A handler for PostgreSQL Foreign Key Constraint Exceptions to insert in your ApplicationController

application_controller.rb

class ApplicationController < ActionController::Base

  # A foreign key constraint exception from the database
  rescue_from PG::Error do |exception|
    message = exception.message
    if message.include?('foreign key constraint')
      logger.warn(message)
      # shorten the message
      message = message.match(/DETAIL: .+/).to_s
      
      redirect_to :back,
        :flash => {:error => "Whatever you tried to do - the server is unable to process your request because of a foreign key constraint. (#{message})" }
    else
      # anything else
      raise exception
    end
  end

end