projects_controller.rb

class ProjectsController < ApplicationController
  def create
    @project = Project.new(project_params)

    # authorization checking - this depends on the library you're using
    # this example is for cancan
    authorize! :create, @project

    if @project.save
      redirect_to projects_path, notice: 'Yay!'
    else
      # Do bad stuff
      # This will re-render the form, and in theory show the error messages on the @project object
      render :new
    end
  end
end