From dc7d4560a0b13cd3a097522fb08fc682ba75e31b Mon Sep 17 00:00:00 2001
From: Vladimir Maksimovski <vlade.maksimovski@gmail.com>
Date: Sun, 30 Mar 2025 22:51:58 -0400
Subject: [PATCH] Null values are not discarded

---
 core/engine/tests/decision.rs       | 13 ++++++++++
 core/expression/src/variable/mod.rs | 23 +++++-------------
 test-data/test-null.json            | 37 +++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 17 deletions(-)
 create mode 100644 test-data/test-null.json

diff --git a/core/engine/tests/decision.rs b/core/engine/tests/decision.rs
index cc5609b0..3e8b9e83 100644
--- a/core/engine/tests/decision.rs
+++ b/core/engine/tests/decision.rs
@@ -88,3 +88,16 @@ fn decision_validation() {
         DecisionGraphValidationError::InvalidInputCount(_)
     ));
 }
+
+#[tokio::test]
+#[cfg_attr(miri, ignore)]
+async fn nulls_are_preserved_when_merging_incoming_nodes() {
+    let table_content = load_test_data("test-null.json");
+    let decision = Decision::from(table_content);
+
+    let context = json!({ "b": null, "c": { "d": null } });
+    let result = decision.evaluate(context.into()).await;
+
+    assert_eq!(result.unwrap().result, json!({ "b": null, "c": { "d": null } }).into());
+}
+
diff --git a/core/expression/src/variable/mod.rs b/core/expression/src/variable/mod.rs
index 2c60b024..6c1df379 100644
--- a/core/expression/src/variable/mod.rs
+++ b/core/expression/src/variable/mod.rs
@@ -268,12 +268,8 @@ fn merge_variables(
             MergeStrategy::InPlace => {
                 let mut map = doc_ref.borrow_mut();
                 for (key, value) in patch.deref() {
-                    if value == &Variable::Null {
-                        map.remove(key.as_str());
-                    } else {
-                        let entry = map.entry(key.to_string()).or_insert(Variable::Null);
-                        merge_variables(entry, value, false, strategy);
-                    }
+                    let entry = map.entry(key.to_string()).or_insert(Variable::Null);
+                    merge_variables(entry, value, false, strategy);
                 }
 
                 return true;
@@ -292,17 +288,10 @@ fn merge_variables(
                         new_map.as_mut().unwrap()
                     };
 
-                    if value == &Variable::Null {
-                        // Remove null values
-                        if map.remove(key.as_str()).is_some() {
-                            changed = true;
-                        }
-                    } else {
-                        // Handle nested merging
-                        let entry = map.entry(key.to_string()).or_insert(Variable::Null);
-                        if merge_variables(entry, value, false, strategy) {
-                            changed = true;
-                        }
+                    // Handle nested merging
+                    let entry = map.entry(key.to_string()).or_insert(Variable::Null);
+                    if merge_variables(entry, value, false, strategy) {
+                        changed = true;
                     }
                 }
 
diff --git a/test-data/test-null.json b/test-data/test-null.json
new file mode 100644
index 00000000..6ff402f5
--- /dev/null
+++ b/test-data/test-null.json
@@ -0,0 +1,37 @@
+{
+  "contentType": "application/vnd.gorules.decision",
+  "nodes": [
+    {
+      "id": "8f1971cc-1658-44b4-a64c-b6c734697e3d",
+      "name": "request",
+      "type": "inputNode",
+      "content": {
+        "schema": ""
+      },
+      "position": {
+        "x": 110,
+        "y": 114
+      }
+    },
+    {
+      "id": "aff016a9-1d4f-4d5d-98a5-adf4f367f371",
+      "name": "response",
+      "type": "outputNode",
+      "content": {
+        "schema": ""
+      },
+      "position": {
+        "x": 750,
+        "y": 114
+      }
+    }
+  ],
+  "edges": [
+    {
+      "id": "8dd114f0-06b3-4f7e-9213-4faaa5a855c8",
+      "type": "edge",
+      "sourceId": "8f1971cc-1658-44b4-a64c-b6c734697e3d",
+      "targetId": "aff016a9-1d4f-4d5d-98a5-adf4f367f371"
+    }
+  ]
+}
\ No newline at end of file