In the past it was always a struggle to create a hierarchy in Coda.
With the new functionality Coda has added to the old Lookup column type, now the Relation column type, it has become EXTREMELY simple to do. Simply create a new column of type Relation, and enable two-way editing.
Done.
In the doc below I show two examples, one a literal parent child family structure, the second a project with WBS elements below.