Rails Migrations - Modify rows based on condition

Question

I need to update a table data in database using RAILS migrations.

Sample:
Table: Table_A(Col_A(number), Col_B(varchar),...)
Query: UPDATE Table_A SET Col_B = "XXX" where Col_B = "YYY"

What would be the best way to do this using RAILS Migrations. I am not even sure if RAILS migrations is the way to go for updating data in database. Any explanation would be helpful.

Answer 1

It's usually better to do these sorts of big data updates in a rake task. I usually write them so they have two versions: rake change_lots_of_data:report and rake change_lots_of_data:update. The 'report' version just executes the where clause and spits out a list of what would be changed. The 'update' version uses the very same where clause but makes the changes.

Some advantages of doing it this way are:

• Migrations are saved for changing the database structure
• You can run the 'report' version as often as you want to make sure the right records are going to be updated.
• It's easier to unit test the class called by the rake task.
• If you ever need to apply the same criteria to make the change again, you can just run the rake task again. It's possible but trickier to do that with migrations.