Fix document reference on empty reference arrays.

This commit fixes an issue caused by empty reference arrays.

Closes #3805
Original pull request: #3807.

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

@@ -19,6 +19,7 @@ import java.lang.annotation.Annotation;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -122,7 +123,9 @@ public final class ReferenceLookupDelegate {
 
 		// Use the first value as a reference for others in case of collection like
 		if (value instanceof Iterable) {
-			value = ((Iterable<?>) value).iterator().next();
+
+			Iterator iterator = ((Iterable) value).iterator();
+			value = iterator.hasNext() ? iterator.next() : new Document();
 		}
 
 		// handle DBRef value
@@ -266,6 +269,10 @@ public final class ReferenceLookupDelegate {
 				ors.add(decoded);
 			}
 
+			if(ors.isEmpty()) {
+				return new ListDocumentReferenceQuery(new Document("_id", new Document("$exists", false)), sort);
+			}
+
 			return new ListDocumentReferenceQuery(new Document("$or", ors), sort);
 		}
 

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

@@ -25,6 +25,7 @@ import lombok.Getter;
 import lombok.Setter;
 import lombok.ToString;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.LinkedHashMap;
@@ -679,6 +680,41 @@ public class MongoTemplateDocumentReferenceTests {
 		assertThat(result.getSimpleValueRef()).containsExactly(new SimpleObjectRef("ref-2", "me-the-2-referenced-object"));
 	}
 
+	@Test // GH-3805
+	void loadEmptyCollectionReference() {
+
+		String rootCollectionName = template.getCollectionName(CollectionRefRoot.class);
+
+		// an empty reference array.
+		Document source = new Document("_id", "id-1").append("value", "v1").append("simplePreinitializedValueRef",
+				Collections.emptyList());
+
+		template.execute(db -> {
+			db.getCollection(rootCollectionName).insertOne(source);
+			return null;
+		});
+
+		CollectionRefRoot result = template.findOne(query(where("id").is("id-1")), CollectionRefRoot.class);
+		assertThat(result.simplePreinitializedValueRef).isEmpty();
+	}
+
+	@Test // GH-3805
+	void loadNoExistingCollectionReference() {
+
+		String rootCollectionName = template.getCollectionName(CollectionRefRoot.class);
+
+		// no reference array at all
+		Document source = new Document("_id", "id-1").append("value", "v1");
+
+		template.execute(db -> {
+			db.getCollection(rootCollectionName).insertOne(source);
+			return null;
+		});
+
+		CollectionRefRoot result = template.findOne(query(where("id").is("id-1")), CollectionRefRoot.class);
+		assertThat(result.simplePreinitializedValueRef).isEmpty();
+	}
+
 	@Test // GH-3602
 	void queryForReference() {
 
@@ -1122,6 +1158,9 @@ public class MongoTemplateDocumentReferenceTests {
 		@DocumentReference(lookup = "{ '_id' : '?#{#target}' }") //
 		List<SimpleObjectRef> simpleValueRef;
 
+		@DocumentReference
+		List<SimpleObjectRef> simplePreinitializedValueRef = new ArrayList<>();
+
 		@DocumentReference(lookup = "{ '_id' : '?#{#target}' }", sort = "{ '_id' : -1 } ") //
 		List<SimpleObjectRef> simpleSortedValueRef;