I’m always confused when using decimal in a Rails migration. Normally I need to store a value that has 2 or 3 numbers behind the comma (or dot), or decimals.
Let’s say you have a Product model with a discount_percentage attribute. This attribute is currently an integer, only allowing non-decimal values. To allow 2 digit decimal values (e.g. 12.54), you can mak the following migration:
change_column :products, :discount_percentage, :decimal, precision: 5, scale: 2
This will allow you to store values like 80.00, 99.99 and 100.00. There are five digits in the entire number and of those five, two decimals.
It’s not necessary so specify precision and scale as your database will then allow any decimal value it can store.
Read more about the decimal data type in PostgreSQL here: http://www.postgresql.org/docs/9.1/static/datatype-numeric.html