Hello Jürgen,
BOPF's association definition is built on top of DDIC, thus there is no need for BOPF to have the information stored in the database table's metadata. Of course you could add those information manually on DB level if you would like.
You didn't mentioned your use case in detail, but I guess you would like to automatically generate CDS out of BOPF. In that case you could also use the BOPF configuration in order to get the knowledge about the existence of foreign key association in BOPF instead of evaluating the database metadata:
" get all associations of a BO (including foreign key associations)
DATA(lo_configuration) = /bobf/cl_frw_factory=>get_configuration( iv_bo_key = IF_YOUR_CONSTANT_INTERFACE=>sc_bo_key ).
data(lt_association) = lo_configuration->get_assoc_tab( ).
" now check the kind of associations (field ASSOC_CAT = SC_ASSOCCAT_FOREIGN_KEY)
Best regards
Tilmann
