SQL如何只获得1个真值

Question

我正在构建一个应用程序，我需要能够唱主唱 我需要防止 2 位老师共享特定班级的领导头衔

class CreateClassroomTeachers < ActiveRecord::Migration[5.2]
  def change
    create_table :classroom_teachers do |t|
      t.belongs_to :classroom
      t.belongs_to :teacher
      t.boolean    :lead, default: false
    end
    add_index :household_people, [:classroom_id, :teacher_id], unique: true
    # Only one teacher in a classroom can be lead 
  end
end

我的模型中有这个

class ClassroomTeacher < ApplicationRecord
  belongs_to :classroom
  belongs_to :teacher


  validate :only_one_is_lead_teacher

  def only_one_is_lead_teacher
    if lead
      if ClassroomTeacher.where(classroom_id: classroom_id, lead: true).count > 0
        errors.add(:lead, "There can only be one (1) lead teacher per classroom")
      end
    end
  end
end

这方面的问题是，在创建时我可以让 2 位或更多位老师担任领导

感谢您的帮助

Answer 1

有几种方法可以通过约束、触发器等来实现这一点 - 取决于您各自的数据库服务器支持什么。

应该至少在 Postgres 中起作用（即使它可能有点 hacky）是在 %i[classroom_id lead] 上设置唯一索引并确保 lead 是 true 或 @ 987654324@。这应该可行，因为 Postgres 将 NULL 值视为不同的，这意味着如果多个 NULL 值存储在具有唯一性约束的列中，它不会报错。

---

如果您想在代码中解决它（我个人不建议这样做，因为您的数据库可能会被您的代码以外的东西访问，甚至您的代码也可以解决它，例如直接写入数据库而不是使用ActiveRecord 的高级方法），这是我过去的做法：

class ClassroomTeacher < ActiveRecord::Base
  before_save :ensure_only_one_lead_teacher

  private

  def ensure_only_one_lead_teacher
    # We don't have to do this unless the record is the one who should be the (new) lead.
    return unless lead?

    # Set all other records for the class room to lead = false.
    self.class.where(classroom_id: classroom_id).update_all(lead: false)

    # Now if the record gets persisted, it will be the only one with lead = true.
  end
end

一个可能稍微“正确”的方法是在记录被持久化后确保唯一性：

class ClassroomTeacher < ActiveRecord::Base
  after_commit :ensure_only_one_lead_teacher

  private

  def ensure_only_one_lead_teacher
    # We don't have to do this unless the record is the one who should be the (new) lead.
    return unless lead?

    # Set all other records for the class room to lead = false. Note that we now have to exclude
    # the record itself by id because it has already been saved.
    self.class.where.not(id: id).where(classroom_id: classroom_id).update_all(lead: false)
  end
end

Answer 2

根据迁移，模型的属性是 ClassroomTeacher：classroom_id、teacher_id、lead

考虑将教师添加到课堂：

/控制器文件

def create
  ClassroomTeacher.create(teacher_id: data1, classroom_id: data2, lead: data3)
end

具有理想值的可能样本数据是：

id classroom_id teacher_id lead
1     1             3       false
2     2             4       true
3     1             2       false
4     1             5       true

现在您需要避免任何新教师被添加到课堂作为领导。模型验证代码可以是

validate :only_one_is_lead_teacher

def only_one_is_lead_teacher
  if self.lead
    class_obj = ClassroomTeacher.where(classroom_id: self.classroom_id, lead: true).first
    if class_obj.present?
      self.errors.add(:lead, "There can only be one (1) lead teacher per classroom")
      return false
    end
  end
end