Add Migrations to a Rails Engine's Install Generator

Question

In my rails 5 engine I want to include the installation of the engine's migrations with the custom install generator I have at:

myengine/lib/generators/myengine/install_generator.rb

This generator currently looks like this:

module Myengine
  module Generators

    class InstallGenerator < Rails::Generators::Base
      source_root File.expand_path("../../templates", __FILE__)

      desc "Creates a Myengine initializer and copys template files to your application."

      def copy_initializer
        template "myengine.rb", "config/initializers/myengine.rb"
      end

    end
  end
end

When I add the engine to a rails app, instead of having to call:

rails g myengine:install

then

rails myengine:install:migrations

How can I add the creation of those migrations to the custom generator?

Answer 1

Fun question! And one that I hope I can answer. Let's say you have an engine named "Buttafly" and a generator living at:

#lib/generators/buttafly/install/install_generator.rb

At the top of your generator, require the 'date' library like so:

require 'date'

Then in the body of your generator, add a method definition like:

  def copy_migrations

    # get an array of the migrations in your engine's db/migrate/ 
    # folder:

    migrations = Dir[Buttafly::Engine.root.join("db/migrate/*.rb")]
    migrations.each_with_index do |migration, i|

    # The migrations will be created with the same timestamp if you 
    # create them all at once. So if you have more than one migration 
    # in your engine, add one second to the second migration's file
    # timestamp and a third second to the third migration's timestamp 
    # and so on:

    seconds = (DateTime.now.strftime("%S").to_i + i).to_s
    seconds = seconds.to_s.length == 2 ? seconds : "0#{seconds}"
    timestamp = (DateTime.now.strftime "%Y%m%d%H%M") + seconds

    # get the filename from the engine migration minus the timestamp:
    name = migration.split("/").split("_").last

    # See if a the name of your engine migration is already in your
    # host app's db/migrate folder:

    if Rails.root.join("db/migrate/*#{name}").exist?

      # do nothing:
      puts "Migration #{name} has already been copied to your app"
    else

      # copy your engine migration over to the host app with a new 
      # timestamp:
      copy_file m, "db/migrate/#{timestamp}_buttafly_#{name}"
    end
  end
end