Question

I have an ActiveRecord relationship between Trade and Execution. I can get

Trade.executions  #returns all exeuctions realated to the Trade

If I do

Trade.executions.last

It seems seems to return the last execution record based on ID.

Is this the correct way to retrieve the last execution record related to Trade based on ID?

Answer 1

No, that’s not guaranteed to give you the Execution with the highest id. If you don’t specify an explicit ordering then the records can come out of the database in any order. The fact that they look like they’re sorted by id is just a convenient accident.

You should do one of these:

highest_id_execution = trade.executions.order(:id).last
highest_id_execution = trade.executions.order('id desc').first

That will give you the execution for trade that has the highest id. If you really want the most recently created one then you should order(:created_at) instead:

most_recent_execution = trade.executions.order(:created_at).last
most_recent_execution = trade.executions.order('created_at desc').first

The id and created_at columns will almost always come in the same order but you should say what you mean to make things clearer to the people that maintain your code.

In both cases, the order(:x).last and order('x desc').first are exactly the same and even resolve to exactly the same SQL so use whichever one makes the most sense to you.