When we create a web application we generally create models which are referred to as tables in the database. Sometimes we need to add a column to the existing table or remove an unused column from the table or just change the column. The way to do all this through migrations are -:

Adding a Column

Suppose we have a user table and we need to add a column "last_name" to it . 

$ rails generate migration add_column_last_name_to_user last_name:string

After writing this run the migration command.

$ rake db:migrate

The migration file generated will be like :

class AddColumnLastNameToUser < ActiveRecord::Migration
  def change
    add_column :users, :last_name, :string
  end
end

Changing a Column

If we want to change the datatype of last_name to text , so we need to write the following in our migration file :

class ChangeColumnLastNameInUser < ActiveRecord::Migration
  def change
    change_column :users, :last_name, :text
  end
end

After this run the migration command.

$ rake db:migrate

Removing a Column

If we want to remove the last_name column from our user table then we write:

$ rails generate migration remove_column_last_name_from_user last_name:text

After this run the migration command.

$ rake db:migrate

Our migration file will be like :

class RemoveColumnLastNameFromUser < ActiveRecord::Migration
  def change
    remove_column :users, :last_name, :text
  end
end