问题描述
说明
我正在使用 Ruby 2.6.6 和 Ruby on Rails 6.0.3.2 。
模型
- 书
- 作者
协会
一本书属于作者。
作者有很多书。
目标
我做了什么
我按照以下顺序创建并运行了3个迁移:
AddAuthorToBooks
class AddAuthorToBooks < ActiveRecord::Migration[6.0]
def change
add_column :books,:author,:string
end
end
DropAuthors
class DropAuthors < ActiveRecord::Migration[6.0]
def change
drop_table :authors do |t|
t.string "full_name",null: false
t.timestamps null: false
end
end
end
RemoveAuthorForeignKeyFromBooks
class RemoveAuthorForeignKeyFromBooks < ActiveRecord::Migration[6.0]
def change
remove_foreign_key :books,:authors
end
end
架构
不幸的是,在运行迁移之前,我没有架构。 (我尝试签出较旧的提交,但是架构文件顽固地拒绝更改。)
这是当前版本:
ActiveRecord::Schema.define(version: 2020_08_11_125724) do
create_table "books",force: :cascade do |t|
t.string "title"
t.text "description"
t.string "cover_url"
t.decimal "price"
t.datetime "created_at",precision: 6,null: false
t.datetime "updated_at",null: false
t.integer "author_id",null: false
t.string "author"
t.index ["author_id"],name: "index_books_on_author_id"
end
create_table "books_genres",id: false,force: :cascade do |t|
t.integer "book_id",null: false
t.integer "genre_id",null: false
t.index ["book_id","genre_id"],name: "index_books_genres_on_book_id_and_genre_id"
t.index ["genre_id","book_id"],name: "index_books_genres_on_genre_id_and_book_id"
end
create_table "genres",force: :cascade do |t|
t.string "name"
t.datetime "created_at",null: false
end
create_table "reviews",force: :cascade do |t|
t.string "username"
t.decimal "rating"
t.text "body"
t.integer "book_id",null: false
t.datetime "created_at",null: false
t.index ["book_id"],name: "index_reviews_on_book_id"
end
add_foreign_key "reviews","books"
end
问题
author表已被删除,add_foreign_key "books","authors"
(我认为是这样)也消失了,但是author_id
固执地保留在books
表中。
我正在计划什么
我考虑过通过另一次迁移删除这2列,但是我不知道这是否会...
解决方法
关于异常的`schema.rb`
行为
我尝试签出一个较旧的提交,但是架构文件顽固地拒绝更改。
这很不寻常。请确保使用git跟踪db/schema.rb
文件。如果被跟踪,则没有理由为什么签出较旧的提交不应将其返回到较早的状态。此时,您应该能够:
$ rails db:drop
$ rails db:create
$ rails db:schema:load
...将旧模式加载到数据库中。然后,您应该能够使用git返回最新的代码,并在创建较旧模式的日期之后运行挂起的迁移。
关于实现此目的的更清洁方式
在编写下面的迁移之前,第一步是删除在Book
类中编写的所有现有关系。例如:
# app/models/book.rb
class Book < ApplicationRecord
# The line below should be deleted! Otherwise,it will probably interfere
# with the `book.update!(author: ...)` line in the migration.
belongs_to :author
end
由于所有相关的迁移,我都将它们写在单个文件中。对我来说,这看起来像:
class MoveAuthorToBooks < ActiveRecord::Migration[6.0]
class Author < ApplicationRecord
end
class Book < ApplicationRecord
end
def up
# Start by adding a string column.
add_column :books,:author,:string
# Let's preserve existing author names.
Book.all.each do |book|
author = Author.find(book.author_id)
book.update!(author: author.name)
end
# Now that the names have been moved to the books table,we don't
# need the relationship to `authors` table anymore. This should
# also delete any related foreign keys - manual foreign key deletion
# should not be required.
remove_column :books,:author_id
# Alternative: If you'd created the `authors_id` column using the
# `add_reference` command,then it's probably best to use the opposite
# `remove_reference` command.
#
#remove_reference :books,index: true,foreign_key: true
# Finally,remove the `authors` table.
drop_table :authors
end
def down
# This can be technically be reversed,but that'll need some more code that
# reverses the action of the `up` function,and it may not be needed.
raise ActiveRecord::IrreversibleMigration
end
end