scope :activity,   -> (activity_id) { where activity_id: activity_id }
scope :city,       -> (city_id)     { where city_id: city_id }

def self.created_between(start_time, end_time)
  where('starts_at >= ? AND ends_at <= ?', start_time, end_time)
end

def index
  @activities = Activity.order(:name)
  @cities     = City.order(:name)

  @schedules  = Schedule.where(nil)
  @schedules  = @schedules.activity(params[:activity_id]) if params[:activity_id].present?
  @schedules  = @schedules.city(params[:city_id]) if params[:city_id].present?
  @schedules  = @schedules.created_between(start_time, end_time) if params[:starts_at].present?
end

private

def start_time
  date = params[:starts_at]
  starting_hour = params[:date][:starting_hour]
  starting_time = [date, starting_hour].join(" ")
  Time.zone.parse(starting_time)
end

def end_time
  date = params[:starts_at]
  ending_hour = params[:date][:ending_hour]
  ending_time = [date, ending_hour].join(" ")
  Time.zone.parse(ending_time)
end

<%= form_tag schedules_path, method: 'get' do %>

  <%= collection_select nil, :city_id, @cities, :id, :name, include_blank: 'All', selected: params[:city_id] %>

  <%= collection_select nil, :activity_id, @activities, :id, :name, include_blank: 'All', selected: params[:activity_id] %>

  <%= select_tag :starts_at, options_for_select([
        ['Today', Time.zone.today],
        ['Tomorrow', Time.zone.tomorrow],
        [(Time.zone.today + 2).strftime("%d %b"), Time.zone.today + 2],
        [(Time.zone.today + 3).strftime("%d %b"), Time.zone.today + 3],
        [(Time.zone.today + 4).strftime("%d %b"), Time.zone.today + 4],
        [(Time.zone.today + 5).strftime("%d %b"), Time.zone.today + 5],
        [(Time.zone.today + 6).strftime("%d %b"), Time.zone.today + 6]
      ], selected: params[:starts_at]), include_blank: true %>

  <%= select_hour Time.zone.now, field_name: "starting_hour", start_hour: 7, end_hour: 23, ampm: true %>

  <%= select_hour Time.zone.now, field_name: "ending_hour", start_hour: 7, end_hour: 23, ampm: true %>

  <%= submit_tag "Filter" %>
<% end %>

undefined method `[]' for nil:NilClass