Add support for DISTINCT ON

Resolves #31

This uses the new multimethod added to honeysql in
seancorfield/honeysql@509df19
to add DISTINCT ON support.

Tests are included which cover the behavior.

This commit updates deps.edn to point to the github repo for honeysql
which will obviously have to be changed once honeysql is released.

Author: b-ryan

deps.edn

@@ -1 +1 @@
-{:deps {honeysql {:mvn/version "0.9.4" :exclusions [org.clojure/clojurescript]}}}
+{:deps {honeysql {:git/url "https://github.com/jkk/honeysql" :sha "be203e9086808c47ab7e42520950db0bfd9efcc1" :exclusions [org.clojure/clojurescript]}}}

src/honeysql_postgres/format.cljc

@@ -1,6 +1,6 @@
 (ns ^{:doc "Extension of the honeysql format functions specifically for postgreSQL"}
     honeysql-postgres.format
-  (:require [honeysql.format :as sqlf :refer [fn-handler format-clause]] ;; multi-methods
+  (:require [honeysql.format :as sqlf :refer [fn-handler format-clause format-modifiers]] ;; multi-methods
             [honeysql-postgres.util :as util]))
 
 (def ^:private custom-additions
@@ -209,3 +209,6 @@
   (str  "INSERT INTO " (sqlf/to-sql table-name) " AS " (sqlf/to-sql table-alias)))
 
 (override-default-clause-priority)
+
+(defmethod format-modifiers :distinct-on [[_ & fields]]
+  (str "DISTINCT ON(" (sqlf/comma-join (map sqlf/to-sql fields)) ")"))

test/honeysql_postgres/postgres_test.cljc

@@ -8,7 +8,8 @@
                                                          create-table rename-table drop-table
                                                          window create-view over with-columns]]
             [honeysql.helpers :as sqlh :refer [insert-into values where select columns
-                                               from order-by update sset query-values]]
+                                               from order-by update sset query-values
+                                               modifiers]]
             [honeysql.core :as sql]
             [clojure.test :as test :refer [deftest is testing]]))
 
@@ -219,3 +220,11 @@
               (from :products)
               (where [:not-ilike :name "%name%"])
               sql/format)))))
+
+(deftest select-distinct-on
+  (testing "select distinct on"
+    (is (= ["SELECT DISTINCT ON(\"a\", \"b\") \"c\" FROM \"products\" "]
+           (-> (select :c)
+              (from :products)
+              (modifiers :distinct-on :a :b)
+              (sql/format :quoting :ansi))))))