Need help with first migration

I need some help with my first migration using this module. I read bear.inc but I didn't understand some things. Below (in bold) I have some questions to clarify some things. I would like to document this migration to help new users to Migrate module. I would really appreciate the help of an advanced user.

Here are my tables:

1. test_category (taxonomy)
2. test_subcategory (taxonomy)
3. test_users (user)
4. test_questions (node)

I attached the structure of each table in png files (also sql files in the zip archive).

1. Migration of test_category

class TestParentTermMigration extends TestMigration {
    public function __construct() {
        parent::__construct();
        $this->description = t('Migrate parent categories from the source database to taxonomy terms');

        $this->map = new MigrateSQLMap($this->machineName,
            array(
                'id' => array('type' => 'int',
                    'length' => 10,
                    'not null' => TRUE,
                    'description' => 'Category ID',
                )
            ),
            MigrateDestinationTerm::getKeySchema()
        );

        $query = db_select('test_category', 'c')->fields('c', array('id', 'title'));
        $this->source = new MigrateSourceSQL($query);

        // Set up our destination - terms in the category vocabulary
        $this->destination = new MigrateDestinationTerm('category');

        // Assign mappings TO destination fields FROM source fields. To discover
        $this->addFieldMapping('name', 'title');

        // Unmapped fields
        $this->addUnmigratedDestinations(array('path', 'description', 'format', 'weight', 'parent', 'parent_name'));
    }
}

No problems so far, just a simple migration of two fields.

2. Migration of test_subcategory

class TestChildTermMigration extends TestMigration {
    public function __construct() {
        parent::__construct();
        $this->description = t('Migrate child categories from the source database to taxonomy terms');
        $this->dependencies = array('TestParentTerm');

        $this->map = new MigrateSQLMap($this->machineName,
            array(
                'id' => array('type' => 'int',
                    'length' => 10,
                    'not null' => TRUE,
                    'description' => 'Subcategory ID',
                )
            ),
            MigrateDestinationTerm::getKeySchema()
        );

        $query = db_select('test_subcategory', 's')->fields('s', array('id', 'title', 'category_id', 'description'));
        $this->source = new MigrateSourceSQL($query);

        // Set up our destination - terms in the category vocabulary
        $this->destination = new MigrateDestinationTerm('category');

        // Assign mappings TO destination fields FROM source fields. To discover
        $this->addFieldMapping('name', 'title');
        $this->addFieldMapping('description', 'description');

        // Translate the old ID to Drupal identifier
        $this->addFieldMapping('parent', 'category_id')
            ->sourceMigration('TestParentTerm')
            ->defaultValue(0);

        // Unmapped fields
        $this->addUnmigratedDestinations(array('path', 'format', 'weight', 'parent_name'));
    }
}

Question: When importing the child terms, I have to map them to the ID of the imported taxonomy term. Is this the correct way, using sourceMigration('TestParentTerm')?

3. Migration of test_users

class TestUserMigration extends TestMigration {
    public function __construct() {
        // The basic setup is similar to BeerTermMigraiton
        parent::__construct();
        $this->description = t('Users from test migration');
        $this->map = new MigrateSQLMap($this->machineName,
            array('id' => array(
                'type' => 'int',
                'not null' => TRUE,
                'description' => 'User ID'
            )
            ),
            MigrateDestinationUser::getKeySchema()
        );
        $query = db_select('test_users', 'u')
            ->fields('u', array('id', 'username', 'password', 'email', 'created_at'));
        $this->source = new MigrateSourceSQL($query);
        $this->destination = new MigrateDestinationUser();

        $this->addFieldMapping('mail', 'email');
        $this->addFieldMapping('init', 'email');
        // Dedupe assures that value is unique. Use it when source data is non-unique.
        // Pass the Drupal table and column for determining uniqueness.
        $this->addFieldMapping('name', 'username')
            ->dedupe('users', 'name');

        // The migrate module automatically converts date/time strings to UNIX timestamps.
        $this->addFieldMapping('created', 'created_at');
        $this->addFieldMapping('pass', 'password');
        $this->addFieldMapping('roles')
            ->defaultValue(2);
        $this->addFieldMapping('status')
            ->defaultValue(1);

        // Unmapped destination fields
        $this->addUnmigratedDestinations(array('theme', 'signature', 'signature_format', 'access', 'login',
            'timezone', 'language', 'picture', 'is_new', 'path'));
    }
}

First problem: Some users do not have email address. I know Drupal needs it, so I would like to generate on-the-fly something like current_username@example.com. How can I prepare each row using Migrate?

4. Migration of test_questions

class TestNodeMigration extends TestMigration {
    public function __construct() {
        parent::__construct();
        $this->description = t('User questions');

        // You may optionally declare dependencies for your migration - other migrations
        // which should run first. In this case, terms assigned to our nodes and
        // the authors of the nodes should be migrated before the nodes themselves.
        $this->dependencies = array('TestUser', 'TestChildTerm');

        $this->map = new MigrateSQLMap($this->machineName,
            array(
                'id' => array(
                    'type' => 'int',
                    'not null' => TRUE,
                    'description' => 'Question ID'
                )
            ),
            MigrateDestinationNode::getKeySchema()
        );

        $query = db_select('test_questions', 'q')
            ->fields('q', array('id', 'content', 'user_id', 'subcategory_id', 'status', 'created_at'));
        $this->source = new MigrateSourceSQL($query);

        // Set up our destination - nodes of type question
        $this->destination = new MigrateDestinationNode('question');

        // Mapped fields
        $this->addFieldMapping('field_article_state', 'status');
        $this->addFieldMapping('status')->defaultValue(1);
        $this->addFieldMapping('comment')->defaultValue(2);
        $this->addFieldMapping('created', 'created_at');
        // Data too long for column 'title'
        /*$this->addFieldMapping('title', 'content')
            ->description(t('Mapping question content in source to node title'));*/
        $this->addFieldMapping('title', NULL)->defaultValue('Question title');
        $this->addFieldMapping('sticky')->defaultValue(0);
        $this->addFieldMapping('is_new')->defaultValue(TRUE);
        $this->addFieldMapping('body', 'content');

        // Translate old user_id to new Drupal UID
        $this->addFieldMapping('uid', 'user_id')
            ->sourceMigration('TestUser');

        // Map to subcategory
        $this->addFieldMapping('field_category', 'subcategory_id')
            ->sourceMigration('TestChildTerm');

        // Unmapped destination fields
        $this->addUnmigratedDestinations(array('changed','promote', 'revision', 'language', 'path', 'revision_uid', 'log', 'tnid'));
    }
}

Second problem: I don't have a title field for the question to import into the title field of the node. I know the title is varchar(255) in node table, so I can't dump the content of the question into the title field (Migrate throws errors like: Data is too long). I have to prepare that title with substr() function, so how can I create a function for that?
The real problem: Although the relation to each user is correctly maintain, I can't assign the correct child term. I'm using the same sourceMigration which should take the old ID of the subcategory, look in the map and get the new taxonomy term to attach it to the node: $this->addFieldMapping('field_category','subcategory_id')->sourceMigration('TestChildTerm');

The structure of the tables is quite different from beer.inc migrate_example, but this shouldn't arise any problems for some experienced Migrate user. I'm new so I really need some answers to continue.

Any help is appreciated. Thanks