Rails 6 parallel minitest with sqlite3 :memory: databases.

sqlite_test_db_loader.rb

#### evals the schema into the current process
class SqliteTestDbLoader
  
  # assumes your schema is generated for MySQL
  # tweak for Postgres!
  MYSQL_REPLACEMENTS = {
    /ENGINE=InnoDB DEFAULT CHARSET=[a-z0-9]*/ => '',
    /, collation: "[^"]*"/ => ''
  }

  class << self
    def reload!(context)
      new.reload!
    ensure
      # puts "Reloaded Schema: #{context}"
    end
  end

  # grab schema from fs
  def extract
    File.read(
      Rails.root.join(
        "db", "schema.rb"))
  end

  # Get rid of non-standard SQL unimportables
  def transform(schema)
    MYSQL_REPLACEMENTS
      .reduce(schema) do |res, replacement|
        res.gsub(*replacement)
    end
  end

  # load into sqlite's :memory: db
  def load(schema)
    eval(schema)
  end

  def reload! ; load(transform(extract)) ; end
end

# Always load schema into :memory: after a new connection
module SqlitePostConnectionLoad
  def establish_connection(*args, &block)
    super
    connection
      .migration_context
      .needs_migration? &&
      SqliteTestDbLoader.reload!("#{self.class.name}#establish_connection")
  end
end

# "rails/test_help" needs a connection early on in the test_helper load process
module CheckPendingSoftly
  def check_pending!(conn = ActiveRecord::Base.connection)
    conn
      .migration_context
      .needs_migration?
      .yield_self do |needs_migration|
      needs_migration &&
        SqliteTestDbLoader.reload!("#{self.class.name}#check_pending!")
    end
  end
end

module PreserveParallelMemoryDatabase
  def create_and_load_schema(i, env_name:)
    if ActiveRecord::Base
        .configurations
        .configs_for(env_name: env_name)
        .to_s.match(/:memory:/m)
      puts "Memory database found. Skipping db creation"
      ActiveRecord::Base.establish_connection(Rails.env.to_sym)
      return
    end
    super
  end
end

ActiveSupport.on_load(:active_record) do
  ActiveRecord::Migration.singleton_class.prepend CheckPendingSoftly
  ActiveRecord::TestDatabases.singleton_class.prepend PreserveParallelMemoryDatabase
end

test_helper.rb

ENV['RAILS_ENV'] ||= 'test'
require_relative '../config/environment'

# must load BEFORE rails/test_help
require_relative './support/sqlite_test_db_loader.rb'
require 'rails/test_help'
require "minitest/rails"

# This assumes you're sharing config between unit/integration
module TestSetup
  extend ActiveSupport::Concern

  included do
    # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
    fixtures :all

    # Use Jest-like CI flag
    parallelize(workers: ENV['CI'] ? 1 : :number_of_processors, with: :processes)

    # Make sure that you reload the sqlite when starting processes
    parallelize_setup do
      # slightly more efficient than a direct call to establish_connection
      ActiveRecord::Migration.check_pending!
    end
  end
end

class ActiveSupport::TestCase
  include TestSetup
end

class ActionDispatch::IntegrationTest
  include TestSetup
end