Hi,

I'm trying to understand the difference between relationship and a reference in Haystack 4.0

For example https://project-haystack.dev/example/alpha/a-024d there's an airRef from the VAV back to the parent AHU, the VAV is itself a member of children collection of the AHU.

Are ref tags themselves relationships - i.e. https://project-haystack.dev/doc/index-relationship - the documentation for airRef (https://project-haystack.dev/doc/lib-phIoT/airRef) doesn't indicate this but perhaps this is a limitation of the tool which generates the html doc from the ontology definition?

Regards David

Probably the best way to explain is to look at two examples:

// this is definition for a subtype of relationship
def: ^inputs
is: ^relationship
reciprocalOf: ^outputs
transitive
doc: "The entity inputs a substance from the referent.
  The value of this tag is what is received."

// this is definition for a subtype of ref
def: ^airRef
is: ^ref
of: ^air-output
tagOn: ^air-input
inputs: ^air
doc: "Air flows from the referent to this entity"

The relationship defines the semantics of an entity-to-entity relationship: what does the relationship mean in prose, is it transitive, does it have a reciprocal. These aspects are useful for inference engines, mapping to RDF OWL, etc.

The ref is a definition for a tag you stick on entities to model a relationship in your actual instance models. In this case when you use the airRef tag you are defining a relationship that "air is input from the referent".

The ref tag uses a relationship tag in its def - its a meta-model concept. We designed it this way for a couple of reasons:

1. A given ref tag could implement multiple relationships
2. Most relationships in our domain are substance/energy flows and we wanted to capture the concept of "what" flows with one tag versus some complicated design using indirection