Fetching Minimum/Maximum for each group in ActiveRecord

Question

This is an age-old question where given a table with attributes \'type\', \'variety\' and \'price\', that you fetch the record with the minimum price for each type there is.

Answer 1

While this question is pretty stale, I was asking the same question today. Here's a gist of a solution to compose the SQL needed to accomplish the goal with minimal (2) queries.

Please lmk if there are better ways these days!

Using Security and Price models, where Securities have many (historical) Prices, and you are after the Securities' most recent price:

module MostRecentBy
  def self.included(klass)
    klass.scope :most_recent_by, ->(group_by_col, max_by_col) {
      from(
        <<~SQL
          (
            SELECT #{table_name}.*
            FROM #{table_name} JOIN (
               SELECT #{group_by_col}, MAX(#{max_by_col}) AS #{max_by_col}
               FROM #{table_name}
               GROUP BY #{group_by_col}
            ) latest
            ON #{table_name}.date = latest.#{max_by_col}
            AND #{table_name}.#{group_by_col} = latest.#{group_by_col}
          ) #{table_name}
        SQL
      )
    }
  end
end

class Price < ActiveRecord::Base
  include MostRecentBy

  belongs_to :security

  scope :most_recent_by_security, -> { most_recent_by(:security_id, :date) }
end

class Security < ActiveRecord::Base
  has_many :prices
  has_one :latest_price, 
    -> { Price.most_recent_by_security },
    class_name: 'Price'
end

now you can call the following in your controller code:

def index
  @resources = Security.all.includes(:latest_price)

  render json: @resources.as_json(include: :latest_price)
end

which results in two queries:

  Security Load (4.4ms)  SELECT "securities".* FROM "securities"
  Price Load (140.3ms)  SELECT "prices".* FROM (
    SELECT prices.*
    FROM prices JOIN (
       SELECT security_id, MAX(date) AS date
       FROM prices
       GROUP BY security_id
    ) latest
    ON prices.date = latest.date
    AND prices.security_id = latest.security_id
  ) prices
  WHERE "prices"."price_type" = $1 AND "prices"."security_id" IN (...)

for reference: https://gist.github.com/pmn4/eb58b036cc78fb41a36c56bcd6189d68