backend/app/models/tag_similarity.rb

@@ -0,0 +1,4 @@
+class TagSimilarity < ApplicationRecord
+  belongs_to :tag, class_name: 'Tag', foreign_key: 'tag_id'
+  belongs_to :target_tag, class_name: 'Tag', foreign_key: 'target_tag_id'
+end

backend/lib/tasks/calc_tag_similarities.rake

@@ -0,0 +1,28 @@
+namespace :tag_similarity do
+  desc '関聯タグ・テーブル作成'
+  task calc: :environment do
+    dot = -> a, b { (a.keys & b.keys).sum { |k| a[k] * b[k] } }
+    norm = -> v { Math.sqrt(v.values.sum { |e| e * e }) }
+    cos = -> a, b do
+      na = norm.(a)
+      nb = norm.(b)
+      if na.zero? || nb.zero?
+        0.0
+      else
+        dot.(a, b) / na / nb
+      end
+    end
+
+    tags = Tag.includes(:posts).to_a
+    tags.each_with_index do |tag, i|
+      existence_of_posts = tag.posts.index_with(1)
+      ((i + 1)...tags.size).each do |j|
+        target_tag = tags[j]
+        existence_of_target_posts = target_tag.posts.index_with(1)
+        TagSimilarity.find_or_initialize_by(tag:, target_tag:).tap { |ts|
+          ts.cos = cos.(existence_of_posts, existence_of_target_posts)
+        }.save!
+      end
+    end
+  end
+end