What’s does the [5.0] in Rails 5’s ActiveRecord::Migration mean?

问题

A migration I created in a Rails 5 application had 5.0 passed into a method:

class CreateVariableKeys < ActiveRecord::Migration[5.0]
  ...
end

I would like to know what the [5.0] means.

回答1:

It is a class method of ActiveRecord::Migration and is defined here.

It allows us to select the version of migrations we wish to use between 4.2 and 5.0. The method throws a:

"Unknown migration version ... "

error if an incompatible version is passed as an argument.

Production ready versions of ActiveRecord don’t have that method so it should go away as soon as Rails 5 goes out of beta.

回答2:

This blog has more info too

It seems to be there so that you don't have to upgrade old migrations, when moving from rails 4 to rails 5. (There are some small changes in the migrations API).

来源：https://stackoverflow.com/questions/35302197/what-s-does-the-5-0-in-rails-5-s-activerecordmigration-mean