model.js

const API_URL = 'http://localhost:3011/'

const OfficeModel = {
  getCollection(){
    return fetch(`${API_URL}offices`)
      .then(res => res.json())
  },
  async __extendOfficeWithEmployees(office){
    const employees = await EmployeeModel.getCollection(office.city)
    return { ...office, employees }
  },
  async getCollectionWithEmployees(){
    const offices = await this.getCollection()
    const promises = offices.map(this.__extendOfficeWithEmployees)
    return Promise.all(promises)
  },
  async getItemWithEmployees(city){
    const officeRes = await fetch(`${API_URL}offices?city=${city}`)
    const office =  await officeRes.json()
    return this.__extendOfficeWithEmployees(office[0])
  }
}

const EmployeeModel = {
  getCollection(office){
    const query = office ? `?office_like=${office}` : ''
    return fetch(`${API_URL}employees${query}`)
      .then(res => res.json())
  },
  getItem(id){
    return fetch(`${API_URL}employees/${id}`)
      .then(res => res.json())
  }
}

const ProjectModel = {
  getCollection(){
    return fetch(`${API_URL}projects`)
      .then(res => res.json())
  },
  getItem(id){
    return fetch(`${API_URL}projects/${id}`)
      .then(res => res.json())
  },
  async __extendProjectWithEmployees(project){
    const managerPromise = EmployeeModel.getItem(project.manager)
    const teamPromises = project.team.map(member => EmployeeModel.getItem(member.id))
    return { ...project,
      manager: await managerPromise,
      team: await Promise.all(teamPromises)
    }
  },
  async getItemWithEmployees(id){
    return this.__extendProjectWithEmployees(await this.getItem(id))
  }
}

////////////////////////////////

OfficeModel.getCollection()
  .then(offices => console.log('offices', offices))

EmployeeModel.getCollection()
  .then(employees => console.log('employees', employees))

ProjectModel.getCollection()
  .then(projects => console.log('projects', projects))

ProjectModel.getItemWithEmployees("579ef28f-c539-41ff-abe2-e4f6b1c1afed")
  .then(project => console.log('project with employees', project))

OfficeModel.getItemWithEmployees('Dallas')
  .then(office => console.log('office with employees', office))

OfficeModel.getCollectionWithEmployees()
  .then(offices => console.log('all offices with employees', offices))