CODEWITHSUNDEEP
ruby-on-railsruby-on-rails-3
ashr81
ashr81

Reputation: 650

Rails Active Record to make arithmetic calculation over a select statement

I am trying to calculate values from existing table column and use it on an external variable.

Let my table columns be as : ["id","unit_price","quantity","extras_1","extras_2"] I am presenting what i want to do in rails using sql command as reference.

SQL Command:

SELECT unit_price*quantity AS "regular_price", 
       unit_price*quantity-unit_price*quantity*discount AS "price_after_discount"
FROM order_details;

In Rails Active Record Query i tried same as:

OrderDetail.select('unit_price*quantity AS regular_price,unit_price*quantity-unit_price*quantity*discount AS price_after_discount')

From the above query.i tried sorting based on derived attributes.it worked perfectly.But i cannot see the derived attribute values by querying.

The output i got is without the derived attributes as:

[#<OrderDetail >,#<OrderDetail >]

But i need output as:

[#<OrderDetail regular_price: 43, price_after_discount: 54>,#<OrderDetail regular_price: 54, price_after_discount: 76>]

I tried below query to sort the data.It sorted the data perfectly:

OrderDetail.select('unit_price,quantity,unit_price*quantity AS regular_price,unit_price*quantity-unit_price*quantity*discount AS price_after_discount').order('regular_price desc')

I can access the values using below command:

OrderDetail.select('unit_price,quantity,unit_price*quantity AS extras_1,unit_price*quantity-unit_price*quantity*discount AS extras_2')

above commmand worked because extras_1 and extras_2 are table columns.

But it is working when assigned to existing table column.I need derived attribute to be non-existing table column name.

How can i access a derived attributes values from a record.I can access them by assigning them to an existing table column.But i want the attributes name given how i want irrespective of table columns.

Upvotes: 1

Views: 2126

Answers (2)

coorasse
coorasse

Reputation: 5528

Your attributes are perfectly accessible. Just call them! :)

As an additional remark I'd suggest you to use a more modern way of writing the query:

class OrderDetail
  scope :with_calculated_prices, -> do
    regular_price = (arel_table[:unit_price] * arel_table[:quantity])
    price_after_discount = (regular_price - regular_price * arel_table[:discount])
    select(regular_price.as('regular_price'), price_after_discount.as('price_after_discount'))}
end

price_after_discount is better defined as

price_after_discount = (regular_price * (1 - arel_table[:discount]))

Upvotes: 2

dnsh
dnsh

Reputation: 3633

You will not be able to see derived(alias) attributes. But they are present.

OrderDetail.select('unit_price*quantity AS regular_price,unit_price*quantity-unit_price*quantity*discount AS price_after_discount').first.regular_price

Will print regular_price.

What you see in rails console is output of inspect method. inspect method is not implemented to show alias attributes. Hence the confusion arises.

Go through this doc: http://apidock.com/rails/v4.0.2/ActiveRecord/QueryMethods/select

If an alias was specified, it will be accessible from the resulting objects:

Model.select('field AS field_one').first.field_one
#=> "value"

Upvotes: 2

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