A stab at Elastic script filters for a multi-currency price filter

readme.md

Here we are trying to implement price filtering for a currency-per-instance model (i.e. an item is priced at ₺100, another at $100 etc).

Option 1 - Use a base currency

1. Convert all prices to a base price (price_cents_try here)
2. Periodically update that field as the exchange rates are updated

Pros

• Simpler to implement
• Better query performance (no script filter)

Cons

• The cron job can become unwieldy if there are lots of records and/or the update frequency is high

Option 2 - Use scripting

1. Use script fields and script filter (Option 2 part in the code).

Pros

• No need for a cron job

Cons

• Needs scripting enabled
• Possible performance degradation

Chewy_price_filter.rb

# Assumes gems: Chewy, Money and eu_central_bank
class SearchForm
  include ActiveModel::Model

  DEFAULTS = { 'currency' => 'TRY' }

  attr_accessor :price_min, :price_max, :currency

  # ...

  def price_filter
    currency = self.currency || DEFAULTS[:currency]

    # Option 1
    body = {}.tap do |h|
      h.merge!(gte: Money.new(price_min, currency).exchange_to('TRY').cents) if price_min
      h.merge!(lte: Money.new(price_max, currency).exchange_to('TRY').cents) if price_max
    end
    index.filter(range: { price_cents_try: body }) if body.present?

    # Option 2
    # TODO (Halil Özgür): Note: this method is probably faulty for currencies with subunit_to_unit != 100
    script_rules = []
    params = {}
    if price_min
      script_rules << 'price_cents_try >= price_min'
      params[:price_min] = Money.new(price_min, currency).exchange_to('TRY').cents
    end
    if price_max
      script_rules << 'price_cents_try <= price_max'
      params[:price_max] = Money.new(price_max, currency).exchange_to('TRY').cents
    end
    if script_rules.present?
      eur_to_try = Money.default_bank.rates['EUR_TO_TRY'].to_f
      rates = {}
      Product::CURRENCIES.each do |currency|
        rates[currency] = eur_to_try / Money.default_bank.rates["EUR_TO_#{currency}"].to_f
      end

      index
        .filter
        .script_fields(
          price_cents_try: {
            params: { rates: rates },
            script: 'doc["price_cents"].value * rates[doc["price_currency"].value]'
          }
        )
        .filter { s(script_rules.join(' && '), params) }
    end
  end
end

Product.rb

class Product < ActiveRecord::Base
  CURRENCIES = %w(TRY USD EUR)

  monetize :price_cents, with_model_currency: :currency, numericality: { greater_than: 0 }
end

ProductIndex.rb

class ProductIndex < Chewy::Index
  define_type Product do
    field :price_cents, type: 'integer'
    field :price_currency, index: 'not_analyzed'

    # Only needed for the Option 1
    field :price_cents_try, type: 'integer', value: -> { price.exchange_to('TRY').cents }
  end
end