form.rb

class ResourceSearchForm
  include ActiveModel::Model

  attr_accessor :keywords, :categories, :types, :start_date, :end_date

  def model_name
    ActiveModel::Name.new(self, nil, "search")
  end

  def initialize(params)
    self.keywords = params[:keywords]
    self.categories = Array.wrap(params[:categories]).select(&:present?).map(&:to_i)
    self.types = Array.wrap(params[:types]).select(&:present?)
    self.start_date = Date.parse(params[:start_date]) if params[:start_date].present?
    self.end_date = Date.parse(params[:end_date]) if params[:end_date].present?
  end
end

searcher.rb

class ResourceSearcher
  attr_accessor :form, :scope

  def initialize(form, scope)
    self.form = form
    self.scope = scope
  end

  def results
    results = scope
    results = results.search(form.keywords) if form.keywords.present?
    results = with_date_range(results, form.start_date, form.end_date)
    results = with_type_filter(results, form.types)
    results = with_category_filter(results, form.categories)
    results
  end

private

  def with_date_range(results, start_date, end_date)
    results = results.where("published_at >= ? OR published_at IS NULL", start_date) if start_date.present?
    results = results.where("published_at <= ? OR published_at IS NULL", end_date) if end_date.present?
    results
  end

  def with_type_filter(results, types)
    results = results.where(type: types) if types.any?
    results
  end

  def with_category_filter(results, categories)
    if categories.any?
      results.where(id: results.joins(:categories).where(resource_categories: {id: categories}))
    else
      results
    end
  end
end