refactoring join_sql to return nil if there are no sources

lib/arel/select_manager.rb

@@ -128,6 +128,8 @@ module Arel
     end
 
     def join_sql
+      return nil unless @ctx.froms
+
       viz = Visitors::JoinSql.new @engine
       Nodes::SqlLiteral.new viz.accept @ctx
     end

lib/arel/table.rb

@@ -49,40 +49,39 @@ module Arel
     end
 
     def join relation, klass = Nodes::InnerJoin
-      return select_manager unless relation
+      return from(self) unless relation
 
-      sm = SelectManager.new(@engine)
       case relation
       when String, Nodes::SqlLiteral
         raise if relation.blank?
-        sm.from Nodes::StringJoin.new(self, relation)
+        from Nodes::StringJoin.new(self, relation)
       else
-        sm.from klass.new(self, relation, nil)
+        from klass.new(self, relation, nil)
       end
     end
 
     def group *columns
-      select_manager.group(*columns)
+      from(self).group(*columns)
     end
 
     def order *expr
-      select_manager.order(*expr)
+      from(self).order(*expr)
     end
 
     def where condition
-      select_manager.where condition
+      from(self).where condition
     end
 
     def project *things
-      select_manager.project(*things)
+      from(self).project(*things)
     end
 
     def take amount
-      select_manager.take amount
+      from(self).take amount
     end
 
     def having expr
-      select_manager.having expr
+      from(self).having expr
     end
 
     def columns
@@ -98,7 +97,7 @@ module Arel
     end
 
     def select_manager
-      SelectManager.new(@engine, self)
+      SelectManager.new(@engine)
     end
 
     private

spec/select_manager_spec.rb

@@ -299,6 +299,11 @@ module Arel
         manager.join_sql.should be_like %{ 'hello' }
         check manager.joins(manager).should == manager.join_sql
       end
+
+      it 'returns nil join sql' do
+        manager = Arel::SelectManager.new Table.engine
+        manager.join_sql.should be_nil
+      end
     end
 
     describe 'order_clauses' do

spec/table_spec.rb

@@ -12,6 +12,13 @@ module Arel
       end
     end
 
+    describe 'select_manager' do
+      it 'should return an empty select manager' do
+        sm = @relation.select_manager
+        sm.to_sql.should be_like 'SELECT'
+      end
+    end
+
     describe 'having' do
       it 'adds a having clause' do
         mgr = @relation.having @relation[:id].eq(10)