title: Automatically map foreignKey ID with sequelize in Node.JS
body: <p>Is it possible that sequelize automatically finds foreignKey IDs?</p>
  <p>Or is it necessary to find out existing IDs manually before a new insert.</p>
  <p>e.g. if I have a user table with mapping from user-id to username.
  From an API request I only know the users name but in all tables I need the ID.</p>
  <pre><code>const Users = dbConn.define('Users', {
      id: {
          field: 'usersId',
          type: DataTypes.NUMBER,
          primaryKey: true,
      },
      userName: {
          field: 'usersName',
          type: DataTypes.NUMBER,
          allowNull: false,
      },
  });
  
  const ChartSettings = dbConn.define('ChartSettings', {
      id: {
          field: 'CS_ID',
          type: DataTypes.NUMBER,
          primaryKey: true,
      },
      userId: {
          field: 'UserId',
          type: DataTypes.NUMBER,
          allowNull: false,
      },
      clientType: {
          field: 'ClientType',
          type: DataTypes.STRING,
          allowNull: false,
      },
      cS_Name: {
          field: 'CS_Name',
          type: DataTypes.STRING,
          allowNull: false,
      },
      settings: {
          field: 'Settings',
          type: 'VARBINARY(MAX)',
          allowNull: false,
      },
  }, {
      timestamps: false
  });
  
  // 1:M mapping
  User.hasMany(ChartSettings, { foreignKey: 'userId' });
  ChartSettings.belongsTo(User, { foreignKey: 'id' });
  
  // actually I do it like this
  const userId = await getUserIdByName('admin'); // e.g. this results in 154
  
  const row = ChartSettings.build({
      userId: 154,
      clientType: 'some text',
      cS_Name: 'some text',
      settings: 0,
  });
  row.save();
  
  
  // ... but I would like to insert a new entry without having the users-ID
  const row = ChartSettings.build({
      // userId: ?
      userName: 'admin', // will be automatically mapped. possible?
      clientType: 'some other txt',
      cS_Name: 'some other txt',
      settings: 0,
  });
  row.save();
  </code></pre>