ActiveRecord has_one with a dynamic scope

Question

I have this

class Student
  has_many :assignments
end

class Assignment
  belongs_to :student
  belongs_to :book
end

class Book
  has_many :assignments
end

To find the student's "Harry Potter" assignment, I am doing this

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

Is there a way to make this into a has_one association for the Student model?

Something like this (pseudo-code here)

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

If not with a has_one , what is a better way?

Answer 1

If I'm not wrong, the following should work

#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

You can try to define it like this:

class Student < ActiveRecord::Base
  has_many :assignments

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

Now querying:

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

produces the following SQL:

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

It's not has_one relation though. There might be zero, one or more assignments for given student and book 'Harry Potter'.

Answer 3

If you want to make book's title dynamic, you can do the following:

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

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

And call it this way:

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