In Rails 4, you can add computed columns to Active Record models using virtual attributes and custom methods. A virtual attribute is an attribute that does not have a corresponding column in the database but can be used as if it were a regular attribute of the model.

Here's how you can add a computed column to an Active Record model in Rails 4:

Assuming you have a model called Product and you want to add a computed column total_price, which is the product of price and quantity:

  1. Define Virtual Attributes: In your Product model, define virtual attributes for price and quantity. Virtual attributes are just regular Ruby instance variables with getter and setter methods:

    ruby
    class Product < ActiveRecord::Base attr_accessor :price, :quantity end
  2. Create a Custom Method: Add a custom method called total_price that calculates the computed column value:

    ruby
    class Product < ActiveRecord::Base attr_accessor :price, :quantity def total_price price.to_f * quantity.to_i end end

    In this example, we assume that price is a string, so we use to_f to convert it to a float for multiplication. We also convert quantity to an integer using to_i to ensure it is treated as a number.

  3. Use the Computed Column: Now you can use the total_price method as if it were a regular attribute in your application:

    ruby
    product = Product.new(price: "10.5", quantity: "2") puts product.total_price # Output: 21.0

    Note that the total_price attribute is not stored in the database. It is only available in memory during the current request or session.

By defining virtual attributes and custom methods, you can add computed columns to your Active Record models in Rails 4. These computed columns are calculated based on other attributes in the model and are available for use within your application without the need to store them in the database.

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