How to add counter cache with custom column in Rails?

Counter caches are a handy way to maintain and optimize the performance of counting associated records in Rails. By using a counter cache, Rails keeps a separate column in the parent table to track the number of associated child records, updating it automatically whenever child records are created or destroyed.

In this post, we will cover:

• How to add a counter cache using the default column name.
• How to customize the column name for the counter cache.

Adding a counter cache with the default column name

When adding a counter cache, Rails expects the column to follow the naming convention #{model_name}_count in the parent table.

For example, if a Post model has many Comments, Rails expects a comments_count column in the posts table.

Here’s how you can set it up:

1. Add the counter cache

   Define the association in the child model and enable the counter cache:

    class Post < ApplicationRecord
      has_many :comments
    end
   
    class Comment < ApplicationRecord
      belongs_to :post, counter_cache: true
    end
   

2. Add the comments_count column

   Generate a migration to add the comments_count column to the posts table:

    rails generate migration AddCommentsCountToPosts comments_count:integer
   

   Update the migration file to add default value:

    class AddCommentsCountToPosts < ActiveRecord::Migration[7.1]
      def change
        add_column :posts, :comments_count, :integer, default: 0
      end
    end
   

   Run the migration:

    rails db:migrate
   

3. Initialize existing records

   If you already have data in your database, you’ll need to populate the counter cache column for existing records.

   Use the reset_counters method:

    Post.find_each do |post|
      Post.reset_counters(post.id, :comments)
    end
   

That’s it! Now, Rails will automatically update the comments_count column whenever a comment is created or destroyed.

Adding a counter cache with a custom column name

If you want to use a different column name for your counter cache (e.g., custom_comments_count), you can customize it like this:

1. Specify the custom column name

   In the child model, provide the column name in the counter_cache option:

    class Comment < ApplicationRecord
      belongs_to :post, counter_cache: :custom_comments_count
    end
   

2. Add the custom column

   Generate a migration to add the custom column to the posts table:

    rails generate migration AddCustomCommentsCountToPosts custom_comments_count:integer
   

   Update the migration file to add default value:

    class AddCustomCommentsCountToPosts < ActiveRecord::Migration[7.1]
      def change
        add_column :posts, :custom_comments_count, :integer, default: 0
      end
    end
   

   Run the migration:

    rails db:migrate
   

3. Initialize existing records

   Similar to the default setup, initialize the column for existing records using the reset_counters method:

    Post.find_each do |post|
      Post.reset_counters(post.id, :comments) # Use the association name, not the column name
    end
   

4. Verify the counter cache

   Now, whenever a Comment is created or destroyed, Rails will automatically update the custom_comments_count column in the posts table.

When to use counter caches

Counter caches are particularly useful when you frequently need to display or query counts of associated records. Instead of calculating these counts on the fly, which can be slow and resource-intensive, Rails maintains the count for you in a separate column, ensuring faster lookups.