Semantic link is missing between Geometry and thematic data for 3DBAG dataset

[jravani]

Problem Description

When importing 3DBAG into 3DCityDB, the dataset assigns all thematic attributes to the Building object (objectclass_id = 901), while all geometries (including roofs, walls, and ground surfaces) are linked to BuildingPart objects (objectclass_id = 902).

This creates a structural mismatch:

• Thematic attributes belong to Building (901)
• Geometry and surfaces belong to BuildingPart (902)

In the current CityDB schema, there is no direct parent–child link connecting a BuildingPart to its corresponding Building. As a result:

• It becomes impossible to retrieve thematic attributes for a BuildingPart using only its feature_id.
• Attribute extraction workflows break because the pipeline expects the geometry and thematic attributes to belong to the same feature.
• For 3DBAG specifically, the reconstruction logic attaches LoD2.1/LoD2.2 geometry to BuildingPart, but all BAG/3DBAG metadata remains attached to Building.

Impact

• Attribute extraction fails for 3DBAG because pipeline operations depend on matching geometry to the correct thematic record.
• Any downstream process requiring height, construction year, or BAG metadata cannot be linked to the geometry of the building.
• This breaks consistency with other datasets (DE/AT/CZ) where building geometry is attached directly to the Building object.

What Needs to Be Solved

We need a reliable way to:

1. Identify which BuildingPart belongs to which Building.
2. Propagate or map thematic attributes from Building → BuildingPart.
3. Ensure the pipeline can extract both attributes and geometry using a single feature_id.