Make template work for existing applications

Not every RES adopter wants to generate new app from scratch. Some of
them potentially wants to put it in the existing application.

Template can be used in that manner:
  - cd to your rails app directory
  - bin/rails app:template LOCATION=https://railseventstore.org/new
  - that's it, RES is installed and preconfigured in existing app

run_after_bundle_callbacks had to be used explicitly since if the bundle
is present (it is in exising app), the after_bundle hook won't be run.

Conditional run_bundle is also necessary to install RES after adding it
to Gemfile in existing app, so we can generate the migration and execute
it.

Determining database kind based on options only makes sense for the new
app, since it lives in generator options (e.g. --database=postgresql),
but for the existing app, we need to figure it out based on ActiveRecord
connection settings. Those are available for the generator already.
Generator options will contain default value which is "sqlite3".

Co-authored-by: Łukasz Reszke <lukaszreszke93@gmail.com>

APP_TEMPLATE

@@ -1,9 +1,10 @@
 class Profile
-  def initialize(options)
+  def initialize(options, location)
     @options = options.dup
+    @location = location
   end
 
-  private attr_reader :options
+  private attr_reader :options, :location
 
   def client
     postgres? ? "RailsEventStore::JSONClient.new" : "RailsEventStore::Client.new"
@@ -13,18 +14,22 @@ class Profile
     postgres? ? "jsonb" : "binary"
   end
 
+  def existing_app?
+    !!location
+  end
+
   private
 
   def postgres?
     database.downcase.eql?("postgresql")
   end
 
   def database
-    options.fetch(:database)
+    existing_app? ? ActiveRecord::Base.connection_db_config.adapter : options.fetch(:database)
   end
 end
 
-profile = Profile.new(options)
+profile = Profile.new(options, ENV["LOCATION"])
 
 gem "rails_event_store", "~> 2.8.1"
 
@@ -62,8 +67,12 @@ CODE
 
 route "mount RailsEventStore::Browser => '/res' if Rails.env.development?"
 
+run_bundle unless run "bundle check"
+
 after_bundle do
   rails_command 'db:create'
   generate "rails_event_store_active_record:migration --data-type=#{profile.data_type}"
   rails_command 'db:migrate'
 end
+
+run_after_bundle_callbacks if profile.existing_app?