Make it the responsibility of the connection to hold on to a visitor for generating SQL, rather than the TreeManager. (There is a related commit coming in Active Record.)

lib/arel/nodes/node.rb

@@ -32,8 +32,7 @@ module Arel
       #
       # Maybe we should just use `Table.engine`?  :'(
       def to_sql engine = Table.engine
-        viz = Visitors.for engine
-        viz.accept self
+        engine.connection.visitor.accept self
       end
 
       # Iterate through AST, nodes will be yielded depth-first

lib/arel/select_manager.rb

@@ -149,13 +149,13 @@ module Arel
 
     def wheres
       warn "#{caller[0]}: SelectManager#wheres is deprecated and will be removed in ARel 3.0.0 with no replacement"
-      Compatibility::Wheres.new @engine, @ctx.wheres
+      Compatibility::Wheres.new @engine.connection_pool, @ctx.wheres
     end
 
     def where_sql
       return if @ctx.wheres.empty?
 
-      viz = Visitors::WhereSql.new @engine
+      viz = Visitors::WhereSql.new @engine.connection_pool
       Nodes::SqlLiteral.new viz.accept @ctx
     end
 
@@ -205,12 +205,13 @@ module Arel
     def join_sql
       return nil if @ctx.source.right.empty?
 
-      sql = @visitor.dup.extend(Visitors::JoinSql).accept @ctx
+      sql = visitor.dup.extend(Visitors::JoinSql).accept @ctx
       Nodes::SqlLiteral.new sql
     end
 
     def order_clauses
-      Visitors::OrderClauses.new(@engine).accept(@ast).map { |x|
+      visitor = Visitors::OrderClauses.new(@engine.connection_pool)
+      visitor.accept(@ast).map { |x|
         Nodes::SqlLiteral.new x
       }
     end

lib/arel/tree_manager.rb

@@ -4,21 +4,23 @@ module Arel
     include Arel::Relation
     include Arel::FactoryMethods
 
-    attr_accessor :visitor
     attr_reader :ast, :engine
 
     def initialize engine
-      @engine  = engine
-      @visitor = Visitors.visitor_for @engine
-      @ctx     = nil
+      @engine = engine
+      @ctx    = nil
     end
 
     def to_dot
       Visitors::Dot.new.accept @ast
     end
 
+    def visitor
+      engine.connection.visitor
+    end
+
     def to_sql
-      @visitor.accept @ast
+      visitor.accept @ast
     end
 
     def initialize_copy other

lib/arel/visitors/to_sql.rb

@@ -4,17 +4,15 @@ require 'date'
 module Arel
   module Visitors
     class ToSql < Arel::Visitors::Visitor
-      def initialize engine
-        @engine         = engine
+      def initialize pool
+        @pool           = pool
         @connection     = nil
-        @pool           = nil
         @quoted_tables  = {}
         @quoted_columns = {}
       end
 
       def accept object
         self.last_column = nil
-        @pool = @engine.connection_pool
         @pool.with_connection do |conn|
           @connection = conn
           super

test/nodes/test_bin.rb

@@ -8,13 +8,13 @@ module Arel
       end
 
       def test_default_to_sql
-        viz  = Arel::Visitors::ToSql.new Table.engine
+        viz  = Arel::Visitors::ToSql.new Table.engine.connection_pool
         node = Arel::Nodes::Bin.new(Arel.sql('zomg'))
         assert_equal 'zomg', viz.accept(node)
       end
 
       def test_mysql_to_sql
-        viz  = Arel::Visitors::MySQL.new Table.engine
+        viz  = Arel::Visitors::MySQL.new Table.engine.connection_pool
         node = Arel::Nodes::Bin.new(Arel.sql('zomg'))
         assert_equal 'BINARY zomg', viz.accept(node)
       end

test/nodes/test_select_core.rb

@@ -23,7 +23,7 @@ module Arel
       def test_set_quantifier
         core = Arel::Nodes::SelectCore.new
         core.set_quantifier = Arel::Nodes::Distinct.new
-        viz = Arel::Visitors::ToSql.new Table.engine
+        viz = Arel::Visitors::ToSql.new Table.engine.connection_pool
         assert_match 'DISTINCT', viz.accept(core)
       end
     end

test/nodes/test_sql_literal.rb

@@ -3,6 +3,10 @@ require 'helper'
 module Arel
   module Nodes
     describe 'sql literal' do
+      before do
+        @visitor = Visitors::ToSql.new Table.engine.connection_pool
+      end
+
       describe 'sql' do
         it 'makes a sql literal node' do
           sql = Arel.sql 'foo'
@@ -13,38 +17,33 @@ module Arel
       describe 'count' do
         it 'makes a count node' do
           node = SqlLiteral.new('*').count
-          viz = Visitors::ToSql.new Table.engine
-          viz.accept(node).must_be_like %{ COUNT(*) }
+          @visitor.accept(node).must_be_like %{ COUNT(*) }
         end
 
         it 'makes a distinct node' do
           node = SqlLiteral.new('*').count true
-          viz = Visitors::ToSql.new Table.engine
-          viz.accept(node).must_be_like %{ COUNT(DISTINCT *) }
+          @visitor.accept(node).must_be_like %{ COUNT(DISTINCT *) }
         end
       end
 
