[Tile-serving] [osm2pgsql-dev/osm2pgsql] How to omit certain relation members with flex output? (Discussion #2204)

[mboeringa]

@joto 

I think I have a question that relates to the issue @cdauth is struggling with.

What I attempted to do is to build a table with geometries from waterway relations in OpenStreetMap.

Now I use the **object.as_geometrycollection()** function to generate a geometry collection object of e.g. the Alpine Rhine waterway relation (https://www.openstreetmap.org/relation/21222156), and subsequently insert each of its member ways into a table using either **geom:geometries()** or **geom.geometry_n()** and looping over the members.

According to the documentation, object.as_geometrycollection()

>  It will contain all geometries which can be generated from node and way members of the relation in **order of those members**.

will return the members in the order as defined in the relation, if I understand well.

However, if I e.g. want to know the osm ID of the n-th member way, I cannot use the **object.members**. I would have expected object.members to return the members of the relation _**in order of those members****_ as well. However, if I attempt to grab e.g. the n-th geometry of the geometry collection using **geom.geometry_n(n)** and also try to grab the n-th member as returned by **object.members** by using **object.members[n]** and subsequently retrieve the osm ID by using the **ref** field of the member table, I get back an osm ID that does not correspond with the n-th way member of the relation, but another osm ID. So it seems that the geometry collection, and the **object.members** Lua table are out of sync, and **object.members** not sorted by the order in which the relation members are stored in the relation.

Note that the relations I noticed this on, like the Alpine Rhine, do not contain any sub-relations, just way members, and no nodes. So I would expect it would be possible to have the geometry collections members and **object.members** to be in sync.

Of course, it would be nice if I could somehow grab the geometry from the Lua member table, similar to type,ref,role, e.g. **member_table.geometry**, similar to **member_table.type/ref/role**, so that the it would be guaranteed to be in sync.

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