Idempotent Seeds

You Reap What You Sow rake db:seed

seeds

When working with database based applications more often than not, some seed data needs to be created. For example, I’ve used seed data to create an admin_user and a few dummy_users so the site can be used upon a fresh install. Some other common cases include things like, list of names of countries/states, list of user roles, list of permissions etc.
This post is about some ideas to manage seed data in a Ruby on Rails project.

In a Rails application seeds can be put at two places

  • db/seeds.rb [Standard practice]
  • db/migrate/232422_some_migration.rb [*its a possibility]

db/seeds.rb is the obvious place for seeds. Don’t mess around with it unless absolutely necessary.

Some points to keep in mind when sowing seeds:

  • Handling large seed data
    • Split seed files into multiple files clearly indicating your objects and business logic. A sample might look like
              |-app
                |-db
                  -seeds.rb
                  |-seeds
                    -user.rb
                    -roles.rb
                    -plans.rb
    •  Inside seeds.rb require sub seed-files as
      require 'seeds/user.rb'
      require 'seeds/roles.rb'
      require 'seeds/plans.rb'
    • If number of seeds for a business object is large, consider loading it from yaml files. Keeping business logic separate from data is a good practice.
  • Idempotent seeds
    • Idempotent,  a mathematical term that means that something remains the same when some operation is applied to it, using itself as input.
    • For example, multiplication by identity element  5 X 1 = 5.
    • Having idempotent seed files means, they can be run multiple times without causing any issue.
    • Instead of creating or updating objects blindly, it first tries to find the object and check its state.
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temporary Files in Ruby

Working with Ruby on Rails applications, many a times  such as in case of file upload services, generating/processing csv data, uploading data to external services like amazon there is a need to create temporary files.
A very common solution is to create an usual file object and delete it later. Imagine a scenario where you had created a large data file (say a 2GB) for temporary usage and forgot to delete it.

The Solution… Ruby Tempfile Class

Tempfile is a ruby utility class for managing temporary files. The class can be used to create temporary files. The file is generated with a unique name each time and is garbage collected when it goes out of scope. This save you the trouble to have to remove them explicitly.
Since explicitly temporary deleting files is good idea you can still do it with Tempfile object, Tempfile#unlink .
All actions on a File object are also valid on a Tempfile object, hence no loss of functionality.

Creating a using a temporary file with Tempfile class

For complete documentation of class ref Ruby Tempfile .