Fix document reference on empty reference arrays.

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.3.0-SNAPSHOT</version>
+	<version>3.3.0-GH-3805-SNAPSHOT</version>
 	<packaging>pom</packaging>
 
 	<name>Spring Data MongoDB</name>

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.3.0-SNAPSHOT</version>
+		<version>3.3.0-GH-3805-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -14,7 +14,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.3.0-SNAPSHOT</version>
+		<version>3.3.0-GH-3805-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -11,7 +11,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.3.0-SNAPSHOT</version>
+		<version>3.3.0-GH-3805-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

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

@@ -19,6 +19,7 @@
 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;
@@ -119,7 +120,9 @@ private ReferenceCollection computeReferenceContext(MongoPersistentProperty prop
 
 		// 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
@@ -247,6 +250,10 @@ DocumentReferenceQuery computeFilter(MongoPersistentProperty property, Object va
 				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.Setter;
 import lombok.ToString;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.LinkedHashMap;
@@ -678,6 +679,41 @@ void loadCollectionReferenceWithMissingRefs() {
 		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() {
 
@@ -1094,6 +1130,9 @@ static class CollectionRefRoot {
 		@DocumentReference(lookup = "{ '_id' : '?#{#target}' }") //
 		List<SimpleObjectRef> simpleValueRef;
 
+		@DocumentReference
+		List<SimpleObjectRef> simplePreinitializedValueRef = new ArrayList<>();
+
 		@DocumentReference(lookup = "{ '_id' : '?#{#target}' }", sort = "{ '_id' : -1 } ") //
 		List<SimpleObjectRef> simpleSortedValueRef;