adding Distinct ON node

5 files changed, 15 insertions, 33 deletions

diff --git a/lib/arel/nodes.rb b/lib/arel/nodes.rb
index cf2aeb2ddc..9576930a54 100644
--- a/lib/arel/nodes.rb
+++ b/lib/arel/nodes.rb
@@ -5,6 +5,10 @@ require 'arel/nodes/select_core'
 require 'arel/nodes/insert_statement'
 require 'arel/nodes/update_statement'
 
+# terminal
+
+require 'arel/nodes/terminal'
+
 # unary
 require 'arel/nodes/unary'
 require 'arel/nodes/unqualified_column'
diff --git a/lib/arel/nodes/terminal.rb b/lib/arel/nodes/terminal.rb
new file mode 100644
index 0000000000..c6b4f4e1e2
--- /dev/null
+++ b/lib/arel/nodes/terminal.rb
@@ -0,0 +1,6 @@
+module Arel
+  module Nodes
+    class Distinct < Arel::Nodes::Node
+    end
+  end
+end
diff --git a/lib/arel/nodes/unary.rb b/lib/arel/nodes/unary.rb
index e6e40e6b13..5c4add4792 100644
--- a/lib/arel/nodes/unary.rb
+++ b/lib/arel/nodes/unary.rb
@@ -20,14 +20,9 @@ module Arel
       On
       Top
       Lock
+      DistinctOn
     }.each do |name|
       const_set(name, Class.new(Unary))
     end
-
-    class Distinct < Unary
-      def initialize expr = nil
-        super
-      end
-    end
   end
 end
diff --git a/lib/arel/visitors/postgresql.rb b/lib/arel/visitors/postgresql.rb
index c423dc6fc6..377a65a216 100644
--- a/lib/arel/visitors/postgresql.rb
+++ b/lib/arel/visitors/postgresql.rb
@@ -6,25 +6,6 @@ module Arel
         visit o.expr
       end
 
-      def visit_Arel_Nodes_SelectStatement o
-        if !o.orders.empty? && using_distinct_on?(o)
-          subquery        = o.dup
-          subquery.orders = []
-          subquery.limit  = nil
-          subquery.offset = nil
-
-          sql = super(subquery)
-          [
-            "SELECT * FROM (#{sql}) AS id_list",
-            "ORDER BY #{aliased_orders(o.orders).join(', ')}",
-            (visit(o.limit) if o.limit),
-            (visit(o.offset) if o.offset),
-          ].compact.join ' '
-        else
-          super
-        end
-      end
-
       def visit_Arel_Nodes_Matches o
         "#{visit o.left} ILIKE #{visit o.right}"
       end
@@ -33,12 +14,8 @@ module Arel
         "#{visit o.left} NOT ILIKE #{visit o.right}"
       end
 
-      def using_distinct_on?(o)
-        o.cores.any? do |core|
-          core.projections.any? do |projection|
-            /DISTINCT ON/ === projection
-          end
-        end
+      def visit_Arel_Nodes_DistinctOn o
+        "DISTINCT ON ( #{visit o.expr} )"
       end
 
       def aliased_orders orders
diff --git a/lib/arel/visitors/to_sql.rb b/lib/arel/visitors/to_sql.rb
index 6aba31d94d..5319aeb418 100644
--- a/lib/arel/visitors/to_sql.rb
+++ b/lib/arel/visitors/to_sql.rb
@@ -126,8 +126,8 @@ key on UpdateManager using UpdateManager#key=
           "SELECT",
           (visit(o.top) if o.top),
           (visit(o.set_quantifier) if o.set_quantifier),
-          "#{o.projections.map { |x| visit x }.join ', '}",
-          visit(o.source),
+          ("#{o.projections.map { |x| visit x }.join ', '}" unless o.projections.empty?),
+          (visit(o.source) if o.source),
           ("WHERE #{o.wheres.map { |x| visit x }.join ' AND ' }" unless o.wheres.empty?),
           ("GROUP BY #{o.groups.map { |x| visit x }.join ', ' }" unless o.groups.empty?),
           (visit(o.having) if o.having),