ruby-on-rails - Rails 5 Postgresql has_many 通过关联

Question

在我的应用中，我有 3 个模型

class User < ApplicationRecord
 has_many :specialties, class_name: "Specialty",
                        foreign_key:"teacher_id",
                        dependent: :destroy
 has_many :subjects, through: :specialties, source: :subject

class Specialty < ApplicationRecord
 belongs_to :teacher, class_name: "User"
 belongs_to :subject, class_name: "Subject"

class Subject < ApplicationRecord
 has_many :teachers, through: :specialties, source: :teacher
 has_many :specialties, class_name: "Specialty",
                        foreign_key:"subject_id",
                        dependent: :destroy

在我的 rails 控制台中，以前当我使用 SQLite 时， 没有问题 user = User.first 然后 user.subjects 会返回所有用户的主题。 但是，在我更改为 PostgreSQL 之后，它返回给我

ActiveRecord::StatementInvalid: PG::UndefinedFunction: ERROR:  operator does not exist: integer = character varying
LINE 1: ...ects" INNER JOIN "specialties" ON "subjects"."id" = "special...
                                                             ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
: SELECT "subjects".* FROM "subjects" INNER JOIN "specialties" ON "subjects"."id" = "specialties"."subject_id" WHERE "specialties"."teacher_id" = $1

有什么办法可以返回 user.subjects 吗？ 我注意到在遵循 following 和 followed 的 rails 教程模型时没有发生此问题。 (推特模型)

谢谢

编辑，specialties 不是整数列，我会更改它并返回，非常感谢!

Answer 1

因为你的错误已经给出了提示:

HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

Postgresql 对要比较的列的类型有严格的限制。

看起来你有外键 subject_id ， teacher_id 是 string 类型。所以当你比较integer类型的主键和string类型的外键时，就会出现这个错误。

您可以更改列类型以避免此错误:

change_column :specialties, :subject_id, :integer
change_column :specialties, :teacher_id, :integer