'use strict';
module.exports = function(sequelize, DataTypes) {
  var User = sequelize.define('User', {
    user_id: DataTypes.STRING,
    fullname: DataTypes.STRING,
    username: DataTypes.STRING,
    comment: DataTypes.TEXT
  }, {
    classMethods: {
      associate: function(models) {
        // associations can be defined here
        User.hasMany(Sshkey)
      }
    }
  });
  return User;
};

'use strict';
module.exports = function(sequelize, DataTypes) {
  var Staff = sequelize.define('Staff', {
    user_id: DataTypes.STRING,
    fullname: DataTypes.STRING,
    username: DataTypes.STRING,
    password: DataTypes.STRING,
    isAdmin: DataTypes.BOOLEAN
  }, {
    classMethods: {
      associate: function(models) {
        // associations can be defined here
        Staff.hasMany(models.Sshkey)
      }
    }
  });
  return Staff;
};

'use strict';
module.exports = function(sequelize, DataTypes) {
  var Sshkey = sequelize.define('Sshkey', {
    sshkey_id: DataTypes.STRING,
    sshkey: DataTypes.TEXT
  }, {
    classMethods: {
      associate: function(models) {
        // associations can be defined here

        // My Problem starts here |
        // Should i write         |
        //                        |
        //                        V

        Sshkey.hasOne(models.User || models.Staff, {foreignKey: 'user_id'})

        // Or maybe:
        // Sshkey.hasOne(models.User, {foreignKey: 'user_id'}) ||
        // Sshkey.hasOne(models.Staff, {foreignKey: 'user_id'})
        // Should i rather rename models.Staffs foreignKey user_id to staff_id?

      }
    }
  });
  return Sshkey;
};