Activity -> Destination (via destination_id)
Activity -> Itinerary (via itinerary_id)
Itinerary -> Period (via period_template_id)
Day -> ItineraryDay (via itinerary_day_id)
Activity -> Contract (via on_demand_contract_id)
Activity -> Contract (via default_contract_id)
Activity -> Activity (via pool_activity_id)
Tour -> ComissionCategoryType (via comission_category_type_id)
Activity -> Service (via on_demand_service_id)
Activity -> Service (via default_service_id)
Activity -> Service (via service_template_id)
Activity -> Provider (via on_demand_provider_id)
Activity -> Provider (via default_provider_id)