Unfortunately, we have a deadline approaching, so we have to go with the fastest solution. The quickest we could think of is to write a script to directly transfer the data from the old backed up database to our new database – thankfully the database structures are very similar. Our rake task will connect to the old database get the data we want, store it in a variable and then connect to the new database and import the data.

Database setup

Before I touched code, I dusted off an old backup of the database before the refactored work. Created a new Postgres database with the data; for this example, we will call this “old_app_development”, and we will call out current one “app_development.”

When the rake task boots up, it is connected to the database specified in the config file. So I needed a way to create a new database connection, we can archive this using the establish connection method on ActiveRecord::ConnectionHandling class.

The rake task with the connection snippet looks like this

desc "Import old data in to new database"
task import_data: :environment  do |t, args|

  ActiveRecord::Base.establish_connection(
    adapter:  "postgresql",
    host:     "localhost",
    database: "old_app_development"
  )

  puts ActiveRecord::Base.connection.current_database
end

ActiveRecord::Base.connection.current_database to confirm which connection is currently connected.

Upon running the rake task bin/rake import_data, “old_app_development” shows up successfully.

Config file

Now we need to connect back to our “app_development” in order for the data to be transferred. However, I’m sure you can see a problem emerging. I’ll have to copy the contents of the database.yml. In an efforts to not repeat myself and duplicate the connection settings I can use the database config file to specify the connection.

ActiveRecord::Base.establish_connection(:development)

That looks a lot cleaner and reads much better, for that reason I’ll add the connection settings of “old_app_development” to the config and revise the rake task.

default: &default
  adapter: postgresql
  encoding: unicode

development:
  <<: *default
  database: app_development

old_development:
  <<: *default
  database: old_app_development

test:
  <<: *default
  database: app_test

production:
  <<: *default
  url: <%= ENV['DATABASE_URL'] %>

We can then revise the rake task using the new method to connect to both databases.

desc "Import old data in to new database"
task import_data: :environment  do |t, args|

  ActiveRecord::Base.establish_connection(:old_app_development")

  puts ActiveRecord::Base.connection.current_database

  ActiveRecord::Base.establish_connection(:app_development")

  puts ActiveRecord::Base.connection.current_database
end

Tip

A really quick tip – if you like me have removed some models and using active record is no longer an option, I suggest using straight up SQL and maybe have something like this…

old_development = ActiveRecord::Base.establish_connection(:old_development).connection

old_records = old_development.execute("SELECT * from records").to_a

You should then be able to loop through the ‘old_records’ as this is retained in memory and import them in the new database.

Happy Coding!

Photo by Jacob Miller on Unsplash