Fix `@DocumentReference` resolution for properties used in constructor.

This commit fixes an issue that prevented referenced entities from being used as constructor arguments.

Closes: #3806
Original pull request: #3810.

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

@@ -541,7 +541,9 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			} else {
 
 				accessor.setProperty(property,
-						dbRefResolver.resolveReference(property, new DocumentReferenceSource(documentAccessor.getDocument(), documentAccessor.get(property)), referenceLookupDelegate, context::convert));
+						dbRefResolver.resolveReference(property,
+								new DocumentReferenceSource(documentAccessor.getDocument(), documentAccessor.get(property)),
+								referenceLookupDelegate, context::convert));
 			}
 			return;
 		}
@@ -875,10 +877,12 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			if (property.isAssociation()) {
 
 				List<Object> targetCollection = collection.stream().map(it -> {
-						return documentPointerFactory.computePointer(mappingContext, property, it, property.getActualType()).getPointer();
+					return documentPointerFactory.computePointer(mappingContext, property, it, property.getActualType())
+							.getPointer();
 				}).collect(Collectors.toList());
 
-				return writeCollectionInternal(targetCollection, ClassTypeInformation.from(DocumentPointer.class), new ArrayList<>());
+				return writeCollectionInternal(targetCollection, ClassTypeInformation.from(DocumentPointer.class),
+						new ArrayList<>());
 			}
 
 			if (property.hasExplicitWriteTarget()) {
@@ -931,7 +935,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				if (property.isDbReference()) {
 					document.put(simpleKey, value != null ? createDBRef(value, property) : null);
 				} else {
-					document.put(simpleKey, documentPointerFactory.computePointer(mappingContext, property, value, property.getActualType()).getPointer());
+					document.put(simpleKey, documentPointerFactory
+							.computePointer(mappingContext, property, value, property.getActualType()).getPointer());
 				}
 
 			} else {
@@ -1814,6 +1819,11 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				return (T) dbRefResolver.resolveDbRef(property, dbref, callback, dbRefProxyHandler);
 			}
 
+			if (property.isDocumentReference()) {
+				return (T) dbRefResolver.resolveReference(property, accessor.get(property), referenceLookupDelegate,
+						context::convert);
+			}
+
 			return super.getPropertyValue(property);
 		}
 	}
@@ -2044,7 +2054,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 					return (S) mapConverter.convert(this, BsonUtils.asBson(source), typeHint);
 				}
 
-				throw new IllegalArgumentException(String.format("Expected map like structure but found %s", source.getClass()));
+				throw new IllegalArgumentException(
+						String.format("Expected map like structure but found %s", source.getClass()));
 			}
 
 			if (source instanceof DBRef) {

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

@@ -733,6 +733,52 @@ public class MongoTemplateDocumentReferenceTests {
 		assertThat(result.simplePreinitializedValueRef).isEmpty();
 	}
 
+	@Test // GH-3806
+	void resolveReferenceWhenUsedAsCtorArgument() {
+
+		Publisher publisher = new Publisher();
+		publisher.id = "p-111";
+		publisher.name = "ppp";
+
+		template.save(publisher);
+
+		WithRequiredArgsCtor source = new WithRequiredArgsCtor("id-1", publisher);
+
+		template.save(source);
+
+		WithRequiredArgsCtor target = template.findOne(query(where("id").is(source.id)), WithRequiredArgsCtor.class);
+		assertThat(target.publisher).isNotNull();
+	}
+
+	@Test // GH-3806
+	void resolveLazyReferenceWhenUsedAsCtorArgument() {
+
+		Publisher publisher = new Publisher();
+		publisher.id = "p-111";
+		publisher.name = "ppp";
+
+		template.save(publisher);
+
+		WithLazyRequiredArgsCtor source = new WithLazyRequiredArgsCtor("id-1", publisher);
+
+		template.save(source);
+
+		WithLazyRequiredArgsCtor target = template.findOne(query(where("id").is(source.id)), WithLazyRequiredArgsCtor.class);
+
+		// proxy not yet resolved
+		LazyLoadingTestUtils.assertProxy(target.publisher, (proxy) -> {
+
+			assertThat(proxy.isResolved()).isFalse();
+			assertThat(proxy.currentValue()).isNull();
+		});
+
+		// resolve the proxy by invoking a method on it
+		assertThat(target.getPublisher().getName()).isEqualTo("ppp");
+		LazyLoadingTestUtils.assertProxy(target.publisher, (proxy) -> {
+			assertThat(proxy.isResolved()).isTrue();
+		});
+	}
+
 	@Test // GH-3602
 	void queryForReference() {
 
@@ -1371,6 +1417,30 @@ public class MongoTemplateDocumentReferenceTests {
 		String id;
 		String acronym;
 		String name;
+
+		public String getId() {
+			return id;
+		}
+
+		public void setId(String id) {
+			this.id = id;
+		}
+
+		public String getAcronym() {
+			return acronym;
+		}
+
+		public void setAcronym(String acronym) {
+			this.acronym = acronym;
+		}
+
+		public String getName() {
+			return name;
+		}
+
+		public void setName(String name) {
+			this.name = name;
+		}
 	}
 
 	@Data
@@ -1401,4 +1471,40 @@ public class MongoTemplateDocumentReferenceTests {
 		@DocumentReference(lookup="{'publisherId':?#{#self._id} }")
 		List<OneToManyStyleBook> books;
 	}
+
+	static class WithRequiredArgsCtor {
+
+		final String id;
+
+		@DocumentReference
+		final Publisher publisher;
+
+		public WithRequiredArgsCtor(String id, Publisher publisher) {
+
+			this.id = id;
+			this.publisher = publisher;
+		}
+	}
+
+	static class WithLazyRequiredArgsCtor {
+
+		final String id;
+
+		@DocumentReference(lazy = true)
+		final Publisher publisher;
+
+		public WithLazyRequiredArgsCtor(String id, Publisher publisher) {
+
+			this.id = id;
+			this.publisher = publisher;
+		}
+
+		public String getId() {
+			return id;
+		}
+
+		public Publisher getPublisher() {
+			return publisher;
+		}
+	}
 }