Filter Look up with Java Script

I have a requirement to filter a look up based on relationships , have figured only java script can do this.

So in this scenario I have 3 entities, Events, Hotels and Rooms.

Hotels has many rooms and Hotels can have many events

A room can also have many events.

So on the event form I have two look ups one to Hotel and another to room .

So if I pick the hotel look up which is required.

I want to be able to only see the rooms associated to that hotel.

so Hotel has a sub grid for rooms where the rooms will be created.

SO on the event form i want to filter the look up on the room field to only display the rooms that were created for the hotel I chose.

Will appreciate any hint , I figured it will be Java script already.

Regards.