How to show error message on rails views?

Question

I am newbie in rails and want to apply validation on form fields.

myviewsnew.html.erb

<%= form_for :simulation, url: simulations_path do |f|  %>

<div class="form-group">
  <%= f.label :Row %>
  <div class="row">
    <div class="col-sm-2">
      <%= f.text_field :row, class: 'form-control' %>
    </div>
  </div>
</div>
.....

Simulation.rb

class Simulation < ActiveRecord::Base
 belongs_to :user
 validates :row, :inclusion => { :in => 1..25, :message => 'The row must be between 1 and 25' }
end

simulation_controller.rb

class SimulationsController < ApplicationController

  def index
    @simulations = Simulation.all
  end

  def new
  end

  def create
    @simulation = Simulation.new(simulation_params)
    @simulation.save
    redirect_to @simulation
  end

  private
   def simulation_params
   params.require(:simulation).permit(:row)
  end

I want to check the integer range of row field in model class and return the error message if it's not in the range. I can check the range from above code but not able to return the error message

Thanks in advance

Answer 1

Do this -

 <%= form_for :simulation, url: simulations_path do |f|  %>
    <% if f.object.errors.any? %>
      <ul>
        <% if f.object.errors.each do |message| %>
          <li><%= message %></li>
        <% end %>
      </ul>
    <% end %>

   ..........
 <% end %> 

Answer 2

You just need to add this code to the view file (myviewsnew.html.erb):

<%= error_messages_for :simulation %>

Check complete syntax of error_messages_for in http://apidock.com/rails/ActionView/Helpers/ActiveRecordHelper/error_messages_for

Answer 3

The key is that you are using a model form, a form that displays the attributes for an instance of an ActiveRecord model. The create action of the controller will take care of some validation (and you can add more validation).

Controller re-renders new View when model fails to save

Change your controller like below:

def new
  @simulation = Simulation.new
end

def create
  @simulation = Simulation.new(simulation_params)
  if @simulation.save
    redirect_to action: 'index'
  else
    render 'new'
  end
end

When the model instance fails to save (@simulation.save returns false), then the new view is re-rendered.

new View displays error messages from the model that failed to save

Then within your new view, if there exists an error, you can print them all like below.

<%= form_for @simulation, as: :simulation, url: simulations_path do |f|  %>
  <% if @simulation.errors.any? %>
    <% @simulation.errors.full_messages.each do |message| %>
      <li><%= message %></li>
    <% end %>
  <% end %>
  <div class="form-group">
    <%= f.label :Row %>
    <div class="row">
      <div class="col-sm-2">
        <%= f.text_field :row, class: 'form-control' %>
      </div>
    </div>
  </div>
<% end %>

The important part here is that you're checking whether the model instance has any errors and then printing them out:

<% if @simulation.errors.any? %>
  <%= @simulation.errors.full_messages %>
<% end %>