RailsCounter Cache lưu sẵn số lượng associated records vào cột trong bảng cha — tránh phải COUNT(*) mỗi lần hiển thị.
ruby
# Migration
add_column :posts, :comments_count, :integer, default: 0
# Model
class Comment < ApplicationRecord
belongs_to :post, counter_cache: true
# Rails tự increment posts.comments_count khi tạo comment
# và decrement khi xóa
end
# Query: không cần COUNT query nữa
post.comments_count # đọc từ cột, O(1)
# vs
post.comments.count # SELECT COUNT(*) FROM comments WHERE post_id = ...Lưu ý: nếu counter lệch (import trực tiếp vào DB), dùng Post.reset_counters(post_id, :comments) để recalculate.
Counter Cache stores the count of associated records in a column on the parent table — avoids issuing a COUNT(*) query each time the count is displayed.
ruby
# Migration
add_column :posts, :comments_count, :integer, default: 0
# Model
class Comment < ApplicationRecord
belongs_to :post, counter_cache: true
# Rails auto-increments posts.comments_count on create
# and decrements on destroy
end
# Query: no COUNT query needed
post.comments_count # reads from column, O(1)
# vs
post.comments.count # SELECT COUNT(*) FROM comments WHERE post_id = ...Note: if the counter drifts (e.g. direct DB imports), use Post.reset_counters(post_id, :comments) to recalculate.