title: Sequelize: Or-condition over multiple tables
body: <p>I want to add an or condition over multiple tables with sequelizejs.
  My problem is that I don't know how to use the or operators ($or and Sequelize.or) over more than one table.</p>
  
  <p>Let's say I want to implement the following sql-query:</p>
  
  <pre><code>select * from A as a, B as b, C as c where (A.b_id = b.id and b.x = 7) or (A.c_id = C.id and c.z = "test")
  </code></pre>
  
  <p>I would implement only the first condition with sequelize like this:</p>
  
  <pre><code>A.findAll({
     include: [{ model: B, where: { x: 7 } }]
  })
  </code></pre>
  
  <p>And only the second condition like this:</p>
  
  <pre><code>A.findAll({
     include: [{ model: C, where: { z: "test" } }]
  })
  </code></pre>
  
  <p>But how do I combine both queries into the one I want?</p>