add a truncate method to the connection

it doesn't work on SQLite3 since it doesn't support truncate, but that's
OK.  If you call truncate on the connection, you're now bound to that
database (same as if you use hstore or any other db specific feature).

activerecord/CHANGELOG.md

@@ -1,3 +1,5 @@
+*   Add a truncate method to the connection.
+
 *   Don't autosave unchanged has_one through records.
 
     *Alan Kennedy*, *Steve Parrington*

activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb

@@ -83,6 +83,11 @@ module ActiveRecord
         exec_query(sql, name, binds)
       end
 
+      # Executes the truncate statement.
+      def truncate(table_name, name = nil)
+        raise NotImplementedError
+      end
+
       # Executes update +sql+ statement in the context of this connection using
       # +binds+ as the bind substitutes. +name+ is logged along with
       # the executed +sql+ statement.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -389,6 +389,10 @@ module ActiveRecord
         end
       end
 
+      def truncate(table_name, name = nil)
+        execute "TRUNCATE TABLE #{quote_table_name(table_name)}", name
+      end
+
       def table_exists?(name)
         return false unless name.present?
         return true if tables(nil, nil, name).any?

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -255,6 +255,10 @@ module ActiveRecord
         @statements.clear
       end
 
+      def truncate(table_name, name = nil)
+        exec_query "TRUNCATE TABLE #{quote_table_name(table_name)}", name, []
+      end
+
       # Is this connection alive and ready for queries?
       def active?
         @connection.query 'SELECT 1'

activerecord/test/cases/adapters/mysql2/connection_test.rb

@@ -4,6 +4,8 @@ require 'support/connection_helper'
 class MysqlConnectionTest < ActiveRecord::TestCase
   include ConnectionHelper
 
+  fixtures :comments
+
   def setup
     super
     @subscriber = SQLSubscriber.new
@@ -24,6 +26,17 @@ class MysqlConnectionTest < ActiveRecord::TestCase
     end
   end
 
+  def test_truncate
+    rows = ActiveRecord::Base.connection.exec_query("select count(*) from comments")
+    count = rows.first.values.first
+    assert_operator count, :>, 0
+
+    ActiveRecord::Base.connection.truncate("comments")
+    rows = ActiveRecord::Base.connection.exec_query("select count(*) from comments")
+    count = rows.first.values.first
+    assert_equal 0, count
+  end
+
   def test_no_automatic_reconnection_after_timeout
     assert @connection.active?
     @connection.update('set @@wait_timeout=1')

activerecord/test/cases/adapters/postgresql/connection_test.rb

@@ -8,6 +8,8 @@ module ActiveRecord
     class NonExistentTable < ActiveRecord::Base
     end
 
+    fixtures :comments
+
     def setup
       super
       @subscriber = SQLSubscriber.new
@@ -20,6 +22,14 @@ module ActiveRecord
       super
     end
 
+    def test_truncate
+      count = ActiveRecord::Base.connection.execute("select count(*) from comments").first['count'].to_i
+      assert_operator count, :>, 0
+      ActiveRecord::Base.connection.truncate("comments")
+      count = ActiveRecord::Base.connection.execute("select count(*) from comments").first['count'].to_i
+      assert_equal 0, count
+    end
+
     def test_encoding
       assert_not_nil @connection.encoding
     end