Store ID of replied and original PM

[hvorragend]

problem

If you want to show a Conversation History for one PM, but there is no way, because there is no ID of the replied PM. For example see the Facebook Messaging system.
analysis

Two missing columns in the DB
enhancement recommendation

Store also the ID of the replied PM in the DB. It makes also sense to store the ID of the "original" top PM for all replies. So when showing the PM History, the select can be done like this: show me all replies for a specific PMid.

[Kaik]

This can be done with new field 'conversation' with id of first message in conversation or false for first message. Using doctrine we can do array collection with left join to self I guess.