From f1760b0f8530a7aea9804669b1d5d77d34258325 Mon Sep 17 00:00:00 2001
From: jmarkerink <jmarkerink@outlook.com>
Date: Fri, 27 Feb 2026 19:55:30 +0100
Subject: [PATCH] fix: #231 lookup across collections

---
 .../aggregation/stage/LookupStage.java        |  2 +-
 .../aggregation/stage/LookupStageTest.java    | 16 ++++
 .../backend/AbstractAggregationTest.java      | 87 +++++++++++++++++++
 3 files changed, 104 insertions(+), 1 deletion(-)

diff --git a/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStage.java b/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStage.java
index c8d110dfe..ff8dd77a3 100644
--- a/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStage.java
+++ b/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStage.java
@@ -48,7 +48,7 @@ public Stream<Document> apply(Stream<Document> stream) {
     }
 
     private Document resolveRemoteField(Document document) {
-        Object value = Utils.getSubdocumentValue(document, localField);
+        Object value = Utils.getSubdocumentValueCollectionAware(document, localField);
         List<Document> documents = lookupValue(value);
         Document result = document.clone();
         result.put(as, documents);
diff --git a/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStageTest.java b/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStageTest.java
index f5abe98bf..d76cb26f8 100644
--- a/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStageTest.java
+++ b/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/stage/LookupStageTest.java
@@ -148,6 +148,22 @@ public void testLookupWithNestedField() throws Exception {
         );
     }
 
+    @Test
+    public void testLookupWithNestedFieldArray() throws Exception {
+        prepareAuthorsCollectionMock();
+
+        LookupStage lookupStage = buildLookupStage("from: 'authors', 'localField': 'author.id', foreignField: '_id', as: 'author'");
+        configureAuthorsCollection("_id: 3", "_id: 3, name: 'Uncle Bob'");
+        configureAuthorsCollection("_id: 4", "_id: 4, name: 'Aunt Bobby'");
+        Document document = json("title: 'Clean Code', author: [{ id: 3 }, {id: 4}]");
+
+        Stream<Document> result = lookupStage.apply(Stream.of(document));
+
+        assertThat(result).containsExactly(
+            json("title: 'Clean Code', author: { id: 3 }, author: [{_id: 3, name: 'Uncle Bob'}, {_id: 4, name: 'Aunt Bobby'}]")
+        );
+    }
+
     private LookupStage buildLookupStage(String jsonDocument) {
         return new LookupStage(json(jsonDocument), database);
     }
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 208cdd95a..f5cbc65b1 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
@@ -1484,6 +1484,93 @@ void testAggregateWithLookupAndUncorrelatedSubqueries() {
             );
     }
 
+    @Test
+    void testLookupPersonWithSubscription() {
+        MongoCollection<Document> personCollection = db.getCollection("Person");
+        MongoCollection<Document> subscriptionCollection = db.getCollection("Subscription");
+
+        personCollection.insertOne(json("""
+              "_id": 666,
+              "personId": 666,
+              "Relation": [
+                {
+                  "organizationId": "org1",
+                  "active": true
+                },
+                {
+                  "organizationId": "org3",
+                  "active": false
+                }
+              ]
+            """));
+
+        subscriptionCollection.insertOne(json("""
+              "_id": 123,
+              "organizationId": "org1",
+              "Subscription": [
+                {
+                  "Id": 1,
+                  "Name": "Netflix"
+                },
+                {
+                  "Id": 12,
+                  "Name": "Disney+"
+                }
+              ]
+            """));
+        subscriptionCollection.insertOne(json("""
+              "_id": 456,
+              "organizationId": "org2",
+              "Subscription": [
+                {
+                  "Id": 2,
+                  "Name": "HBO Max"
+                },
+                {
+                  "Id": 11,
+                  "Name": "SkyShowtime"
+                }
+              ]
+            """));
+
+        List<Document> pipeline = jsonList(
+            "$match: { 'personId': 666 }",
+            "$lookup: {from: 'Subscription', localField: 'Relation.organizationId', foreignField: 'organizationId', as: 'Subscriptions'}"
+        );
+
+        assertThat(personCollection.aggregate(pipeline))
+            .containsExactly(json("""
+                  "_id": 666,
+                  "personId": 666,
+                  "Relation": [
+                    {
+                      "organizationId": "org1",
+                      "active": true
+                    },
+                    {
+                      "organizationId": "org3",
+                      "active": false
+                    }
+                  ],
+                  "Subscriptions": [
+                    {
+                      "_id": 123,
+                      "organizationId": "org1",
+                      "Subscription": [
+                        {
+                          "Id": 1,
+                          "Name": "Netflix"
+                        },
+                        {
+                          "Id": 12,
+                          "Name": "Disney+"
+                        }
+                      ]
+                    }
+                  ]
+                """));
+    }
+
     @Test
     void testAggregateWithReplaceRoot() {
         List<Document> pipeline = jsonList("$replaceRoot: { newRoot: '$a.b' }");