Fixes #319 by making register-clause! idempotent

Technically, it removes any instance of the clause from the ordering
before it attempts to add it back in, allowing you to correct the
order if you got it wrong.

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changes
 
 * 2.0.next in progress
+  * Fix #319 by ensuring `register-clause!` is idempotent.
   * Fix #317 by dropping qualifiers in `:set` clauses (just like we do with `:insert` columns). Note that you can still use explicit _dotted_ names if you want table qualification.
   * Fix #312 by adding `:raw` as a clause. There is no helper function equivalent (because it would be ambiguous whether you meant a function form -- `[:raw ..]` -- or a clause form -- `{:raw ..}`; and for the same reason, there is no `nest` helper function since that also works as a clause and as a function/special syntax).
 

src/honey/sql.cljc

@@ -60,29 +60,32 @@
    :with-data])
 
 (defn- add-clause-before
-  "Low-level helper just to insert a new clause."
+  "Low-level helper just to insert a new clause.
+
+  If the clause is already in the list, remove it."
   [order clause before]
-  (if before
-    (do
-      (when-not (contains? (set order) before)
-        (throw (ex-info (str "Unrecognized clause: " before)
-                        {:known-clauses order})))
-      (reduce (fn [v k]
-                (if (= k before)
-                  (conj v clause k)
-                  (conj v k)))
-              []
-              order))
-    (conj order clause)))
+  (let [clauses (set order)
+        order   (if (contains? clauses clause)
+                  (filterv #(not= % clause) order)
+                  order)]
+    (if before
+      (do
+        (when-not (contains? clauses before)
+          (throw (ex-info (str "Unrecognized clause: " before)
+                          {:known-clauses order})))
+        (reduce (fn [v k]
+                  (if (= k before)
+                    (conj v clause k)
+                    (conj v k)))
+                []
+                order))
+      (conj order clause))))
 
 (def ^:private dialects
   {:ansi      {:quote #(str \" % \")}
    :sqlserver {:quote #(str \[ % \])}
    :mysql     {:quote #(str \` % \`)
-               :clause-order-fn (fn [order]
-                                  ;; MySQL :set has different priority
-                                  (-> (filterv (complement #{:set}) order)
-                                      (add-clause-before :set :where)))}
+               :clause-order-fn #(add-clause-before % :set :where)}
    :oracle    {:quote #(str \" % \") :as false}})
 
 ; should become defonce

test/honey/sql_test.cljc

@@ -623,3 +623,11 @@ ORDER BY id = ? DESC
            (format {:insert-into :table
                     :values [{:name name
                               :enabled enabled}]})))))
+
+(deftest issue-319-test
+  (testing "that registering a clause is idempotent"
+    (is (= ["FOO"]
+           (do
+             (sut/register-clause! :foo (constantly ["FOO"]) nil)
+             (sut/register-clause! :foo (constantly ["FOO"]) nil)
+             (format {:foo []}))))))