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
这篇关于Rails db:迁移关系不存在的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,WP2