So, what happens when you want to change an existing value in your database?

For example, in my projects table, I want to change the data type of the field stage from an integer to a string data type because I don't want to have numbers, I'd prefer to have words instead.

To do that, go to your console and type:

rails g migration change_data_type_for_stage

This command will create a migration file.

Now, open the up the migration file 20151030005157_change_data_type_for_stage.rb

Unlike how the migration generator worked when we were adding a column to a table, this method is not going to have any code inside the change method for us, so we have to add it manually. The code you need to add inside the change method will look like this:

def change
  change_column :projects :stage :string
end

In this code, we are calling the method change_column, and passing the parameters projects which is the table name, stage which is the field name and string is the new data type we want.

If you go back to rails, and run the migrate command

rake db:migrate

Now, if you go to your schema file, the data type of the stage field has been changed.

You're likely to be doing this task many times in your projects, so it's good to know how to do it. An important item to note here. DO NOT do this on a production application, it will erase all of the data in that column. If you are doing this for a production application, create a new column, then write a script that copies the values over to that column and convert them to the new type. Then you can delete the old column. If you follow this process you won't lose any data.