Rails db:migrate关系不存在 [英] Rails db:migrate relation does not exist

问题描述

我有这样的模型:

学生:

class Student < ActiveRecord::Base
  has_many :participations, dependent: :destroy
  has_many :subject_item_notes, dependent: :destroy
  has_many :payments, dependent: :destroy
  has_many :subject_items, dependent: :destroy, through: :participations
  has_many :subject_items, dependent: :destroy

  validates :first_name, :last_name, presence: true

  accepts_nested_attributes_for :subject_items
end

和subject_item:

and subject_item:

class SubjectItem < ActiveRecord::Base
  has_many :participations, dependent: :destroy
  has_many :students, through: :participations
  has_many :subject_item_notes
  belongs_to :teacher
  belongs_to :student


  validates :title, presence: true

  scope :not_assigned_or_assigned_to_teacher, -> (teacher) { where('teacher_id IS ? or teacher_id = ?', nil, teacher) }
end

迁移:

class AddStudentToSubjectItems < ActiveRecord::Migration
  def change
    add_reference :subject_items, :student, index: true
  end
end

但是当我做rake db:migrate

我收到错误消息:

== 20151121045103 AddStudentToSubjectItems: migrating =========================
-- add_reference(:subject_items, :student, {:index=>true})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::UndefinedTable: ERROR:  relation "subject_items" does not exist
: ALTER TABLE "subject_items" ADD "student_id"

在恐慌中，我多次重建数据库，并可能使模式混乱…….::

In panic I was rebuilding the db many times and probobly made some mess in the schema .... :(

目前，我尝试制作:

rake db:drop然后rake db:create和rake db:migrate，然后发生此错误.

rake db:drop then rake db:create and rake db:migrate, and then this error ocurs.

推荐答案

首先，检查您的迁移顺序.看来您正在尝试创建表subject_items本身之前的引用.

First, check your sequence of migrations. It seems that you are trying to create a reference before creating the table subject_items itself.

您应该首先创建学生表，然后创建subject_items表，然后添加参考.或者，您也可以在创建表时自行添加引用列，这些表就是 有.

You should first create students table then your subject_items table then add the reference. Or alternatively add the reference columns yourself when creating the tables which is all what add_reference does.

此外，您有两个从Student到SubjectItem的关系.

Also, You have two relations from Student to SubjectItem.

has_many :subject_items, dependent: :destroy, through: :participations
has_many :subject_items, dependent: :destroy

您需要先建立关系，以便ActiveRecord能够先标识正确的关系，然后再进行查询并进行迁移.想想如果您调用@student.subject_items将会生成的查询.它会经历第一个关系还是第二个关系.

You need to your first relation in order for ActiveRecord to be able to identify the correct one first for querying then for your migration. Think of the query that will be generated if you called @student.subject_items for example. Will it go through the first relation or the second one.

您的关系应该是:

has_many :participation_subject_items, dependent: :destroy, through: :participations
has_many :subject_items, dependent: :destroy

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