如何为add_column进行可逆迁移

如何解决如何为add_column进行可逆迁移

此迁移效果很好

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,default: 0
      end
      dir.down do
        remove_column :users,:courses_count
      end
    end
  end
end

但是，当我运行它时，它会添加courses_count，但是不会在rails db:rollback上将其删除。使用def change方法进行可逆迁移的目的不是反转add_column吗？

class CreateCourses < ActiveRecord::Migration[6.0]
  def change
    create_table :courses do |t|
      t.references :user,null: false
     
      add_column :users,default: 0
    end    
  end
end

% rails db:migrate
== 20200815181736 CreateCourses: migrating ====================================
-- create_table(:courses)
-- add_column(:users,: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,: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

解决方法

add_column needs to be outside create_table块

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,default: 0
end