Parent id message for migration

In Lithium, the parent id represents the message id of the message to which the message is a reply.

In some systems, you might have a root message A, and then replies B, C, D, and E all to the root message A. So a thread might look like:

A
--> B
--> C
--> D
--> E

In Lithium, you can reply to another reply. So instead, you might have the root message A and messages B and E as replies to A. And then perhaps C and D as replies to B. So a thread might look like this:

A
--> B
------> C
------> D
--> E

If you're viewing the community in linear, you can't really see this threading or tell exactly who is replying to who. But it's much more clear if you turn on the topic view ("List by Topics" in the "Board Options" menu). 

So to get back to the original question... In the second example, the parent for message B would be A. The parent for messages C and D would be B. And the parent for message E would also be A. The root message A has no parent.

I hope this helps!