在Rails中可以为模型创建自定义订购方法吗？

Question

I would like to create something like a new order method for a model in my rails app. This order method should be able to compare records by one column. At the beginning of the list should be records with the highest( > 250) and the lowest( < 80) value of that column and others(with avarage value between 80 and 250) should be at the end of list. Is it possible to do it in rails ?

Answer 1

With plain ruby

# when order between 0 - 80 and > 250 does not matter
YourModel.all.to_a.sort_by do  |record| 
   if record.order_col < 80 || record.order_col >250
     0 
   else
     1
   end
end

# when order between 0 - 80 and > 250 does matter
YourModel.all.to_a.sort_by do  |record| 
   if record.order_col < 80 || record.order_col >250
     record.order_col
   else
     record.order_col + YourModel.max(:order_col) + 1
   end
end

With SQL

# with sql

scope :ordered, find_by_sql(%(
  SELECT your_model.* FROM your_model where value < 80 OR value > 250 ORDER BY id desc
  UNION
  SELECT your_model.* FROM your_model where value >= 80 and value <= 250 ORDER BY id desc
 ))

# can work with mysql
YourModel.select(:field, ..., 'IF (column < 80 OR column > 250, column, column + (SELECT max(column) FROM yourtable ) + 1) as order_column').order('order_column desc')