Rails migration for change column

Question

We have script/generate migration add_fieldname_to_tablename fieldname:datatype syntax for adding new columns to a model.

On the same line, do we have a

Answer 1

This is all assuming that the datatype of the column has an implicit conversion for any existing data. I've run into several situations where the existing data, let's say a String can be implicitly converted into the new datatype, let's say Date.

In this situation, it's helpful to know you can create migrations with data conversions. Personally, I like putting these in my model file, and then removing them after all database schemas have been migrated and are stable.

/app/models/table.rb
  ...
  def string_to_date
    update(new_date_field: date_field.to_date)
  end

  def date_to_string
    update(old_date_field: date_field.to_s)
  end
  ...

    def up
        # Add column to store converted data
        add_column :table_name, :new_date_field, :date
        # Update the all resources
        Table.all.each(&:string_to_date)
        # Remove old column
        remove_column :table_name, :date_field
        # Rename new column
        rename_column :table_name, :new_date_field, :date_field
    end

    # Reversed steps does allow for migration rollback
    def down
        add_column :table_name, :old_date_field, :string
        Table.all.each(&:date_to_string)
        remove_column :table_name, :date_field
        rename_column :table_name, :old_date_field, :date_field
    end