我有一个Book模型，我有一个User模型。我正在尝试在author_id表中创建一个名为books的新列，该列实际上是users表的外键。

我关注了this article by Joshua Frankel。但是我遇到了以下错误

  

activeRecord :: InvalidForeignKey：PG :: ForeignKeyViolation：错误`

我正在使用Rails4.2

db / migrate / 20191112111409_add_author_to_books.rb：

class AddAuthorToBooks < activeRecord::Migration
  def up
    add_reference :books,:author,references: :users,index: false
    commit_db_transaction
    add_index :books,:author_id,algorithm: :concurrently
    add_foreign_key :books,:users,column: :author_id
  end

  def down
    remove_column :books,:author_id
  end
end

app / models / book.rb：

class Book < activeRecord::Base
  belongs_to :author,class_name: :User,foreign_key: :author_id
end

app / models / user.rb：

class User < activeRecord::Base
  has_many :books,foreign_key: :author_id
end

Rails控制台中的错误：

> user = FactoryBot.create(:user)
> book = FactoryBot.create(:book)
> book.author = user
> book.save
activeRecord::InvalidForeignKey: PG::ForeignKeyViolation: ERROR:  insert or update on table "books" violates foreign key constraint "fk_rails_13be98de92"
DetaIL:  Key (author_id)=(1) is not present in table "users".
: UPDATE "books" SET "author_id" = $1,"updated_at" = $2 WHERE "books"."id" = $3
from ~/.ruby-gemset/ruby/2.3.0/gems/activerecord-4.2.11.1/lib/active_record/connection_adapters/postgresql_adapter.rb:602:in `exec_prepared'
Caused by PG::ForeignKeyViolation: ERROR:  insert or update on table "books" violates foreign key constraint "fk_rails_13be98de92"
DetaIL:  Key (author_id)=(1) is not present in table "users".

from ~/.ruby-gemset/ruby/2.3.0/gems/activerecord-4.2.11.1/lib/active_record/connection_adapters/postgresql_adapter.rb:602:in `exec_prepared'
>