#59 関聯投稿一覧の追加

backend/app/models/post.rb

@@ -7,6 +7,12 @@ class Post < ApplicationRecord
   has_many :post_tags, dependent: :destroy
   has_many :tags, through: :post_tags
   has_many :user_post_views, dependent: :destroy
+  has_many :post_similarities_as_post,
+           class_name: 'PostSimilarity',
+           foreign_key: :post_id
+  has_many :post_similarities_as_target_post,
+           class_name: 'PostSimilarity',
+           foreign_key: :target_post_id
   has_one_attached :thumbnail
 
   def as_json options = { }
@@ -18,6 +24,21 @@ class Post < ApplicationRecord
     super(options).merge(thumbnail: nil)
   end
 
+  def related(limit: nil)
+    ids_with_cos =
+        post_similarities_as_post.select(:target_post_id, :cos)
+                                 .map { |ps| [ps.target_post_id, ps.cos] } +
+        post_similarities_as_target_post.select(:post_id, :cos)
+                                        .map { |ps| [ps.post_id, ps.cos] }
+
+    sorted = ids_with_cos.sort_by { |_, cos| -cos }
+
+    ids = sorted.map(&:first)
+    ids = ids.first(limit) if limit
+
+    Post.where(id: ids)
+  end
+
   def resized_thumbnail!
     return unless thumbnail.attached?