It’s still here. 
I’m curios why everyone tries to link the medical facts using the relationship_ids developed for the vocabularies to link the concepts between themselves.
Even specification doesn’t ask to do it.
A couple of examples from there:
- Person relationships (parent-child);
- care site relationships (hierarchical organizational structure of facilities within a health system;
- indication relationship (between drug exposures and associated conditions);
- usage relationships (of devices during the course of an associated procedure);
- facts derived from one another (measurements derived from an associated specimen).
We’re not gonna use ‘Is a’, ‘Subsumes’ and ‘Maps to’ for individuals, right?
So my suggestion would be:
You have the domain_concept_id_1 and domain_concept_id_2 so it’s already from ‘Observation’ to ‘Observation’.
For the rest of the meaning try to find the closest concept within any vocabulary/domain.
I know that we have some, i.e. Diastolic to systolic blood pressure measurement, Child to Parent Measurement.
But please, let’s stop mixing the things. If we need to provide the relationships for the fact_relationship table here, let’s make it a separate vocabulary and at least support the usecases from the specs.