データの取得方法を変更

app/controllers/courses/books_controller.rb

@@ -3,6 +3,16 @@
 class Courses::BooksController < ApplicationController
   def index
     @course = Course.find(params[:course_id])
-    @books = Book.filtered_books(params[:status], @course)
+    @books = Book.with_attached_cover
+                 .eager_load(:practices_books, practices: { categories: :courses_categories })
+                 .where(courses_categories: { course_id: @course.id })
+                 .where(must_read_status)
+                 .order(updated_at: :desc, id: :desc)
+  end
+
+  private
+
+  def must_read_status
+    params[:status] == 'mustread' ? { practices_books: { must_read: true } } : {}
   end
 end

app/models/book.rb

@@ -24,22 +24,4 @@ def cover_url
   rescue ActiveStorage::FileNotFoundError, ActiveStorage::InvariableError
     image_url default_image_path
   end
-
-  def coursebook?(course_practices)
-    practices.any? do |practice|
-      course_practices.any? { |course_practice| practice.id == course_practice.id }
-    end
-  end
-
-  def self.filtered_books(status, course)
-    books = Book
-            .with_attached_cover
-            .includes(:practices)
-            .order(updated_at: :desc, id: :desc)
-
-    course_practices = course.extract_practices
-    mustread_or_all =
-      status == 'mustread' ? books.select { |book| book.practices_books.any?(&:must_read) } : books
-    mustread_or_all.select { |book| book.coursebook?(course_practices) }
-  end
 end

app/models/course.rb

@@ -14,14 +14,4 @@ class Course < ApplicationRecord
   def self.default_course
     find_by(title: DEFAULT_COURSE)
   end
-
-  def extract_practices
-    categories = Category
-                 .joins(:courses_categories)
-                 .where(courses_categories: { course_id: id })
-                 .includes(:practices)
-                 .order('courses_categories.position')
-
-    categories.map(&:practices).flatten
-  end
 end