Rails 4 Has_many：通过加入关联与选择

Question

我正在尝试将 rails 3.0 应用程序升级到 rails 4.0。我注意到的一种行为是模型之间的关系停止工作。

假设我们有以下模型：

class Student < ActiveRecord::Base
  has_many :teacher_students
  has_many :teachers, :through => :teacher_students, :select => 'teacher_students.met_with_parent, teachers.*'

  # The Rails 4 syntax
  has_many :teachers, -> { select('teacher_students.met_with_parent, teachers.*') }, :through => :teacher_students

end

class Teacher < ActiveRecord::Base
  has_many :teacher_students
  has_many :students, :through => :teacher_students, :select => 'teacher_students.met_with_parent, students.*'
end

class TeacherStudent < ActiveRecord::Base
  belongs_to :teacher
  belongs_to :student
  # Boolean column called 'met_with_parent'
end

现在我们可以这样做了：

teacher = Teacher.first
students = teacher.students
students.each do |student|
  student.met_with_parent     # Accessing this column which is part of the join table
end

这适用于 Rails 3.0，但现在在 Rails 4.0 上我得到了Unknown column 'met_with_parent' in 'field list' 我相信 Rails 4 正在努力变得聪明，而不是加载整个给定的连接表。

Answer 1

我个人会推荐以下方法，使用范围：

class Student < ActiveRecord::Base
  has_many :teacher_students
  has_many :teachers, :through => :teacher_students
end

class Teacher < ActiveRecord::Base
  has_many :teacher_students
  has_many :students, :through => :teacher_students

  scope :met_with_parent, -> { joins(:teacher_students).where('teacher_students.met_with_student = ?', true) }
end

class TeacherStudent < ActiveRecord::Base
  belongs_to :teacher
  belongs_to :student
end

然后您可以执行以下操作：

Teacher.first.students.met_with_parent

这允许您在需要时维护关系和过滤。