Magento 2 Installation – Adding Constraints in the Install Script

I'm trying to build a custom module in Magento 2 v0.74.0-beta1.
This module should contain 2 simple entities, each one with its table and a many to many relation between them through a third table.
Nothing fancy..see the image below.

When running the install script (php setup/index.php update via cli) the main tables are created but i get an error when creating the link table Error Code : 1215 Cannot add foreign key constraint.

Here is my code from app/code/Testing/Demo/Setup/InstallSchema.php (Testing – vendor name, Demo – module name):

<?php

namespace Testing\Demo\Setup;

use Magento\Framework\DB\Adapter\AdapterInterface;
use Magento\Framework\DB\Ddl\Table;
use Magento\Framework\Setup\InstallSchemaInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\SchemaSetupInterface;

class InstallSchema implements InstallSchemaInterface
{
    /**
     * install tables
     *
     * @param SchemaSetupInterface $setup
     * @param ModuleContextInterface $context
     * @return void
     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
     */
    public function install(SchemaSetupInterface $setup, ModuleContextInterface $context)
    {
        $installer = $setup;
        $installer->startSetup();
        if (!$installer->tableExists('testing_demo_author')) {
            $table = $installer->getConnection()->newTable(
                $installer->getTable('testing_demo_author')
            )
            ->addColumn(
                'author_id',
                Table::TYPE_INTEGER,
                null,
                [
                    'identity' => true,
                    'nullable' => false,
                    'primary' => true
                ],
                'Author ID'
            )
            ->addColumn(
                'name',
                Table::TYPE_TEXT,
                255,
                ['nullable => false'],
                'Author Name'
            )
            ->addColumn(
                'created_at',
                Table::TYPE_TIMESTAMP,
                null,
                [],
                'Author Created At'
            )
            ->addColumn(
                'updated_at',
                Table::TYPE_TIMESTAMP,
                null,
                [],
                'Author Updated At'
            )
            ->setComment('Author Table');
            $installer->getConnection()->createTable($table);
        }
        if (!$installer->tableExists('testing_demo_article')) {
            $table = $installer->getConnection()->newTable(
                $installer->getTable('testing_demo_article')
            )
            ->addColumn(
                'article_id',
                Table::TYPE_INTEGER,
                null,
                [
                    'identity' => true,
                    'nullable' => false,
                    'primary' => true
                ],
                'Article ID'
            )
            ->addColumn(
                'title',
                Table::TYPE_TEXT,
                255,
                ['nullable => false'],
                'Article Title'
            )
            ->addColumn(
                'description',
                Table::TYPE_TEXT,
                '64k',
                ['nullable => false'],
                'Article Description'
            )

            ->addColumn(
                'created_at',
                Table::TYPE_TIMESTAMP,
                null,
                [],
                'Article Created At'
            )
            ->addColumn(
                'updated_at',
                Table::TYPE_TIMESTAMP,
                null,
                [],
                'Article Updated At'
            )
            ->setComment('Article Table');
            $installer->getConnection()->createTable($table);
        }

