Use call_helper to set rails config.load_defaults

In Rails, config.load_defaults loads default configuration values for a target version and all versions prior. For example, config.load_defaults 6.0 will load defaults for all versions up to and including version 6.0. So every time we upgrade rails version, we need to change config.load_defaults to the new version.

Let’s use synvert to set the config.load_defaults. For example, we want to set config.load_defaults to 6.0, we can write the following snippet:

within_file 'config/application.rb' do
  with_node node_type: 'class', name: 'Application' do
    exists = false
    with_node node_type: 'send', receiver: 'config', message: 'load_defaults', arguments: { size: 1 } do
      exists = true
      replace_with "config.load_defaults 6.0"
    end
    unless exists
      prepend "config.load_defaults 6.0"
    end
  end
end

First, the snippet searches for the file config/application.rb. Once found, it looks for class Application within that file. After locating the class, it searches for the code containing config.load_defaults and replaces it with config.load_defaults 6.0 if it exists. In cases where config.load_defaults is not found, it adds config.load_defaults 6.0 at the beginning of the class.

While it works, it becomes cumbersome when we need to upgrade rails to version 6.1 and 7.0. In such cases, we would have to manually copy and paste the snippet and modify 6.0 to 6.1 and 7.0.

To overcome this duplication issue, we introduced the concept of Helper. By doing so, we can extract the above snippet into a reusable Helper:

Synvert::Helper.new 'rails/set_load_defaults' do |options|
  rails_version = options[:rails_version]

  within_file 'config/application.rb' do
    with_node node_type: 'class', name: 'Application' do
      exists = false
      with_node node_type: 'send', receiver: 'config', message: 'load_defaults', arguments: { size: 1 } do
        exists = true
        replace_with "config.load_defaults #{rails_version}"
      end
      unless exists
        prepend "config.load_defaults #{rails_version}"
      end
    end
  end
end

This snippet defines a Helper called rails/set_load_defaults. This Helper takes an option called rails_version as input.

To utilize this Helper, we can use the call_helper API to execute the Helper and provide rails_version option as a parameter:

Synvert::Rewriter.new 'rails', 'upgrade_5_2_to_6_0' do
  call_helper 'rails/set_load_defaults', rails_version: '6.0'
end

It sets config.load_defaults to 6.0.

Synvert::Rewriter.new 'rails', 'upgrade_6_0_to_6_1' do
  call_helper 'rails/set_load_defaults', rails_version: '6.1'
end

It sets config.load_defaults to 6.1.

Helpers are extremely beneficial as they enable us to extract commonly used code from different snippets. This means that instead of repeating the same code across different snippets, we can place it in a Helper and reuse it wherever needed. This approach enhances code reusability and helps in keeping the codebase concise and organized.