.lang.Nullable;
@@ -197,6 +198,10 @@ public final class ReferenceLookupDelegate {
 			return (T) codec.decode(value, bindingContext);
 		}
 
+		if(!value.startsWith("#") && ExpressionUtils.detectExpression(value) == null) {
+			return (T) value;
+		}
+
 		T evaluated = (T) bindingContext.evaluateExpression(value);
 		return evaluated != null ? evaluated : defaultValue.get();
 	}

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

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+import java.util.Arrays;
+import java.util.Collections;
+
+import org.assertj.core.api.Assertions;
+import org.bson.Document;
+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.ReferenceLoader.DocumentReferenceQuery;
+import org.springframework.data.mongodb.core.convert.ReferenceLookupDelegate.LookupFunction;
+import org.springframework.data.mongodb.core.convert.ReferenceResolver.MongoEntityReader;
+import org.springframework.data.mongodb.core.convert.ReferenceResolver.ReferenceCollection;
+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;
+
+/**
+ * @author Christoph Strobl
+ */
+@ExtendWith(MockitoExtension.class)
+class ReferenceLookupDelegateUnitTests {
+
+	@Mock MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
+	@Mock SpELContext spELContext;
+	@Mock EvaluationContext evaluationContext;
+	@Mock MongoEntityReader entityReader;
+
+	private ReferenceLookupDelegate lookupDelegate;
+
+	@BeforeEach
+	void beforeEach() {
+
+		lookupDelegate = new ReferenceLookupDelegate(mappingContext, spELContext);
+		when(spELContext.getParser()).thenReturn(new SpelExpressionParser());
+	}
+
+	@Test // GH-3842
+	void shouldComputePlainStringTargetCollection() {
+
+		DocumentReference documentReference = mock(DocumentReference.class);
+		MongoPersistentEntity entity = mock(MongoPersistentEntity.class);
+		MongoPersistentProperty property = mock(MongoPersistentProperty.class);
+
+		doReturn(entity).when(mappingContext).getRequiredPersistentEntity((Class) any());
+
+		when(property.isDocumentReference()).thenReturn(true);
+		when(property.getDocumentReference()).thenReturn(documentReference);
+		when(documentReference.collection()).thenReturn("collection1");
+
+		lookupDelegate.readReference(property, Arrays.asList("one"), new LookupFunction() {
+			@Override
+			public Iterable<Document> apply(DocumentReferenceQuery referenceQuery, ReferenceCollection referenceCollection) {
+
+				assertThat(referenceCollection.getCollection()).isEqualTo("collection1");
+				return Collections.emptyList();
+			}
+		}, entityReader);
+	}
+}