title: Sequelize include is not working with associated table
body: <p>I'm using sequelize 5.21.2</p>
  
  <p>I have a users model:</p>
  
  <pre class="lang-js prettyprint-override"><code>'use strict';
  module.exports = (sequelize, Sequelize) =&gt; {
    const users = sequelize.define('users', {
      id: {
        type: Sequelize.INTEGER,
        primaryKey: true,
        autoIncrement: true,
        allowNull: false
      },
      firstname: {
        type: Sequelize.STRING,
        allowNull: false,
      },{
        timestamp: true,
        logging: console.log,
    });
    users.associate = function(models) {
      models.users.hasMany(models.messages);
    };
    return users;
  };
  </code></pre>
  
  <p>A messages model:</p>
  
  <pre class="lang-js prettyprint-override"><code>'use strict';
  module.exports = (sequelize, Sequelize) =&gt; {
      const messages = sequelize.define('messages', {
          id: {
              type: Sequelize.INTEGER,
              primaryKey: true,
              autoIncrement: true,
              allowNull: false
          },
          message:{
              type: Sequelize.TEXT,
              allowNull: false
          }
      }, {
          timestamp: true,
          logging: console.log,
          scopes:{
              withUser:{
                  include: [
                      {
                          model: sequelize.models.users
                      }
                  ]
              }
          }
      });
      messages.associate = function(models) {
          models.messages.belongsTo(models.users);
      };
      return messages;
  };
  </code></pre>
  
  <p>My controller:</p>
  
  <pre class="lang-js prettyprint-override"><code>const db = require('../models');
  const Messages = db.messages
  exports.getOneMessage = (req, res) =&gt;{
      Messages.scope(['withUser']).findByPk(req.query.id).then(message =&gt; {
          res.status(200).json({
              "description": "getOne - " + req.query.id,
              "message": message
          });
      }).catch(err =&gt; {
          res.status(500).json({
              "description": "Can not access",
              "error": err
          });
      });
  };
  </code></pre>
  
  <p>I have this error:</p>
  
  <pre><code>Error: Include unexpected. Element has to be either a Model, an Association or an object.
  </code></pre>
  
  <ul>
  <li>I've tried with hasOne in my users model instead of hasMany - No change</li>
  <li>I've tried with hasMany in my users model and removed the belongsTo of the messages model - No change</li>
  <li>I've tried with belongsTo in the messages model and removed the hasMany of the users model - No change</li>
  <li>I've dropped all tables and retried everything - No change</li>
  </ul>
  
  <p>What am I doing wrong?</p>