Render model validation errors inline by adding a simple presenter class

_form.html.erb

<%= form_with(model: post) do |form| %>
<% error_handler = ::InlineErrorHandler.new(post, self) %>  # Initialize the error handler presenter here, or consider doing this in the controller
  <div>
    <%= form.label :title, style: "display: block" %>
    <%= form.text_field :title %>
    <%= error_handler.error_message_for(:title) %>  # Display inline error for the title field here
  </div>

  <div>
    <%= form.label :body, style: "display: block" %>
    <%= form.text_area :body %>
    <%= error_handler.error_message_for(:body) %>  # Display inline error for the body field here
  </div>

  <div>
    <%= form.submit %>
  </div>
<% end %>

application.css

 .error {
   color: red;
 }

inline_error_handler.rb

class InlineErrorHandler
  def initialize(model, view_context)
    @model = model
    @view_context = view_context
  end

  def error_message_for(attribute)
    if errors[attribute].present?
      @view_context.content_tag(:span, errors[attribute].join(", "), class: "error")
    end
  end

  private

  attr_reader :model, :view_context

  def errors
    @errors ||= model.errors
  end
end

inline_error_handler_spec.rb

RSpec.describe InlineErrorHandler do
  let(:model) { Post.new }
  let(:view_context) { ApplicationController.helpers }
  let(:inline_error_handler) { described_class.new(model, view_context) }

  describe '#error_message_for' do
    context 'when there is an error' do
      before do
        model.valid?
      end

      it 'returns the error message' do
        expect(inline_error_handler.error_message_for(:title)).to eq("<span class=\"error\">can&#39;t be blank</span>")
      end
    end

    context 'when there is no error' do
      it 'returns nil' do
        expect(inline_error_handler.error_message_for(:title)).to be_nil
      end
    end
  end
end