问题描述
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