From a5e00ddcf1008383601c09359691a9fdcf38899f Mon Sep 17 00:00:00 2001
From: Daniel Petranek <dpetranek@flur.ee>
Date: Mon, 20 Jan 2025 14:08:05 -0600
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 1d9a3db2..451f2154 100644
--- a/src/fluree/server/handler.clj
+++ b/src/fluree/server/handler.clj
@@ -93,7 +93,7 @@
 (def QueryResponse
   (m/schema [:orn
              [:select [:sequential [:or coll? map?]]]
-             [:select-one [:or coll? map?]]]))
+             [:select-one [:or coll? map? nil? [:enum "null"]]]]))
 
 (def HistoryQuery
   (m/schema (fqh/history-query-schema [[:from LedgerAlias]])
@@ -248,6 +248,15 @@
                   (handler req*))))
         (handler req)))))
 
+(def wrap-no-result
+  "Handle a nil response from a selectOne query with no results."
+  (fn [handler]
+    (fn [req]
+      (let [{:keys [status body] :as result} (handler req)]
+        (if (and (= 200 status) (nil? body))
+          (assoc result :body "null")
+          result)))))
+
 (defn wrap-set-fuel-header
   [handler]
   (fn [req]
@@ -361,6 +370,7 @@
                                 [200 coercion/coerce-exceptions-middleware]
                                 [300 coercion/coerce-response-middleware]
                                 [400 coercion/coerce-request-middleware]
+                                [450 wrap-no-result]
                                 [500 wrap-policy-metadata]
                                 [600 (wrap-closed-mode root-identities closed-mode)]
                                 [1000 exception-middleware]])))
diff --git a/test/fluree/server/integration/basic_query_test.clj b/test/fluree/server/integration/basic_query_test.clj
index 5019a777..a29dfdfc 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/write-value-as-string
-                        {"@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/read-value)))))
+          txn-res     (api-post :transact txn-req)]
+      (assert (= 200 (:status txn-res)))
+      (testing "with result"
+        (let [query-req {:body
+                         (json/write-value-as-string
+                          {"@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/read-value)))))
+      (testing "without result"
+        (let [query-req {:body
+                         (json/write-value-as-string
+                          {"@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/read-value)))))))
 
   (testing "bind query works"
     (let [ledger-name (create-rand-ledger "query-endpoint-bind-test")