add rescue_from

app/controllers/application_controller.rb

@@ -1,7 +1,26 @@
 class ApplicationController < ActionController::Base
   include DeviseTokenAuth::Concerns::SetUserByToken
-  # Prevent CSRF attacks by raising an exception.
-  # For APIs, you may want to use :null_session instead.
-  protect_from_forgery with: :null_session
   include DeviseTokenAuth::Concerns::SetUserByToken
+
+  protect_from_forgery with: :null_session
+
+  rescue_from ActionController::ParameterMissing, with: :invalid_params
+  rescue_from ActiveRecord::RecordNotFound, with: :not_found
+
+  def render_error_messages(object, status=:unprocessable_entity)
+    render json: { :errors => object.errors.messages, :fields=>object.errors.keys },
+           status: status, head: status
+  end
+
+  def render_blank_success
+    render json: {}, status: :ok
+  end
+
+  def invalid_params(exception)
+    render json: { errors: "Required parameter is missing: #{exception.param}" }, status: :unprocessable_entity, head: :unprocessable_entity
+  end
+
+  def not_found
+    render json: { errors: "Item not found" }, status: :not_found, head: :not_found
+  end
 end

app/controllers/courses_controller.rb

@@ -8,7 +8,7 @@ def index
   def create
     if course=Course.create(params.permit(:name))
       current_user.courses<<course
-      render json: {}, status: :ok
+      render_blank_success
     else
       render json: {error: 'Course not create'}, status: 400
     end