title: Sequelize many to many relationship
body: <p>I need some help to get my head around the sequelize many to many relationship.</p>
  
  <p>I have to models, clients and services (many to many). Now I would like to load all services offered by one client (via client id). How does the condition with include have to look like to achieve this?</p>
  
  <p>Service:</p>
  
  <pre><code>var Service = sequelize.define('service', {
      title: {
        type: DataTypes.STRING(200),
        allowNull: false
      }
    },{
      paranoid: true,
      underscored: true,
      classMethods: {
        associate:function(models){
            Service.belongsToMany(models.client, { through: 'client_services' });
            Service.hasMany(models.rule, { foreignKey: 'service_id' })
        }
      }
    });
  </code></pre>
  
  <p>Client:</p>
  
  <pre><code> var Client = sequelize.define('client', {
      title: {
        type: DataTypes.STRING(200),
        allowNull: false
      },
      company: {
          type: DataTypes.STRING(200),
          allowNull: false
      },
      vendor: {
        type: DataTypes.BOOLEAN,
        allowNull: false,
        defaultValue: false
      },
      consumer: {
        type: DataTypes.BOOLEAN,
        allowNull: false,
        defaultValue: true
      },
      address_id: {
        type: DataTypes.INTEGER,
        allowNull: true
      }
    },{
      paranoid: true,
      underscored: true,
      classMethods: {
        associate:function(models){
            Client.hasMany(models.service, { through: 'client_services'});
        }
      }
    });
  </code></pre>
  
  <p>In my Controller (doesn't work err: [Error: client (client) is not associated to service!]):</p>
  
  <pre><code>var condition = {
        where: { 'client.id': req.user.client_id },
        include: [{ model: Client, as: 'client' }]
      }
  Service.findAll(condition)
  .then(responseWithResult(res))
  </code></pre>