foreignKeyJoin.js

const foreignKeyJoin = (
  idKey,
  populatedKey,
  serviceName,
) => args => async (item, context) => {

  const res = await context.app.services[serviceName].find({
    query: {
      _id: { $in: item[idKey] }
    }
  });

  item[populatedKey] = Array.isArray(item[idKey]) ? res[0] : res;
};

module.exports = foreignKeyJoin;

schedule.hooks.js

const { authenticate } = require('@feathersjs/authentication').hooks;
const {fastJoin } = require('feathers-hooks-common');
const foreignKeyJoin = require('../../util/foreignKeyJoin');

const resolvers={
  joins:{
    items: foreignKeyJoin('itemIds', 'items', 'schedule-items'),
    conditions: foreignKeyJoin('conditionsGroupId', 'conditions', 'condition-groups')
  }
};

const createConditionsGroup = async context => {
  const conditionGroup = await context.app.service('condition-groups').create({});
  context.data.conditionsGroupId = conditionGroup._id.toHexString();
  return context;
};

module.exports = {
  before: {
    create: [createConditionsGroup],
  },

  after: {
    all: [fastJoin(resolvers)],
  },
};

schedules.model.js

module.exports = function (app) {
  const mongooseClient = app.get('mongooseClient');
  const { Schema } = mongooseClient;
  const schedules = new Schema({
    name: {type: String, default: 'New Schedule'},
    enabled: {type: Boolean, default: true},
    conditionsGroupId: {type: Schema.Types.ObjectId, default: null},
    itemIds: {type: [Schema.Types.ObjectId], default: null},
}, {
    timestamps: true,
    lean: true
  });

  return mongooseClient.model('schedules', schedules);
};