        if (!$installer->tableExists('testing_demo_author_article')) {
            $table = $installer->getConnection()
                ->newTable($installer->getTable('testing_demo_author_article'));
            $table->addColumn(
                    'author_id',
                    Table::TYPE_INTEGER,
                    null,
                    [
                        'unsigned' => true,
                        'nullable' => false,
                        'primary'   => true,
                    ],
                    'Author ID'
                )
                ->addColumn(
                    'article_id',
                    Table::TYPE_INTEGER,
                    null,
                    [
                        'unsigned' => true,
                        'nullable' => false,
                        'primary'   => true,
                    ],
                    'Article ID'
                )
                ->addColumn(
                    'position',
                    Table::TYPE_INTEGER,
                    null,
                    [
                        'nullable' => false,
                        'default' => '0'
                    ],
                    'Position'
                )
                ->addIndex(
                    $installer->getIdxName('testing_demo_author_article', ['author_id']),
                    ['author_id']
                )
                ->addIndex(
                    $installer->getIdxName('testing_demo_author_article', ['article_id']),
                    ['article_id']
                )
                ->addForeignKey(
                    $installer->getFkName(
                        'testing_demo_author_article',
                        'author_id',
                        'testing_demo_author',
                        'author_id'
                    ),
                    'author_id',
                    $installer->getTable('testing_demo_author'),
                    'author_id',
                    Table::ACTION_CASCADE,
                    Table::ACTION_CASCADE
                )
                ->addForeignKey(
                    $installer->getFkName(
                        'testing_demo_author_article',
                        'article_id',
                        'testing_demo_article',
                        'article_id'
                    ),
                    'article_id',
                    $installer->getTable('testing_demo_article'),
                    'article_id',
                    Table::ACTION_CASCADE,
                    Table::ACTION_CASCADE
                )
                ->addIndex(
                    $installer->getIdxName(
                        'testing_demo_author_article',
                        [
                            'author_id',
                            'article_id'
                        ],
                        AdapterInterface::INDEX_TYPE_UNIQUE
                    ),
                    [
                        'author_id',
                        'article_id'
                    ],
                    [
                        'type' => AdapterInterface::INDEX_TYPE_UNIQUE
                    ]
                )
                ->setComment('Author To Article Link Table');
            $installer->getConnection()->createTable($table);
        }
        $installer->endSetup();
    }
}

The generated scripts that are executed are:

CREATE TABLE `testing_demo_author` (
  `author_id` int NOT NULL auto_increment COMMENT 'Author ID' ,
  `name` varchar(255) NULL COMMENT 'Author Name' ,
  `created_at` timestamp NULL default NULL COMMENT 'Author Created At' ,
  `updated_at` timestamp NULL default NULL COMMENT 'Author Updated At' ,
  PRIMARY KEY (`author_id`),
  INDEX `IDX_TESTING_DEMO_AUTHOR_ID` (`author_id`)
) COMMENT='Author Table' ENGINE=INNODB charset=utf8 COLLATE=utf8_general_ci;

CREATE TABLE `testing_demo_article` (
  `article_id` int NOT NULL auto_increment COMMENT 'Article ID' ,
  `title` varchar(255) NULL COMMENT 'Article Title' ,
  `description` text NULL COMMENT 'Article Description' ,
  `created_at` timestamp NULL default NULL COMMENT 'Article Created At' ,
  `updated_at` timestamp NULL default NULL COMMENT 'Article Updated At' ,
  PRIMARY KEY (`article_id`),
  INDEX `IDX_TESTING_DEMO_ARTICLE_ID` (`article_id`)
) COMMENT='Article Table' ENGINE=INNODB charset=utf8 COLLATE=utf8_general_ci;

CREATE TABLE `testing_demo_author_article` (
  `author_id` int UNSIGNED NOT NULL COMMENT 'Author ID' ,
  `article_id` int UNSIGNED NOT NULL COMMENT 'Article ID' ,
  `position` int NOT NULL default '0' COMMENT 'Position' ,
  PRIMARY KEY (`author_id`, `article_id`),
  INDEX `IDX_TESTING_DEMO_AUTHOR_ARTICLE_AUTHOR_ID` (`author_id`),
  INDEX `IDX_TESTING_DEMO_AUTHOR_ARTICLE_ARTICLE_ID` (`article_id`),
  UNIQUE `UNQ_TESTING_DEMO_AUTHOR_ARTICLE_AUTHOR_ID_ARTICLE_ID` (`author_id`, `article_id`),
  CONSTRAINT `FK_28E3EEEF7CD69E4787AE4826A331D49F` FOREIGN KEY (`author_id`) REFERENCES `testing_demo_author` (`author_id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `FK_247EC1688F0CAA785ECB3A4BA5398032` FOREIGN KEY (`article_id`) REFERENCES `testing_demo_article` (`article_id`) ON DELETE CASCADE ON UPDATE CASCADE
) COMMENT='Author To Article Link Table' ENGINE=INNODB charset=utf8 COLLATE=utf8_general_ci;

What am I doing wrong?
I know what the error means, but I cannot understand why can't the FK be created.