Solution for ext-auth

app/controllers/application_controller.rb

@@ -1,2 +1,10 @@
 class ApplicationController < ActionController::API
+  rescue_from ActionController::ParameterMissing, with: :render_bad_request
+  rescue_from ActiveRecord::RecordNotFound, with: :render_bad_request
+  rescue_from ArgumentError, with: :render_bad_request
+
+  private
+  def render_bad_request
+    render status: :bad_request
+  end
 end

app/controllers/transaction_controller.rb

@@ -1,5 +1,34 @@
 class TransactionController < ApplicationController
+  before_action :validate_transaction_params, only: [:create]
+  before_action :set_credit_card, only: [:create]
+
   def create
-    raise NotImplementedError
+    if @credit_card.status != 'active' || @credit_card.limit_cents < params[:amount]
+      return render status: :payment_required
+    end
+
+    ZippiMediator.submit_transaction(
+      amount_cents: params[:amount],
+      occurred_at: params[:datetime],
+    )
+
+    updated_limit = @credit_card.limit_cents - params[:amount]
+    @credit_card.update(limit_cents: updated_limit)
+
+    render status: :ok
+  end
+
+  private
+  def validate_transaction_params
+    params.require(%i[id amount datetime])
+
+    params[:amount] = (Float(params[:amount]) * 100).round
+    params[:datetime] = DateTime.
+      strptime(params[:datetime], "%d/%m/%Y %R").
+      change(:offset => "-0300")
+  end
+
+  def set_credit_card
+    @credit_card = CreditCard.find(params[:id])
   end
 end

spec/requests/transaction_request_spec.rb

@@ -76,7 +76,7 @@
     end
 
     it 'for sending an unregistered id' do
-      post '/transaction', params: { amount: 1.99, datetime: '123', id: 1 }
+      post '/transaction', params: { amount: 1.99, datetime: '02/02/2019 10:01', id: 1 }
       expect(response).to have_http_status(:bad_request)
     end
   end