Rails: validates_length_of

Question

I want to validate the Discount of a Sale in my Sale model.

The form for creating a Sale receives the Product data from my Warehouse model and saves it inside the Sale record:

<%= f.select(:product, Warehouse.pluck(:product).uniq, {prompt:true}, {class: 'form-control'}) %>

The Warehouse model has the discount specified for this respective product. Now I want to check if the sale.product is equal to the warehouse.product and then set the discount limit for this sale. Is that possible? Something like this:

validates_length_of :discount, maximum: Warehouse.where(:product => @sales.product).pluck(:discount), message: "Discount is to high"

Many Thanks in advance!

Answer 1

First of all, you should be validating numericality for numeric values.

Custom validation is unnecessary. You don't have to use constants or literals or other class-evaluation time values! Use procs! They will be called!

You pass a key as a comparison and then a proc as a value, so it is called during validation.

valudates_numericality_of :discount,
  less_than_or_equal_to: proc { |model| Something.query(whatever).pluck(:wow) },
  message: "is too high"

Note: you should probably also add a check for whether it's positive.

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Bonus (pure fun, best not use): almost the same code with a spectacular amount of arrows
(in order: symbol <=, hashrocket =>, stabby-lambda -> () {}):

valudates_numericality_of :discount,
  :<= => -> (model) { Something.query(whatever).pluck(:wow) },
  :message => "is too high"

Note that 1.9 hash syntax won't work with a symbol like <= (like <=: value) so you have to stick to a hashrocket with this one, which is... another bit of pure fun.