Consider the following scenario.

Let us say we have a User model and users ,its corresponding table in the database has a column role_id along with other columns.

During the time of the attachment of the role_id to users table ,we could have written the following migration so that it receives a default value :

class AddRoleIdToUsers < ActiveRecord::Migration
  def change

    add_column :users, :role_id, :integer,:default => 2

    add_index :users, :role_id

  end

end

An index has been there for a faster search.

Now if we added the column but  accidently or unknowingly forgot to add default option ,we can do something like this.

generate an empty migartion using rails g migration GiveDefaultValueForRoleId and modify the change method as described below:

class GiveDefautvalueForRoleId < ActiveRecord::Migration

   def change

   change_column :users ,:role_id ,:integer ,default: 2


  end
end

here the name of the migration is of no importance but it should produce some sense.

The change_column method here takes table name,column name,column 's data type and a hash

as its parameters in the same order.

But a caution here ,it won't provide default values for the earilier created records so we have to manually update them using rails console.

Thank you