How to create singleton associations in FactoryGirl

singletonFactory.js

/**
  * For use with https://github.com/aexmachina/factory-girl
  *
  * Helper for defining an association to a record that should only be instantiated once.
  * Uses sequelizes findOrCreate() to ensure the record is created on
  * first invocation, and reused subsequently
  *
  * NOTE: Make sure to use this within a function initialiser or the singleton
  * record will be created when factories are defined which may cause confusion
  *
  * NOTE: Make sure uniqueAttr is the name of a column that has a unique constraint
  * or it's highly likley you will end up with duplicate records in your db.
  *
  * @param {String} factory Name of the factory to use
  * @param {String} uniqueAttr A unique attribute to use to check if the record
  *		has already been created
  * @param {String} attr Attribute to return (otherwise returns the record)
  * @returns {Model|Object} Returns either the record itself or the value of record[attr]
  */
async function singleton(factory, uniqueAttr, attr) {
	// Find the default campaign
	if (!this.factories[factory]) {
		throw new Error(`Unknown factory: ${factory}`);
	}

	const model = this.factories[factory].Model;
	const defaults = await this.attrs(factory);
	const where = {};
	where[uniqueAttr] = defaults[uniqueAttr];
  
	const record = await model.findOrCreate({ where, defaults });

  return attr ? record[0][attr] : record[0];
}

module.exports = singleton;

test.js

const fg = require('factory-girl').factory;
const singletonFactory = require('./mixins/singletonFactory');

const FactoryGirl = fg.FactoryGirl;

const factory = new FactoryGirl();
factory.singleton = singletonFactory;

factory.define('defaultOrganisation', Organisation, {
  name: 'ACME Inc',
  slug: 'acme.com',
});

// NOTE: The initialiser must be a function 
factory.define('department', Department, () => ({
  name: factory.sequence('Department.name', n => `department ${n}`),
  organisationId: factory.singleton('defaultOrganisation', 'slug', 'id'),
}));

async function test() {
  const department1 = await factory.create('department');
  const department2 = await factory.create('department');

  assertEqual(department1.organisationId, department2.organisationId);
}

test();