While creating a web application we create models in our application which act as tables in the database. Sometimes when the development of the application moves further, the need arises to add a column to our existing table or removing a column from the table or just changing it. 

To achieve this, rails active records provide us with migrations with which we can easily do the desired changes in our table. Below are the ways defined to alter our table with the help of migrations.

• Adding a column

Let the table be user table and suppose we need to add address column to the table so to add the address column to the user table we need to write this on our console.

$ rails generate migration add_column_address_to_user address:text

And after that, we need to run 

$ rake db:migrate

so our migration file will look like this

class AddColumnAddressToUser < ActiveRecord::Migration
  def change
    add_column :users, :address, :text
  end
end

• Changing a column

Suppose now we need to change the data type of address column to string so for that we simply need to write this

in our migration file

class ChangeColumnAddressInUser < ActiveRecord::Migration
  def change
    change_column :users, :address, :string
  end
end

and then run 

$ rake db:migrate

• Removing a column

Now, suppose we need to remove this address column from our users table so for that we will write this on console

$ rails generate migration remove_column_address_from_user address:string

and then run

$ rake db:migrate

so our migration file will look like this

class RemoveColumnAddressFromUser < ActiveRecord::Migration
  def change
    remove_column :users, :address, :string
  end
end

So these were the three ways through which we can alter our table with the help of active record migrations.