filter.rb

module FiltrableModel
  def filter_class=(klass)
    @filter_class = klass
  end

  def filter(params = {})
    @filter_class or raise "No filter model specified"
    @filter_class.new(self).apply(params)
  end
end

class Order
  extend FiltrableModel
  filter_class OrderFilter
end

class Filter
  PREDICATES = {
    eq: ->(scope, value) { scope.where(scope.arel_table[@name].eq(value)) },
    cont: ->(scope, value) { scope.where(scope.arel_table[@name].matches("%#{value}%")) },
  }

  class << self
    def mapping
      @mapping ||= {}
    end

    def param(name, &apply)
      mapping[name] = apply
    end

    def column(name, opts = {})
      predicate = PREDICATES[opts.fetch(:predicate, :eq)] or raise "Unknown predicate: #{name}"
      param("#{name}_#{predicate}", &predicate)
    end
  end

  def initialize(scope)
    @scope = scope
  end

  def apply(params = {})
    self.class.mapping.inject(scope) do |scope, (name, apply)|
      apply.call(scope, params[name]) if params.key?(name)
    end
  end
end

class OrderFilter < Filter
  param(:custom) { |scope, value| scope.where(yay: value) }
  column(:ref)
end

class OrderController < ApplicationController
  def index
    @orders = filter_scope.order(created_at: :desc)
  end

  def scope
    Order.where(deleted_at: nil)
  end

  def filter_scope
    scope.filter(params[:filter])
  end
end

You may add another type of predicate for segmented strings with multiple terms (to search something like "La casa de la pradera" with this query "cas dera"), you could add something like
seg: ->(scope, value) { scope.where(scope.arel_table[@name].matches_all(value.split(" ").map{|x| "%#{x}%"})) },