create_calendar.rb

class CreateCalendarTable < ActiveRecord::Migration
  def up
    create_table :calendar, id: false do |t|
      t.date "_date"
      t.string :month_name
      t.integer :month_no
      t.integer :year
      t.integer :day
      t.integer :day_of_week
      t.integer :day_of_year
      t.string :day_name
    end

    execute %Q{ ALTER TABLE calendar ADD PRIMARY KEY (_date); }
  end

  def down
    drop_table :calendar
  end

end

seeds.rb

ActiveRecord::Base.connection.execute("DROP PROCEDURE IF EXISTS filldates;")
ActiveRecord::Base.connection.execute <<-SQL
	CREATE PROCEDURE filldates(dateStart DATE, dateEnd DATE)
	BEGIN
		WHILE dateStart <= dateEnd DO
			INSERT INTO calendar (_date, month_name, month_no, year, day, day_of_week, day_name, day_of_year)
				VALUES (dateStart, monthname(dateStart), month(dateStart), year(dateStart), day(dateStart), dayofweek(dateStart), dayname(dateStart), dayofyear(dateStart));
			SET dateStart = date_add(dateStart, INTERVAL 1 DAY);
		END WHILE;
	END;
SQL
ActiveRecord::Base.connection.execute("CALL filldates('2000-01-01','2020-12-31');")
ActiveRecord::Base.connection.execute("DROP PROCEDURE IF EXISTS filldates;")