I've been asked to sort out a shower issue. The first part was trivial, ordering a new inner assembly(on last page) for the shower waste.

The second part is that the bathroom tends to build up a mild but unpleasant smell overnight with no one using it. There is nothing leaking and all the plumbing is easy to see at head height under the house. I noticed the shower does not have a 'p' trap. With the inner assembly out you can easily hear the sink draining etc etc.

I ordered the new assembly prior to leaving and now I'm miles away and may not get another look at it for a while. The shower waste is a newline side entry.

So my question is: Does this type of shower waste require a 'p' trap or is there something about the waste that automagically creates a trap in there when it is operating correctly?