From 057156938e35288083d75d7219aaad48d46a689d Mon Sep 17 00:00:00 2001
From: Daniel Petranek <dpetranek@flur.ee>
Date: Tue, 28 Oct 2025 10:40:22 -0500
Subject: [PATCH] add support for empty result set with selectOne

---
 src/fluree/server/handler.clj                 | 12 +++++-
 .../server/integration/basic_query_test.clj   | 42 ++++++++++++-------
 2 files changed, 38 insertions(+), 16 deletions(-)

diff --git a/src/fluree/server/handler.clj b/src/fluree/server/handler.clj
index f389d98..a6fce26 100644
--- a/src/fluree/server/handler.clj
+++ b/src/fluree/server/handler.clj
@@ -106,7 +106,7 @@
 (def QueryResponse
   (m/schema [:orn
              [:select [:sequential [:or coll? map?]]]
-             [:select-one [:or coll? map?]]
+             [:select-one [:or coll? map? nil? [:enum "null"]]]
              [:construct map?]]))
 
 (def HistoryQuery
@@ -235,6 +235,15 @@
                :fluree/subscriptions subscriptions)
         handler)))
 
+(defn wrap-no-result
+  "Handle a nil response from a selectOne query with no results."
+  [handler]
+  (fn [req]
+    (let [{:keys [status body] :as result} (handler req)]
+      (if (and (= 200 status) (nil? body))
+        (assoc result :body "null")
+        result))))
+
 (defn wrap-cors
   [cors-origins handler]
   (let [origins (or cors-origins [#".*"])]
@@ -523,6 +532,7 @@
      coercion/coerce-exceptions-middleware
      coercion/coerce-response-middleware
      coercion/coerce-request-middleware
+     wrap-no-result
      wrap-request-header-opts
      (wrap-closed-mode root-identities closed-mode)
      exception-middleware]))
diff --git a/test/fluree/server/integration/basic_query_test.clj b/test/fluree/server/integration/basic_query_test.clj
index 93b9ed6..15a70ea 100644
--- a/test/fluree/server/integration/basic_query_test.clj
+++ b/test/fluree/server/integration/basic_query_test.clj
@@ -118,21 +118,33 @@
                                       "type"    "schema:Test"
                                       "ex:name" "query-test"}]})
                        :headers json-headers}
-          txn-res     (api-post :transact txn-req)
-          _           (assert (= 200 (:status txn-res)))
-          query-req   {:body
-                       (json/stringify
-                        {"@context"  test-system/default-context
-                         "from"      ledger-name
-                         "selectOne" '{?t ["*"]}
-                         "where"     '{"id" ?t, "type" "schema:Test"}})
-                       :headers json-headers}
-          query-res   (api-post :query query-req)]
-      (is (= 200 (:status query-res)))
-      (is (= {"id"      "ex:query-test"
-              "type"    "schema:Test"
-              "ex:name" "query-test"}
-             (-> query-res :body (json/parse false))))))
+          txn-res     (api-post :transact txn-req)]
+      (assert (= 200 (:status txn-res)))
+      (testing "with result"
+        (let [query-req {:body
+                         (json/stringify
+                          {"@context" test-system/default-context
+                           "from" ledger-name
+                           "selectOne" '{?t ["*"]}
+                           "where" '{"id" ?t, "type" "schema:Test"}})
+                         :headers json-headers}
+              query-res (api-post :query query-req)]
+          (is (= 200 (:status query-res)))
+          (is (= {"id"      "ex:query-test"
+                  "type"    "schema:Test"
+                  "ex:name" "query-test"}
+                 (-> query-res :body (json/parse false))))))
+      (testing "without result"
+        (let [query-req {:body
+                         (json/stringify
+                          {"@context" test-system/default-context
+                           "from" ledger-name
+                           "selectOne" '{?t ["*"]}
+                           "where" '{"id" ?t, "type" "schema:Foo"}})
+                         :headers json-headers}
+              query-res (api-post :query query-req)]
+          (is (= 200 (:status query-res)))
+          (is (nil? (-> query-res :body (json/parse false))))))))
 
   (testing "bind query works"
     (let [ledger-name (create-rand-ledger "query-endpoint-bind-test")