Add Determine The Configured Primary Key Type as a Rails TIL

rails/determine-the-configured-primary-key-type.md

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+# Determine The Configured Primary Key Type
+
+I noticed an interesting helper function in the database migration generated by
+`bin/rails active_storage:install`.
+
+```ruby
+class CreateActiveStorageTables < ActiveRecord::Migration[8.0]
+  def change
+    # Use Active Record's configured type for primary and foreign keys
+    primary_key_type, foreign_key_type = primary_and_foreign_key_types
+
+    # ...
+  end
+
+  private
+
+    def primary_and_foreign_key_types
+      config = Rails.configuration.generators
+      setting = config.options[config.orm][:primary_key_type]
+      primary_key_type = setting || :primary_key
+      foreign_key_type = setting || :bigint
+      [ primary_key_type, foreign_key_type ]
+    end
+end
+```
+
+The `primary_and_foreign_key_types` method looks in the generators config for
+the ORM (`:active_record`) to determine the configured `:primary_key_type`. By
+default this will return `nil`. This method then uses `:primary_key` as a
+fallback value which will be `bigint`. That's why the `foreign_key_type` falls
+back to `:bigint`.
+
+If desired, this can be manually configured in `config/application.rb` like
+shown in the [ActiveRecord Migrations
+docs](https://guides.rubyonrails.org/active_record_migrations.html#enabling-uuids-in-rails).