title: Sequelize not creating model association columns
body: <p>I'm a little confused with when Sequelize creates the fields for associations. </p>
  
  <p>I've created my migrations using sequelize-cli. This generated a migration and model file. Then in the model file I populated my associations. Then ran <code>npx sequelize-cli db:migrate</code>. </p>
  
  <p>This creates the tables but not the foreign keys needed for the associations defined in the model. </p>
  
  <p>For example: 
  migration-quesions: </p>
  
  <pre><code>"use strict";
  module.exports = {
    up: (queryInterface, Sequelize) =&gt; {
      return queryInterface.createTable("questions", {
        id: {
          allowNull: false,
          autoIncrement: true,
          primaryKey: true,
          type: Sequelize.INTEGER
        },
        category: {
          type: Sequelize.INTEGER
        },
        question: {
          type: Sequelize.STRING
        },
        createdAt: {
          allowNull: false,
          defaultValue: new Date(),
          type: Sequelize.DATE
        },
        updatedAt: {
          allowNull: false,
          defaultValue: new Date(),
          type: Sequelize.DATE
        }
      });
    },
    down: (queryInterface, Sequelize) =&gt; {
      return queryInterface.dropTable("questions");
    }
  };
  </code></pre>
  
  <p>model-questions: </p>
  
  <pre><code>"use strict";
  module.exports = (sequelize, DataTypes) =&gt; {
    const questions = sequelize.define(
      "questions",
      {
        question: DataTypes.STRING,
        weight: DateTypes.INTEGER
      },
      {}
    );
    questions.associate = function(models) {
      // associations can be defined here
      models.questions.hasOne(models.question_categories);
      models.questions.hasMany(models.question_answers);
    };
    return questions;
  };
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/0eMxk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0eMxk.png" alt="enter image description here"></a></p>