From 3741fc73bdb0b99252518c394fd824339aaecd8d Mon Sep 17 00:00:00 2001
From: jmarkerink <jmarkerink@outlook.com>
Date: Thu, 29 Jan 2026 00:40:38 +0100
Subject: [PATCH] fid: dot notation projection to nested object

---
 .../aggregation/stage/ProjectStage.java       |  6 +-
 .../backend/AbstractAggregationTest.java      | 78 +++++++++++++++++++
 2 files changed, 81 insertions(+), 3 deletions(-)

diff --git a/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/ProjectStage.java b/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/ProjectStage.java
index e5efea0e..e752ef48 100644
--- a/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/ProjectStage.java
+++ b/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/ProjectStage.java
@@ -75,13 +75,13 @@ Document projectDocument(Document document) {
                         .stream()
                         .map(value -> Expression.evaluateDocument(value, document))
                         .collect(Collectors.toList());
-                    result.put(field, resolvedProjectionValues);
+                    Utils.changeSubdocumentValue(result, field, resolvedProjectionValues);
                 } else if (projectionValue == null) {
-                    result.put(field, null);
+                    Utils.changeSubdocumentValue(result, field, null);
                 } else {
                     Object value = Expression.evaluateDocument(projectionValue, document);
                     if (!(value instanceof Missing)) {
-                        result.put(field, value);
+                        Utils.changeSubdocumentValue(result, field, value);
                     }
                 }
             }
diff --git a/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java b/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java
index ab6f9e5a..8a00ae77 100644
--- a/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java
+++ b/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java
@@ -2714,6 +2714,84 @@ void testAggregateWithRedact() {
             .containsOnly(json("_id: 1"));
     }
 
+    @Test
+    void dotNotationProjection() {
+        collection.insertOne(json("""
+            _id: 1,
+            companyName: 'Walt Disney',
+            buildingNumber: 500,
+            streetName: 'South Buena Vista Street',
+            city: 'Burbank',
+            state: 'California',
+            zipCode: 91502
+            """));
+
+        assertThat(collection.aggregate(jsonList("""
+            $project: {
+                Name: "$companyName",
+                "Address.City": "$city",
+                "Address.ZipCode": "$zipCode",
+                "Address.StreetName": "$streetName",
+                "Address.BuildingNumber": "$buildingNumber",
+            }
+            """)))
+            .containsExactly(json("""
+                _id: 1,
+                Name: 'Walt Disney',
+                Address: {
+                   City: 'Burbank',
+                   ZipCode: 91502,
+                   StreetName: 'South Buena Vista Street',
+                   BuildingNumber: 500
+                }
+                """));
+    }
+
+    @Test
+    void dotNotationProjectionDeepNesting() {
+        collection.insertOne(json("""
+            _id: 1,
+            companyName: 'Acme Corp',
+            country: 'USA',
+            state: 'California',
+            city: 'San Francisco',
+            street: 'Market Street',
+            buildingNumber: 123,
+            floor: 5,
+            unit: 'A'
+            """));
+
+        assertThat(collection.aggregate(jsonList("""
+            $project: {
+                Company: "$companyName",
+                "Location.Country": "$country",
+                "Location.Address.State": "$state",
+                "Location.Address.City": "$city",
+                "Location.Address.Details.Street": "$street",
+                "Location.Address.Details.BuildingNumber": "$buildingNumber",
+                "Location.Address.Details.Floor": "$floor",
+                "Location.Address.Details.Unit": "$unit"
+            }
+            """)))
+            .containsExactly(json("""
+                _id: 1,
+                Company: 'Acme Corp',
+                Location: {
+                   Country: 'USA',
+                   Address: {
+                      State: 'California',
+                      City: 'San Francisco',
+                      Details: {
+                         Street: 'Market Street',
+                         BuildingNumber: 123,
+                         Floor: 5,
+                         Unit: 'A'
+                      }
+                   }
+                }
+                """));
+    }
+
     // https://github.com/bwaldvogel/mongo-java-server/issues/191
     @Test
     void testProjectWithCondition() throws Exception {