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With a migration you can create a table or insert data to a table, also there is a possibility to modify existing tables; from renaming tables to changing the type of a column. And what about constraint such as default value or not null?

Yeah, you can change them too, but I was only successful with adding them — removing them was not possible with standard Rails methods.

I tried to create a migration that would remove the NOT NULL constraint from a column.

My first try was:

class ChangeCustomerColumns < ActiveRecord::Migration
  def self.up
    change_column :customers, :surname, :string
  end

  def self.down
    change_column :customers, :surname, :string, :null => false
  end
end

but nothing changed (PostgreSQL: to see the actual structure of a table type in the psql application with the \d table_name command).

Then I tried the following line, but also no difference.

change_column :customers, :surname, :string, :null => true

It seems that the only way how to do it is to call an SQL statement :(

I have a PostgreSQL database, so my migration is the following:

class ChangeCustomerColumns < ActiveRecord::Migration
  def self.up
    execute "alter table customers " +
      "alter column surname drop not null;"
  end

  def self.down
    execute "alter table customers " +
      "alter column surname set not null;"
  end
end

The same applies for the default value constraint: drop default or set default "John Smith".