title: Sequelize Many to Many Query Issue
body: <p>So, I have an existing MySQL database that I'm trying to connect to with Sequelize in Node that has a products table, a categories table and a categories_products table.  What I want to do is return products, with each product containing all of the categories it belongs to.  Here's what I've got:</p>
  
  <pre><code>// Declare Product Model
  const Product = sequelize.define('products', {
      name: Sequelize.STRING,
      description: Sequelize.STRING,
      single_price: Sequelize.BOOLEAN,
      oz_price: Sequelize.FLOAT,
      half_price: Sequelize.FLOAT,
      quarter_price: Sequelize.FLOAT,
      eigth_price: Sequelize.FLOAT,
      gram_price: Sequelize.FLOAT,
      unit_price: Sequelize.FLOAT
  },
  {
      underscored: true
  });
  
  // Declare Category Model
  const Category = sequelize.define('categories', {
      name: Sequelize.STRING,
      parent_id: Sequelize.INTEGER,
      picture_file_name: Sequelize.STRING
  },
  {
      underscored: true
  });
  
  // Join Table
  const ProductCategory = sequelize.define('categories_products', {
      product_id: Sequelize.INTEGER,
      category_id: Sequelize.INTEGER,
  
  }, {  
      timestamps: false,
      underscored: true
  });
  
  // Do this because there is no id column on ProductCategory table
  ProductCategory.removeAttribute('id');
  
  Category.hasMany(Category, { as: 'children', foreignKey: 'parent_id' });
  
  ProductCategory.belongsTo(Product);
  ProductCategory.belongsTo(Category);
  Product.hasMany(ProductCategory);
  Category.hasMany(ProductCategory);
  </code></pre>
  
  <p>Using this setup, I query as follows:</p>
  
  <pre><code>Product.findAll({
      include: [{
          model: ProductCategory,
          include: [ Category ]
      }],
      where: { active: true },
      limit: 10
  }).then(prods =&gt; {
      res.send(prods);
  }).catch(err =&gt; {
      res.status(500).send(err);
  });
  </code></pre>
  
  <p>I get back my products and each one has an array of categories, BUT each product only shows a max of one category.  I have products that should have many categories, but it only shows the first.</p>
  
  <p>Am I missing something?  Any help would be greatly appreciated.</p>