Support D6->D7 migration (patch provided)

Problem/Motivation

It could be nice to be able to migrate Follow data from Drupal 6 to Drupal 7.
migrate and migrate_d2d provide a non-intrusive framework to handle such a migration.

Proposed resolution

The migration is straightforward as Follow data lies in a single table and the schema is the same in D6 and D7.
See patch.

Remaining tasks

The patch can be reviewed.
Test plan (this page can also help):

1. Apply the patch against 7.x-2.x
2. In settings.php of the D7 site, create a reference to the D6 database:
   

   // Customize with your D6 credentials.
   $databases['d6']['default'] = array(
     'driver' => 'mysql',
     'database' => 'myd6database',
     'username' => 'myd6username',
     'password' => 'myd6password',
     'host' => 'myd6localhost',
   );
   

3. In another module, create a file mymodule.migrate.inc and declare the migration. Migrate will load the file automatically:
   

   // mymodule.migrate.inc
   
   /**
    * Implement hook_migrate_api().
    */
   function mymodule_migrate_api() {
     return array(
       'api' => 2,
       'groups' => array(
         'follow' => array(
           'title' => t('Follow'),
         ),
       ),
       'migrations' => array(
         'follow' => array(
           'group_name' => 'follow',
           'source_connection' => 'd6', // Name of Drupal 6 DB connection as in your settings.php
           'source_version' => 6,
           'class_name' => 'FollowMigration',
           'description' => t('Migration of Follow configuration'),
           'machine_name' => 'FollowMigration',
           //'user_migration' => 'MyUserMigration', // Optional, if you have a user migration.
         ),
       ),
     );
   }
   

4. Enable migrate and migrate_d2d
5. Clear the cache
6. Go to admin/content/migrate/configure and register classes
7. Go to admin/content/migrate and run the migration