title: Joining two models in loopback using include filter
body: <p>I have two models Purchase and Products, and productId is common for both.</p>
  
  <p>I need to find the productDetails from product model for a purchaseId.</p>
  
  <p>So I have created a custom endpoint in Purchase model, called getProductDetails.</p>
  
  <p>This is how I am trying to query the models.</p>
  
  <pre><code>         Purchase.find({
              "filter": {
                  include: {
                      relation: 'Product',
                      scope: {
                          fields: ['productDesc'],
                      }
                  }
              },
             where:{
                  id:purchaseId
              },
  </code></pre>
  
  <p>My relation is 
  Purchase belongsTo Product, foreignKey being productId</p>
  
  <p>Product hasMany Purchase</p>
  
  <p><strong>But even when I do the above query, I do not get productDesc in query result</strong></p>
  
  <p><strong>Is my model relation wrong ?</strong></p>