Rails db:迁移关系不存在 [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

以及迁移:

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

但是当我这样做 rake db:migrate

我得到错误:

== 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，并且可能在架构中造成了一些混乱.... :(

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

至于现在我尝试制作:

rake db:drop 然后 rake db:create 和 rake db:migrate，然后出现此错误.

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.您似乎正在尝试在创建表 subject_items 本身之前创建引用.

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，然后创建您的 subject_items 表，然后添加参考.或者在创建表时自己添加参考列，这就是 add_reference 确实.

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.

另外，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

您需要第一个关系，以便 ActiveRecord 能够首先识别正确的关系以进行查询，然后再进行迁移.例如，如果您调用 @student.subject_items，请考虑将生成的查询.它会通过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.

你的关系应该是前任:

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

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