ActiveRecord has_one具有动态范围

Question

我有这个

class Student
  has_many :assignments
end

class Assignment
  belongs_to :student
  belongs_to :book
end

class Book
  has_many :assignments
end

要找到学生的“哈利·波特”作业，我正在这样做

@student.assignments.find_by_book_id(Book.find_by_name('Harry Potter'))

有没有办法使它成为Student模型的has_one关联？

这样的东西（这里是伪代码）

class Student
  has_many :assignments
  has_one  :harry_potter_assigment, -> { where book_id: Book.find_by_name('Harry Potter') }, class: 'Assignment'
end

如果不使用has_one ，还有什么更好的方法？

Answer 1

如果我没记错的话，以下方法应该有效

#student.rb
class Student
  has_many :assignments
  has_one :harry_potter_assigment, -> { where(book_id: Book.find_by_name('Harry Potter').id }, class_name: "Assignment"
end

Answer 2

您可以尝试这样定义它：

class Student < ActiveRecord::Base
  has_many :assignments

  def assignments_named(book_name)
    self.assignments.join(:book).where("books.name=?",name)
  end
end

现在查询：

student = Student.first
student.assignments_named('Harry Potter')

产生以下SQL：

SELECT "assignments".* FROM "assignments"
INNER JOIN "books" ON "books"."id" = "assignments"."book_id"
WHERE "assignments"."student_id" = 1 AND (books.name='Harry Potter')

它不是has_one关系。 对于给定的学生和书“哈利·波特”，可能有零个，一个或多个作业。

Answer 3

如果要使书名动态化，可以执行以下操作：

class Student
  has_many :assignments
  has_many :books, through: :assignments

  def book_by_name(book_name)
    books.find_by!(name: book_name)
  end
end

并这样称呼：

ron = Student.find_by(name: 'Ron')
harry_potter_book = ron.book_by_name('Harry Potter')