[postgis-users] Postgis Topology Hierarchal Topogeometries

[Gordon King]

I'm seeking guidance on how to correctly organise hierarchal topogeometries.

I'm working through the excellent Postgis in Action's chapter on topology and, in summary, it sets up two tables with topogeometries, one called neighbourhoods (which becomes layer 1) based on geometries loaded with toTopoGeom, and one called cities (layer 2), which is derived from the neighbourhoods (layer 1).

The book says this about layer 2.

"...You then define a column called topo in the cities table to define the neighbourhoods that each city is composed of. Each city is defined by a topogeometry. In the example, you only want one city, so there's only one topogeometry”

I find this confusing. I thought the column definition was the topogeometry in which case how can each Row entry (a city) have a different one?

What say I want more than one city? Where do the neighbourhoods go? How do I say that 'this city has these neighbourhoods from layer 1, and this second city, has these different neighbourhoods from layer 1'?

I'm clearly missing something important, because at the moment it seems to me that every 'parent' in the hierarchy basically ends up as a single solitary row in a table.

Thanks
Gordon

n.b. crosspost to https://gis.stackexchange.com/questions/303118/postgis-topology-hierarchal-topogeometries <https://gis.stackexchange.com/questions/303118/postgis-topology-hierarchal-topogeometries> as I wasn’t sure which forum was most active.
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