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

[Candid Dauth]

Thank you for your detailed response. I now finally have the time to look at this again, and what you have written has been very helpful so far.

It seems like I will be able to find a way to achieve this by keeping things the way they are for multipolygon relations, but resolving the geometries of route relations in the second stage only, although this will be a bit of work to implement.

I am not deep enough in the internals of how osm2pgsql actually works to make a qualified suggestion how it could be improved to support my use case. From my novice point of view it seems like the `object:as_multilinestring()` relation method would be the usual approach to achieve what I am trying to do, and it does seem convenient. The specific functionality that I’m missing for my use case is to exclude relation members based on their _tags_. It seems to me like this is something that could be added somehow, since I supposed the method needs to load the individual member objects anyways to resolve their geometries, so it wouldn’t cause an additional performance regression? Interestingly, even if I ignore a way in the `osm2pgsql.process_way(object)` function both in the first and second stage, its geometry is still added to the relation. However, I suspect that I might be able to exclude members of the relation based on their _role_ by simply deleting them from the `members` property before calling `object:as_multilinestring()`. I will have to try this out, and if it works this could be a feasible solution that wouldn’t even require two-stage processing, as railway platforms can be identified both by their member role as well as by their object tags.

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