dm_pack_tbl() converts a parent table to a packed column in its child
table.
The parent table should not have parent tables itself (i.e. it needs to be a
terminal parent table).
Usage
dm_pack_tbl(dm, parent_table, into = NULL)
Arguments
dm
A dm.
parent_table
A terminal table with one child table.
into
The table to pack parent_tables into, optional as it can be guessed
from the foreign keys unambiguously but useful to be explicit.