Rails 4 Has_many：通過加入關聯選擇

Question

我正在嘗試將rails 3.0應用升級到rails 4.0。我注意到的一種行爲是模型之間的關係停止工作。

假設我們有以下型號：

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 

現在，我們能夠做到：

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 

這個工作對Rails的3.0，但現在on Rails的4.0我收到Unknown column 'met_with_parent' in 'field list'我相信Rails的4試圖變得聰明，而不是加載整個給定的連接表。

Answer 1

我個人推薦以下方法，使用範圍：

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 

然後你就可以做到以下幾點：

Teacher.first.students.met_with_parent 

這使您可以在需要時維持關係和過濾器。