       describe 'equality' do
         it 'makes an equality node' do
           node = SqlLiteral.new('foo').eq(1)
-          viz = Visitors::ToSql.new Table.engine
-          viz.accept(node).must_be_like %{ foo = 1 }
+          @visitor.accept(node).must_be_like %{ foo = 1 }
         end
       end
 
       describe 'grouped "or" equality' do
         it 'makes a grouping node with an or node' do
           node = SqlLiteral.new('foo').eq_any([1,2])
-          viz = Visitors::ToSql.new Table.engine
-          viz.accept(node).must_be_like %{ (foo = 1 OR foo = 2) }
+          @visitor.accept(node).must_be_like %{ (foo = 1 OR foo = 2) }
         end
       end
 
       describe 'grouped "and" equality' do
         it 'makes a grouping node with an or node' do
           node = SqlLiteral.new('foo').eq_all([1,2])
-          viz = Visitors::ToSql.new Table.engine
-          viz.accept(node).must_be_like %{ (foo = 1 AND foo = 2) }
+          @visitor.accept(node).must_be_like %{ (foo = 1 AND foo = 2) }
         end
       end
     end

test/support/fake_record.rb

@@ -3,9 +3,9 @@ module FakeRecord
   end
 
   class Connection
-    attr_reader :tables, :columns_hash
+    attr_reader :tables, :columns_hash, :visitor
 
-    def initialize
+    def initialize(visitor)
       @tables = %w{ users photos developers products}
       @columns = {
         'users' => [
@@ -27,6 +27,7 @@ module FakeRecord
         'users' => 'id',
         'products' => 'id'
       }
+      @visitor = visitor
     end
 
     def primary_key name
@@ -78,7 +79,7 @@ module FakeRecord
 
     def initialize
       @spec = Spec.new(:adapter => 'america')
-      @connection = Connection.new
+      @connection = Connection.new(Arel::Visitors::ToSql.new(self))
     end
 
     def with_connection

test/test_select_manager.rb

@@ -42,6 +42,10 @@ module Arel
       @engine.connection.tables
     end
 
+    def visitor
+      @engine.connection.visitor
+    end
+
     def execute sql, name = nil, *args
       @executed << sql
     end

test/visitors/test_ibm_db.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the ibm_db visitor' do
       before do
-        @visitor = IBM_DB.new Table.engine
+        @visitor = IBM_DB.new Table.engine.connection_pool
       end
 
       it 'uses FETCH FIRST n ROWS to limit results' do

test/visitors/test_informix.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the informix visitor' do
       before do
-        @visitor = Informix.new Table.engine
+        @visitor = Informix.new Table.engine.connection_pool
       end
 
       it 'uses LIMIT n to limit results' do

test/visitors/test_join_sql.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the join_sql visitor' do
       before do
-        @visitor = ToSql.new Table.engine
+        @visitor = ToSql.new Table.engine.connection_pool
         @visitor.extend(JoinSql)
       end
 

test/visitors/test_mssql.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the mssql visitor' do
       before do
-        @visitor = MSSQL.new Table.engine
+        @visitor = MSSQL.new Table.engine.connection_pool
         @table = Arel::Table.new "users"
       end
 

test/visitors/test_mysql.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the mysql visitor' do
       before do
-        @visitor = MySQL.new Table.engine
+        @visitor = MySQL.new Table.engine.connection_pool
       end
 
       it 'squashes parenthesis on multiple unions' do

test/visitors/test_oracle.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the oracle visitor' do
       before do
-        @visitor = Oracle.new Table.engine
+        @visitor = Oracle.new Table.engine.connection_pool
       end
 
       it 'modifies order when there is distinct and first value' do

test/visitors/test_postgres.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the postgres visitor' do
       before do
-        @visitor = PostgreSQL.new Table.engine
+        @visitor = PostgreSQL.new Table.engine.connection_pool
       end
 
       describe 'locking' do

test/visitors/test_sqlite.rb

@@ -4,7 +4,7 @@ module Arel
   module Visitors
     describe 'the sqlite visitor' do
       before do
-        @visitor = SQLite.new Table.engine
+        @visitor = SQLite.new Table.engine.connection_pool
       end
 
       it 'defaults limit to -1' do

test/visitors/test_to_sql.rb

@@ -8,7 +8,7 @@ module Arel
   module Visitors
     describe 'the to_sql visitor' do
       before do
-        @visitor = ToSql.new Table.engine
+        @visitor = ToSql.new Table.engine.connection_pool
         @table = Table.new(:users)
         @attr = @table[:id]
       end
@@ -220,7 +220,7 @@ module Arel
             end
           end
           in_node = Nodes::In.new @attr, %w{ a b c }
-          visitor = visitor.new(Table.engine)
+          visitor = visitor.new(Table.engine.connection_pool)
           visitor.expected = Table.engine.connection.columns(:users).find { |x|
             x.name == 'name'
           }
@@ -308,7 +308,7 @@ module Arel
             end
           end
           in_node = Nodes::NotIn.new @attr, %w{ a b c }
-          visitor = visitor.new(Table.engine)
+          visitor = visitor.new(Table.engine.connection_pool)
           visitor.expected = Table.engine.connection.columns(:users).find { |x|
             x.name == 'name'
           }