RoR API how to use the post for asociations - ruby-on-rails

Question

I am string an API project using RoR 6. I have this models:
class Province < ApplicationRecord
validates :province, presence: true, length: {minimum:5}, uniqueness: true
has_many :cities, dependent: :destroy
end
class City < ApplicationRecord
validates :city, presence: true, length: {minimum: 5}, uniqueness: true
belongs_to :province
end
the migrations:
class CreateProvinces < ActiveRecord::Migration[6.1]
def change
create_table :provinces do |t|
t.string :province
t.timestamps
end
end
end
class CreateCities < ActiveRecord::Migration[6.1]
def change
create_table :cities do |t|
t.string :city
t.references :province, null: false, foreign_key: true
t.timestamps
end
end
end
the controller method:
def create
#city = City.new(city_params)
if #city.save
render json: #city, status: :created, location: #city
else
render json: #city.errors, status: :unprocessable_entity
end
end
private
def city_params
params.require(:city).permit(:city, :province_id)
end
and the routes:
Rails.application.routes.draw do
resources :provinces, defaults: { format: :json } do
resources :cities, defaults: { format: :json }
end
end
the thing is this, when running:
curl --location --request POST 'http://127.0.0.1:3000/provinces/1/cities' \
--header 'Content-Type: application/json' \
--data-raw '{
"city": "New City",
"province": 1
}'
{ "province": [ "must exist" ] }
The province exists. But there is no way to insert it. If I use the rails console and run:
p=Province.find(1)
c=City.new({city: 'New City', province: p})
c.save
it works as expected. How to solve this?

Answer 1

When dealing with a nested route you want to find the parent resource first and then create the nested resource off it:
class ProvincesController < ApplicationController
# POST /provinces/1/cities
def create
#province = Province.find(params[:province_id])
#city = #province.cities.new(city_params)
if #city.save
render json: #city, status: :created, location: #city
else
render json: #city.errors, status: :unprocessable_entity
end
end
private
def city_params
params.require(:city).permit(:city)
end
end
Also calling the columns provinces.province and cities.city makes a lot less sense then using provinces.name and cities.name.