Add support for Mysql column positioning via #add_column and #change_column

add_column and change_column in the Mysql adapter now accept some
additional options:
:first => true        # Put the column in front of all the columns
:after => column_name # Put the colmn after 'column_name'

add_column :new_col, :string, :first => true
add_column :another_col, :integer, :default => 0, :after => :new_col

[#3286 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

activerecord/CHANGELOG

 *Edge*
 
+* MySQL: add_ and change_column support positioning.  #3286 [Ben Marini]
+
 * Reset your Active Record counter caches with the reset_counter_cache class method.  #1211 [Mike Breen]
 
 * Remove support for SQLite 2. Please upgrade to SQLite 3+ or install the plugin from git://github.com/rails/sqlite2_adapter.git [Pratik Naik]

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -470,6 +470,13 @@ def rename_table(table_name, new_name)
         execute "RENAME TABLE #{quote_table_name(table_name)} TO #{quote_table_name(new_name)}"
       end
 
+      def add_column(table_name, column_name, type, options = {})
+        add_column_sql = "ALTER TABLE #{quote_table_name(table_name)} ADD #{quote_column_name(column_name)} #{type_to_sql(type, options[:limit], options[:precision], options[:scale])}"
+        add_column_options!(add_column_sql, options)
+        add_column_position!(add_column_sql, options)
+        execute(add_column_sql)
+      end
+
       def change_column_default(table_name, column_name, default) #:nodoc:
         column = column_for(table_name, column_name)
         change_column table_name, column_name, column.sql_type, :default => default
@@ -498,6 +505,7 @@ def change_column(table_name, column_name, type, options = {}) #:nodoc:
 
         change_column_sql = "ALTER TABLE #{quote_table_name(table_name)} CHANGE #{quote_column_name(column_name)} #{quote_column_name(column_name)} #{type_to_sql(type, options[:limit], options[:precision], options[:scale])}"
         add_column_options!(change_column_sql, options)
+        add_column_position!(change_column_sql, options)
         execute(change_column_sql)
       end
 
@@ -529,6 +537,13 @@ def type_to_sql(type, limit = nil, precision = nil, scale = nil)
         end
       end
 
+      def add_column_position!(sql, options)
+        if options[:first]
+          sql << " FIRST"
+        elsif options[:after]
+          sql << " AFTER #{quote_column_name(options[:after])}"
+        end
+      end
 
       # SHOW VARIABLES LIKE 'name'
       def show_variable(name)

activerecord/test/cases/migration_test.rb

@@ -527,6 +527,53 @@ def test_add_remove_single_field_using_symbol_arguments
       assert !Person.column_methods_hash.include?(:last_name)
     end
 
+    if current_adapter?(:MysqlAdapter)
+      def testing_table_for_positioning
+        Person.connection.create_table :testings, :id => false do |t|
+          t.column :first, :integer
+          t.column :second, :integer
+          t.column :third, :integer
+        end
+
+        yield Person.connection
+      ensure
+        Person.connection.drop_table :testings rescue nil
+      end
+      protected :testing_table_for_positioning
+
+      def test_column_positioning
+        testing_table_for_positioning do |conn|
+          assert_equal %w(first second third), conn.columns(:testings).map {|c| c.name }
+        end
+      end
+
+      def test_add_column_with_positioning
+        testing_table_for_positioning do |conn|
+          conn.add_column :testings, :new_col, :integer
+          assert_equal %w(first second third new_col), conn.columns(:testings).map {|c| c.name }
+        end
+        testing_table_for_positioning do |conn|
+          conn.add_column :testings, :new_col, :integer, :first => true
+          assert_equal %w(new_col first second third), conn.columns(:testings).map {|c| c.name }
+        end
+        testing_table_for_positioning do |conn|
+          conn.add_column :testings, :new_col, :integer, :after => :first
+          assert_equal %w(first new_col second third), conn.columns(:testings).map {|c| c.name }
+        end
+      end
+
+      def test_change_column_with_positioning
+        testing_table_for_positioning do |conn|
+          conn.change_column :testings, :second, :integer, :first => true
+          assert_equal %w(second first third), conn.columns(:testings).map {|c| c.name }
+        end
+        testing_table_for_positioning do |conn|
+          conn.change_column :testings, :second, :integer, :after => :third
+          assert_equal %w(first third second), conn.columns(:testings).map {|c| c.name }
+        end
+      end
+    end
+
     def test_add_rename
       Person.delete_all