Question

我有这样的模特：

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：

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

时

我收到错误：

== 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在架构中发现了一些混乱...... :(

至于现在我试着做：

rake db：drop然后rake db：create并rake db：migrate，然后出现此错误。

Answer 1

首先，检查您的迁移顺序。您似乎在创建表subject_items之前尝试创建引用。

首先应创建学生表，然后创建subject_items表，然后添加参考。或者在创建表格时自己添加引用列，这些都是add_reference does。

此外，您有两个从Student到SubjectItem的关系。

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

您需要建立第一个关系，以便ActiveRecord能够首先识别正确的关系，然后进行查询以进行迁移。想想如果您调用@student.subject_items，将会生成的查询。它会通过第一个关系还是第二个关系。

你的关系应该是ex：

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

Rails db：migrate关系不存在

1 个答案: