Migration to change default value for a field and change all existing record's value to new default value only if it has old default value.

Question

I need to change the default value of a field from 0 to 3, but the catch is i have thousands of records already and want those records to change the value to 3 from 0 only if the record has default value 0 but for other values like 1, 2 it should remain the same. how can i do it?

Answer 1

In the migration you should use the method change_column to alter the table settings like this:

change_column :my_models, :attribute_name, :integer, :default => 3 

And then to update all existing records, instead of looping through all records and updating them individually you could use the method update_all like this:

MyModel.update_all({ :attribute_name => 3 }, { :attribute_name => 0 }) 

The first argument tells the method what value to set and the second tells it the condition for which rows to update.