Migrations in Rails

Active records provide us with a very useful feature to change or alter our database schema in ruby on rails.

Through these migrations we can make necessary changes to our database and make changes in our tables and rows.

Active records detects the changes we have made and automatically update the database.

The command to tell active records to update the changes in the schema is 

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rake db:migrate

rake db:migrate

 Migrations which we can use to alter our schema are as fallows :

• Creating a table

To create a table we will have to write this command in console

$ rails generate model user name:string email:string

and then run

$ rake db:migrate

Now, the migration file will look like this

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class ChangeUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.string :email
 
      t.timestamps null: false
    end
  end
end

class ChangeUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.string :email
 
      t.timestamps null: false
    end
  end
end

• Adding a column to an existing table

To add a column to an existing table we need to write this command in console

$ rails generate migration add_column_address_to_users address:text

and then run

$ rake db:migrate

Now, the migration file will look like this

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

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

With this, an address column with data type text will be added to the user table.

• Adding an index to the added column

To add an index to the added column we need to write this command in console

$ rails generate migration add_column_address_to_users address:text:index

and then run

$ rake db:migrate

Now, the migration file will look like this

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

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

• Removing a column from table

To remove a column from an existing table we need to write this command in console

$ rails generate migration remove_column_address_from_users address:text

and then run

$ rake db:migrate

Now, the migration file will look like this

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

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

• Adding a reference

To add a reference to a table of some another table we need to write this command in console

$ rails generate migration add_refrence_user_to_guests

and then run

$ rake db:migrate

Now, the migration file will look like this

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class AddUserRefToGuests < ActiveRecord::Migration
  def change
    add_reference :guests, :user, index: true
  end
end

class AddUserRefToGuests < ActiveRecord::Migration
  def change
    add_reference :guests, :user, index: true
  end
end

In this the user table is added as a reference to the guests table.

This migration will create a user_id column and appropriate index in the guests table.

So these were some of the important migrations of active records which are used frequently while creating we b based applications.