In the output, I will post how to write validation in Rails.
Validation is the verification that the data is correct.
First in the model
class User < ApplicationRecord validates :nickname, presence: true :nickname #Column name presence: true #Not empty
Then the controller
class UsersController < ApplicationController def new @user = User.new end def create @user = User.new(user_params) if @user.valid? #here! !! here! !! !! @user.save redirect_to root_path else render 'new' end end private def user_params params.require(:item).permit( :nickname end end
Validation is executed by the valid? method. Returns true if validation is successful.
As a flow, check with validation whether the value is entered when saving what was generated by the new action with the create action. If you use the if statement, save it as @ user.save. If empty, alert with render'new'. , ,
= f.text_field :nickname, class: "nickname" .error-messages = @user.errors.full_messages_for(:nickname)
In the view, write the nickname to save and the error message display = @ user.errors.full_messages_for (: nickname)  to complete.