WithCounts - dynamic parameterized scope to add count fields for has_many relations.

with_counts.rb

# frozen_string_literal: true

# A dynamic parameterized scope to add a count field. Works with
# direct and polymorphic has_many relations.
#
# Example:
# > post = Post.with_counts(:comments, :likes).first
# > post.comments_count   # contains the count of associated comments
# > post.likes_count      # contains the count of associated likes
#
# See: https://medium.com/@eric.programmer/the-sql-alternative-to-counter-caches-59e2098b7d7
module WithCounts
  extend ActiveSupport::Concern

  included do
    scope :with_counts, lambda { |*relations|
      select (["#{table_name}.*"] + count_queries(relations)).join(',')
    }

    # Just an alias for the scope above.
    scope :with_count, ->(relations) { with_counts(relations) }
  end

  class_methods do
    # Generate the count queries for the given relations according to their type.
    #
    # @param [Array] relations
    # @return [Array]
    def count_queries(relations)
      Array(relations).map do |relation|
        <<~SQL
          (
            SELECT COUNT(#{relation}.id) FROM #{relation}
            #{where_query(relation)}
          ) AS #{relation}_count
        SQL
      end
    end

    private

    # Generate a where query part according to the relation type.
    #
    # @param [Symbol] relation
    # @return [String]
    def where_query(relation)
      if polymorphic?(relation)
        polymorphic_as = reflect_on_association(relation).options[:as]
        <<~SQL
          WHERE #{polymorphic_as}_id = #{table_name}.id
          AND #{polymorphic_as}_type = '#{name}'
        SQL
      else
        "WHERE #{name.underscore.to_sym}_id = #{table_name}.id"
      end
    end

    # Find out if a relation is polymorphic.
    #
    # @param [Symbol] relation
    # @return [Boolean]
    def polymorphic?(relation)
      reflect_on_association(relation).options.key?(:as)
    end
  end
end