Wrote this post because I couldn't find an article in the World Wide Web on how to change the column type and convert its data.✝

So let's say we have a Record model with boolean column master, where a true values means that it's a master release and false means that it's not. Records that aren't master releases can either be re-issues, special editions, remixes. We decided to model this info using just one column, and store it as strings.

Active Record's migration method change_column allows us to change a column's name and type. However it doesn't provide a way to also do data conversion.☦ My suggestion to implement this has three steps:

1. Add the column — release_type — with the new data type;
2. Convert boolean values from master column into respective string in release_type column;
3. Remove master column.

The migration code looks like this:

def up
  add_column :records, :release_type, :string

  cast_values = <<-SQL
    UPDATE records
    SET release_type = 'Re-Issue'
    WHERE master = FALSE;

    UPDATE records
    SET release_type = 'Master Release'
    WHERE master = TRUE;
  SQL
  ActiveRecord::Base.connection.execute(cast_values)

  remove_column :records, :master
end

The down method would reverse this logic and it's left for the reader as exercise.

✝ Must confess that I bumped into many articles changing integer columns into text, or datetime into date. Although data conversion is implied it is something that Rails can handle on its own.

☦ This article does suggest that one can provide a SQL statement for data conversion. However I couldn't find any official documentation to support that.