what does [7.1] mean in migration

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In below ruby code, what does [7.1] mean?

class AddInitialProducts < ActiveRecord::Migration[7.1]
  def up
    5.times do |i|
      Product.create(name: "Product ##{i}", description: "A product.")
    end
  end

  def down
    Product.delete_all
  end
end
ruby-on-rails-7 ruby-on-rails ruby
1 Answers

ActiveRecord::Migration class has the method [] defined here that links to the Compatibility module.

By specifying the version of rails you are running the migration on you make sure that new functionality of the Migration class does not conflict with your migration if you upgrade rails.

There is a good explanation in a comment of the Compatibility module.

      # This file exists to ensure that old migrations run the same way they did before a Rails upgrade.
      # e.g. if you write a migration on Rails 6.1, then upgrade to Rails 7, the migration should do the same thing to your
      # database as it did when you were running Rails 6.1
      #
      # "Current" is an alias for `ActiveRecord::Migration`, it represents the current Rails version.
      # New migration functionality that will never be backward compatible should be added directly to `ActiveRecord::Migration`.
      #
      # There are classes for each prior Rails version. Each class descends from the *next* Rails version, so:
      # 7.0 < 7.1
      # 5.2 < 6.0 < 6.1 < 7.0 < 7.1
      #
      # If you are introducing new migration functionality that should only apply from Rails 7 onward, then you should
      # find the class that immediately precedes it (6.1), and override the relevant migration methods to undo your changes.
      #
      # For example, Rails 6 added a default value for the `precision` option on datetime columns. So in this file, the `V5_2`
      # class sets the value of `precision` to `nil` if it's not explicitly provided. This way, the default value will not apply
      # for migrations written for 5.2, but will for migrations written for 6.0.
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