string join should be a unary op

lib/arel/nodes/string_join.rb

@@ -1,6 +1,9 @@
 module Arel
   module Nodes
     class StringJoin < Arel::Nodes::Join
+      def initialize left, right = nil
+        super
+      end
     end
   end
 end

lib/arel/visitors/depth_first.rb

@@ -67,10 +67,13 @@ module Arel
       alias :visit_Arel_Nodes_Or                 :binary
       alias :visit_Arel_Nodes_Ordering           :binary
       alias :visit_Arel_Nodes_OuterJoin          :binary
-      alias :visit_Arel_Nodes_StringJoin         :binary
       alias :visit_Arel_Nodes_TableAlias         :binary
       alias :visit_Arel_Nodes_Values             :binary
 
+      def visit_Arel_Nodes_StringJoin o
+        visit o.left
+      end
+
       def visit_Arel_Attribute o
         visit o.relation
         visit o.name

lib/arel/visitors/join_sql.rb

@@ -14,13 +14,6 @@ module Arel
       def visit_Arel_Nodes_SelectCore o
         o.source.right.map { |j| visit j }.join ' '
       end
-
-      def visit_Arel_Nodes_StringJoin o
-        [
-          (visit o.left if Nodes::Join === o.left),
-          visit(o.right)
-        ].compact.join ' '
-      end
     end
   end
 end

test/test_select_manager.rb

@@ -367,7 +367,7 @@ module Arel
       it 'returns string join sql' do
         table   = Table.new :users
         manager = Arel::SelectManager.new Table.engine
-        manager.from Nodes::StringJoin.new(table, 'hello')
+        manager.from Nodes::StringJoin.new('hello')
         manager.join_sql.must_be_like %{ 'hello' }
       end
 

test/visitors/test_depth_first.rb

@@ -29,6 +29,7 @@ module Arel
         Arel::Nodes::Grouping,
         Arel::Nodes::Offset,
         Arel::Nodes::Having,
+        Arel::Nodes::StringJoin,
         Arel::Nodes::UnqualifiedColumn,
       ].each do |klass|
         define_method("test_#{klass.name.gsub('::', '_')}") do
@@ -91,7 +92,6 @@ module Arel
         Arel::Nodes::NotEqual,
         Arel::Nodes::NotIn,
         Arel::Nodes::Or,
-        Arel::Nodes::StringJoin,
         Arel::Nodes::TableAlias,
         Arel::Nodes::Values,
         Arel::Nodes::As,
@@ -176,7 +176,8 @@ module Arel
         @visitor.accept core
         assert_equal [
           :a, core.projections,
-          :b,
+          :b, [],
+          core.source,
           :c, core.wheres,
           :d, core.groups,
           :e,

test/visitors/test_join_sql.rb

@@ -8,6 +8,11 @@ module Arel
         @visitor.extend(JoinSql)
       end
 
+      it 'should visit string join' do
+        sql = @visitor.accept Nodes::StringJoin.new('omg')
+        sql.must_be_like "'omg'"
+      end
+
       describe 'inner join' do
         it 'should visit left if left is a join' do
           t    = Table.new :users