如何对 add_column 进行可逆迁移

Question

此迁移工作完美

class CreateCourses < ActiveRecord::Migration[6.0]
  def change
    create_table :courses do |t|
      t.references :user, null: false, foreign_key: true
      t.string :title, limit: 500, null: false
    end    
    reversible do |dir|
      dir.up do
        add_column :users, :courses_count, :integer, null: false, default: 0
      end
      dir.down do
        remove_column :users, :courses_count
      end
    end
  end
end

但是，当我运行这个它添加courses_count，但它并没有删除它rails db:rollback。可逆迁移的重点不是def change它会反转的方法add_column吗？

class CreateCourses < ActiveRecord::Migration[6.0]
  def change
    create_table :courses do |t|
      t.references :user, null: false, foreign_key: true
      t.string :title, limit: 500, null: false
     
      add_column :users, :courses_count, :integer, null: false, default: 0
    end    
  end
end

% rails db:migrate
== 20200815181736 CreateCourses: migrating ====================================
-- create_table(:courses)
-- add_column(:users, :courses_count, :integer)
   -> 0.0037s
   -> 0.0258s
== 20200815181736 CreateCourses: migrated (0.0259s) ===========================

% rails db:rollback
== 20200815181736 CreateCourses: reverting ====================================
-- drop_table(:courses)
   -> 0.0060s
== 20200815181736 CreateCourses: reverted (0.0078s) ===========================

% rails db:migrate 
== 20200815181736 CreateCourses: migrating ====================================
-- create_table(:courses)
-- add_column(:users, :courses_count, :integer)
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DuplicateColumn: ERROR:  column "courses_count" of relation "users" already exists

Answer 1

add_column 需要在create_table块外

def change
  create_table :courses do |t|
    t.references :user, null: false, foreign_key: true
    t.string :title, limit: 500, null: false   
  end    

  add_column :users, :courses_count, :integer, null: false, default: 0
end