Gem City-State is a great library of relationships between countries-states-cities. Thanks this gem you don’t need to resort to using some sort of external API like Google Maps for most basic usecases.

city-state-gem-dynamic-select.gif

However, when adding this gem you will encounter a classic problem of:

  1. selecting state based on country
  2. selecting city based on state

Good validations are most important to making this work.

You want to make it impossible to save an invalid country-state-city combination. Something like country: "Ukraine, city: "New York should be invalid. Oops, there actually exists a city New York, Ukraine😜

1. Basic setup # 

bundle add city-state
rails g scaffold address country state city address_line_1
rails db:migrate

You need to validate that:

  • a selected state belongs to a selected country;
  • city belongs to a selected country-state combination.

Also, condider that some countries don’t have states or cities (Vatican, Antarctica), so you should not validate presence of a city and state for them.

# app/models/address.rb
class Address < ApplicationRecord
  validates :country, presence: true
  validates :address_line_1, presence: true

  # state has to be valid when changing a country
  validates :state, inclusion: { in: ->(record) { record.state_opts.keys }, allow_blank: true }
  validates :state, presence: { if: ->(record) { record.state_opts.present? } }

  # some countries don't have any cities, like Vatican.
  # city has to be valid when changing a country/state
  validates :city, inclusion: { in: ->(record) { record.city_opts }, allow_blank: true }
  validates :city, presence: { if: ->(record) { record.city_opts.present? } }

  def country_opts
    CS.countries.with_indifferent_access
  end

  def state_opts
    CS.states(country).with_indifferent_access
  end

  def city_opts
    CS.cities(state, country) || []
  end

  def country_name
    country_opts[country]
  end

  def state_name
    state_opts[state]
  end
end

Now, the form can look like this:

# app/views/addresses/_form.html.erb
<%= form_with(model: address) do |form| %>
  <%= form.label :country, style: "display: block" %>
  <%= form.select :country, address.country_opts.invert, {include_blank: true}, { onchange: "this.form.requestSubmit();" } %>

  <%= form.label :state, style: "display: block" %>
  <%= form.select :state, address.state_opts.invert, {include_blank: true}, { onchange: "this.form.requestSubmit();" } %>

  <%= form.label :city, style: "display: block" %>
  <%= form.select :city, address.city_opts, {include_blank: true}, {} %>

  <%= form.submit %>
<% end %>

However, this way there is a full page refresh each time you select something.

Let’s improve it.

2. Dynamic form # 

rails g stimulus form-reset
rails g stimulus form-element

To fix a common problem of refreshing the page and still having values in a form:

// app/javascript/controllers/form_reset_controller.js
import { Controller } from "@hotwired/stimulus"

// Connects to data-controller="form"
export default class extends Controller {
  connect() {
    this.element.reset()
  }
}

Considering the learning from the previous post, we will add a stimulus controller that will help us to submit a “remote” button:

// app/javascript/controllers/form_element_controller.js
import { Controller } from "@hotwired/stimulus"

// Connects to data-controller="form"
export default class extends Controller {
  static targets = ["submitbtn"]

  connect() {
    this.submitbtnTarget.hidden = true
  }

  autosumbit() {
    this.submitbtnTarget.click()
  }
}

and in the controller we need to allow passing address_params in the new action:

# app/controllers/addresses_controller.rb
  def new
-    @address = Address.new
+    @address = Address.new address_params
  end

  def address_params
-    params.require(:address).permit(:country, :city, :state, :address_line_1)
+    params.fetch(:address, {}).permit(:country, :city, :state, :address_line_1)
  end

Finally, we will update our form:

  • add a turbo_frame for the part of content that will be reloaded
  • add a button to refresh the turbo_frame when country or state is selected
# app/views/addresses/_form.html.erb
<%= form_with(model: address, data: {controller: "form-reset"}) do |form| %>
  <div data-controller="form-element">
    <%= form.button "Validate", formaction: new_address_path, formmethod: :get, data: {form_element_target: "submitbtn", turbo_frame: :dynamic_fields} %>
    <%= turbo_frame_tag :dynamic_fields do %>
      <%= form.label :country, style: "display: block" %>
      <%= form.select :country, CS.countries.invert, {include_blank: true}, {data: { action: "change->form-element#autosumbit"}} %>

      <%= form.label :state, style: "display: block" %>
      <%= form.select :state, address.state_opts.invert, {include_blank: true}, {data: { action: "change->form-element#autosumbit"}} %>

      <%= form.label :city, style: "display: block" %>
      <%= form.select :city, address.city_opts, {include_blank: true}, {} %>
    <% end %>
  </div>
  <%= form.submit %>
<% end %>

That’s it!

Now your dynamic select should work perfectly: city-state-gem-dynamic-select.gif