Example of how to use the filters option for Relationship fields

Countries.js

// A global file to provide the countries and cities

exports.countries = [{
	name: 'Australia',
	cities: ['Melbourne', 'Sydney', 'Canberra']
}, {
	name: 'España',
	cities: ['Madrid', 'Barcelona', 'Sevilla']
}, {
	name: 'Italia',
	cities: ['Roma', 'Venecia', 'Turin']
}];

Models.js

// Shorthand code to create models with a filtered relationship.

var _ = require('underscore'),
	countries = require('countries').countries;

new keystone.List('City').add({
    name: String,
    country: { Types.Select, options: _.pluck(countries, 'name') }
}).register();

new keystone.List('Person').add({
    name: String,
    country: { Types.Select, options: _.pluck(countries, 'name') },
    city: { type: Types.Relationship, ref: 'City', filters: { country: ':country' }
}).register();

Update.js

// Example of an update script that will populate the Cities model

var _ = require('underscore'),
	countries = require('countries').countries;

var cities = exports.cities = _.flatten(countries.map(function(country) {
	return country.cities.map(function(city) {
		return { country: country.name, name: city.name };
	});
}));

exports.create = {
	City: cities
}

@JedWatson what if I want to get the list of countries from a database? How to asynchronously get data from the database before keystone start?