title: Sequelize get values from belongsTo relationship
body: <p>I am creating an app using an existing database in Mysql. 
  I can retrieve the values from the tables, but when I try to retrieve the values from associate models, doesn't work.</p>
  
  <p>Purchase Order model:</p>
  
  <pre><code>"use strict";
  
  module.exports = (sequelize, DataTypes) =&gt; {
    var PurchaseOrder = sequelize.define('purchase_orders' , {
      id: {
      type: DataTypes.INTEGER(11),
      allowNull: false,
      primaryKey: true,
      autoIncrement: true,
      field: 'id'
    },
    purchaseDate: {
      type: DataTypes.DATE,
      allowNull: true,
      field: 'purchase_date'
    },
    supplierId: {
      type: DataTypes.INTEGER(11),
      allowNull: true,
    //  references: {
  //      model: 'suppliers',
  //      key: 'id'
  //    },
      field: 'supplier_id'
    },
    requestedById: {
      type: DataTypes.INTEGER(11),
      allowNull: true,
      field: 'requested_by_id'
    },
    masterPurchaseOrderId: {
      type: DataTypes.INTEGER(11),
      allowNull: true,
  //    references: {
  //      model: 'master_purchase_orders',
  //      key: 'id'
  //    },
      field: 'master_purchase_order_id'
    },
    createdAt: {
      type: DataTypes.DATE,
      allowNull: false,
      field: 'created_at'
    },
    updatedAt: {
      type: DataTypes.DATE,
      allowNull: false,
      field: 'updated_at'
    }
   },{
    classMethods:{
      associate: (models) =&gt; {
        PurchaseOrder.belongsTo(models.masterPurchaseOrders,{
          foreignkey: 'master_purchase_order_id'
        });
      },
      associate: (models) =&gt; {
        PurchaseOrder.belongsTo(models.supplier,{
          foreignkey: 'supplier_id'
        });
      },
    },
  }, {
      timestamps: true,
      paranoid: false,
      underscored: true,
      freezeTableName: true,
      tableName: 'purchase_orders'
    });
  
    return PurchaseOrder;
  }
  </code></pre>
  
  <p>Purchase Order Controller:</p>
  
  <pre><code>"using strict";
  const purchaseOrder = require('../models/').purchase_orders;
  const Supplier = require('../models/').supplier;
  const Master = require('../models/').masterPurchaseOrders;
  
  module.exports= {
    index(req, res) {
      purchaseOrder
        .findAll({
          include:[Supplier, Master]
        })
        .then((masters) =&gt; {
          res.status(200).json(masters);
        })
        .catch((error) =&gt; {
          res.status(500).json(error);
        });
    },
    create(req, res) {
      purchaseOrder
        .create(req.body)
        .then( master =&gt; {
          res.status(200).json(master);
        })
        .catch( error =&gt; {
          res.status(500).json(error);
        });
    },
  };
  </code></pre>
  
  <p>If I only include the model supplier in the function findall(). I get the data from the purchase and the data from the supplier.
  But when I include the model master purchase order. I get an empty json.</p>