Allow reading already resolved DBRef values.

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>4.1.0-SNAPSHOT</version>
+	<version>4.1.x-GH-4312-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>4.1.0-SNAPSHOT</version>
+		<version>4.1.x-GH-4312-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.1.0-SNAPSHOT</version>
+		<version>4.1.x-GH-4312-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -13,7 +13,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.1.0-SNAPSHOT</version>
+		<version>4.1.x-GH-4312-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

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

@@ -668,9 +668,32 @@ private void readAssociation(Association<MongoPersistentProperty> association, P
 			return;
 		}
 
-		DBRef dbref = value instanceof DBRef ? (DBRef) value : null;
+		if (value instanceof DBRef dbref) {
+			accessor.setProperty(property, dbRefResolver.resolveDbRef(property, dbref, callback, handler));
+			return;
+		}
 
-		accessor.setProperty(property, dbRefResolver.resolveDbRef(property, dbref, callback, handler));
+		/*
+		 * The value might be a pre resolved full document (eg. resulting from an aggregation $lookup).
+		 * In this case we try to map that object to the target type without an additional step ($dbref resolution server roundtrip)
+		 * in between.
+		 */
+		if (value instanceof Document document) {
+			if(property.isMap()) {
+				if(document.isEmpty() || document.values().iterator().next() instanceof DBRef) {
+					accessor.setProperty(property, dbRefResolver.resolveDbRef(property, null, callback, handler));
+				} else {
+					accessor.setProperty(property, readMap(context, document, property.getTypeInformation()));
+				}
+			} else {
+				accessor.setProperty(property, read(property.getActualType(), document));
+			}
+		} else if (value instanceof Collection<?> collection && collection.size() > 0
+				&& collection.iterator().next() instanceof Document) {
+			accessor.setProperty(property, readCollectionOrArray(context, collection, property.getTypeInformation()));
+		} else {
+			accessor.setProperty(property, dbRefResolver.resolveDbRef(property, null, callback, handler));
+		}
 	}
 
 	@Nullable

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

@@ -33,6 +33,7 @@
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 
 import org.bson.Document;
 import org.junit.jupiter.api.BeforeEach;
@@ -110,6 +111,98 @@ void resolvesLazyDBRefOnAccess() {
 		verify(dbRefResolver).bulkFetch(any());
 	}
 
+	@Test // GH-4312
+	void conversionShouldAllowReadingAlreadyResolvedReferences() {
+
+		Document sampleSource = new Document("_id", "sample-1").append("value", "one");
+		Document source = new Document("_id", "id-1").append("sample", sampleSource);
+
+		WithSingleValueDbRef read = converter.read(WithSingleValueDbRef.class, source);
+
+		assertThat(read.sample).isEqualTo(converter.read(Sample.class, sampleSource));
+		verifyNoInteractions(dbRefResolver);
+	}
+
+	@Test // GH-4312
+	void conversionShouldAllowReadingAlreadyResolvedListOfReferences() {
+
+		Document sample1Source = new Document("_id", "sample-1").append("value", "one");
+		Document sample2Source = new Document("_id", "sample-2").append("value", "two");
+		Document source = new Document("_id", "id-1").append("lazyList", List.of(sample1Source, sample2Source));
+
+		WithLazyDBRef read = converter.read(WithLazyDBRef.class, source);
+
+		assertThat(read.lazyList).containsExactly(converter.read(Sample.class, sample1Source),
+				converter.read(Sample.class, sample2Source));
+		verifyNoInteractions(dbRefResolver);
+	}
+
+	@Test // GH-4312
+	void conversionShouldAllowReadingAlreadyResolvedMapOfReferences() {
+
+		Document sample1Source = new Document("_id", "sample-1").append("value", "one");
+		Document sample2Source = new Document("_id", "sample-2").append("value", "two");
+		Document source = new Document("_id", "id-1").append("sampleMap",
+				new Document("s1", sample1Source).append("s2", sample2Source));
+
+		WithMapValueDbRef read = converter.read(WithMapValueDbRef.class, source);
+
+		assertThat(read.sampleMap) //
+				.containsEntry("s1", converter.read(Sample.class, sample1Source)) //
+				.containsEntry("s2", converter.read(Sample.class, sample2Source));
+		verifyNoInteractions(dbRefResolver);
+	}
+
+	@Test // GH-4312
+	void conversionShouldAllowReadingAlreadyResolvedMapOfLazyReferences() {
+
+		Document sample1Source = new Document("_id", "sample-1").append("value", "one");
+		Document sample2Source = new Document("_id", "sample-2").append("value", "two");
+		Document source = new Document("_id", "id-1").append("sampleMapLazy",
+				new Document("s1", sample1Source).append("s2", sample2Source));
+
+		WithMapValueDbRef read = converter.read(WithMapValueDbRef.class, source);
+
+		assertThat(read.sampleMapLazy) //
+				.containsEntry("s1", converter.read(Sample.class, sample1Source)) //
+				.containsEntry("s2", converter.read(Sample.class, sample2Source));
+		verifyNoInteractions(dbRefResolver);
+	}
+
+	@Test // GH-4312
+	void resolvesLazyDBRefMapOnAccess() {
+
+		client.getDatabase(DATABASE).getCollection("samples")
+				.insertMany(Arrays.asList(new Document("_id", "sample-1").append("value", "one"),
+						new Document("_id", "sample-2").append("value", "two")));
+
+		Document source = new Document("_id", "id-1").append("sampleMapLazy",
+				new Document("s1", new com.mongodb.DBRef("samples", "sample-1")).append("s2",
+						new com.mongodb.DBRef("samples", "sample-2")));
+
+		WithMapValueDbRef target = converter.read(WithMapValueDbRef.class, source);
+
+		verify(dbRefResolver).resolveDbRef(any(), isNull(), any(), any());
+
+		assertThat(target.sampleMapLazy).isInstanceOf(LazyLoadingProxy.class);
+		assertThat(target.getSampleMapLazy()).containsEntry("s1", new Sample("sample-1", "one")).containsEntry("s2",
+				new Sample("sample-2", "two"));
+
+		verify(dbRefResolver).bulkFetch(any());
+	}
+
+	@Test // GH-4312
+	void conversionShouldAllowReadingAlreadyResolvedLazyReferences() {
+
+		Document sampleSource = new Document("_id", "sample-1").append("value", "one");
+		Document source = new Document("_id", "id-1").append("sampleLazy", sampleSource);
+
+		WithSingleValueDbRef read = converter.read(WithSingleValueDbRef.class, source);
+
+		assertThat(read.sampleLazy).isEqualTo(converter.read(Sample.class, sampleSource));
+		verifyNoInteractions(dbRefResolver);
+	}
+
 	@Test // DATAMONGO-2004
 	void resolvesLazyDBRefConstructorArgOnAccess() {
 
@@ -164,6 +257,31 @@ List<Sample> getLazyList() {
 		}
 	}
 
+	@Data
+	public static class WithSingleValueDbRef {
+
+		@Id //
+		String id;
+
+		@DBRef //
+		Sample sample;
+
+		@DBRef(lazy = true) //
+		Sample sampleLazy;
+	}
+
+	@Data
+	public static class WithMapValueDbRef {
+
+		@Id String id;
+
+		@DBRef //
+		Map<String, Sample> sampleMap;
+
+		@DBRef(lazy = true) //
+		Map<String, Sample> sampleMapLazy;
+	}
+
 	public static class WithLazyDBRefAsConstructorArg {
 
 		@Id String id;