How I write to readonly databases in tests

We have a Rails app with two databases. There’s the primary database, which is all fine and good. Normal CRUD (create/read/update/delete) life. We have a secondary database with a readonly connection. CRUD without the CUD. Just the reads.

This is all fine and good, too. It’s easy enough to define our records to be readonly. The username/password provided in database.yml establishes a readonly connection at the database level. We reinforce the database level permissions within the Ruby runtime using the ActiveRecord::Base#readonly? convention.

class SecondaryDatabaseRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to(:secondary_database)

  def readonly?
    true
  end
end

class Person < SecondaryDatabase
end

But what happens in tests, where we need to write test data? On the database side, the user in database.yml can have a read connection. But what about the Ruby side?

One approach is to put this knowledge into the class.

def readonly?
  !Rails.env.test
end

With that little change, I can write Person.create!(name: "Jake Peralta") in my tests.

I don’t like this.

Writing to a readonly database should feel surprising. The readonly record should be naive of the environment. It should just read data.

I want writing to the readonly database to stick out like a sore thumb. The code I want to write in my tests is this:

Person.mutate_test_data do
  Person.create!(name: "Jake Peralta")
end

So instead of changing the production system, I throw this little concern in test_helper.rb

SecondaryDatabaseRecord.concerning "MutateTestData" do
  MUTATE_TEST_DATA_CALLS = []

  class_methods do
    def mutate_test_data
      MUTATE_TEST_DATA_CALLS.push(nil)
      yield
    ensure
      MUTATE_TEST_DATA_CALLS.pop
    end
  end

  included do
    undef :readonly?
    define_method :readonly? do
      MUTATE_TEST_DATA_CALLS.size.zero?
    end
  end
end

This little stack allows nesting calls to mutate_test_data. Within this eyesore of a block, mutating test data is allowed. Outside of the block, the readonly reality is relied upon, just like production.

I like this approach to tests assuming the posture of a servant toward the production system. It reinforces the reality relied upon for stability in the real world.

Published: 2021-10-07

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