Diffo.Provider.Extension.InheritedCharacteristicDeclaration (Diffo v0.7.0)

DSL entity for an inherited_characteristic declaration inside characteristics do on an Instance resource.

Generates an Ash calculation that derives a typed characteristic by walking the graph along a via: hop chain, reading the characteristic at a role on the final node(s), and optionally renaming and collapsing the result. The calculation is injected by TransformInheritedRefs at compile time — no direct edge or characteristic record is created on the consuming instance itself.

Unlike inherited_place / inherited_party (which read against universal PlaceRef / PartyRef), the typed characteristic module varies per reached resource and is resolved at runtime via AshNeo4j.worlds/1 on each struct and Diffo.Provider.Extension.Info.provider_characteristics/1 on its outermost resource. Late-binding by design.

Traversal grammar

via: is an ordered list of hops; each walks one edge in one direction (see Diffo.Provider.Extension.Traversal for the full grammar and Diffo.Provider.Calculations.Traversal for the runtime walk):

alias (bare atom) — shorthand for {:reverse, assignment: alias} (inherit from your assigner — the common case). An omitted via: defaults to [name].
{:forward | :reverse, assignment: alias}
`{:forward :reverse, relationship: type}` or
`{:forward :reverse, relationship: [type: t, alias: a]}`

:forward means this instance is the edge source (source_id = me, follow to target); :reverse means this instance is the edge target (target_id = me, follow to source). Mechanism (assignment / relationship) and direction are independent, so chains of any length and any mix compose.

Fields

name — atom; the name of the generated calculation (the Ash load/field handle), and the default read role.
via — the hop list (see grammar above). When nil, defaults to [name].
read — atom; the characteristic role to look up on each reached instance. Defaults to name. (Per-reached-instance the typed module is found at runtime.)
as — atom; renames the surfaced characteristic to this name (both the loaded value and the encoded TMF entry). Defaults to the source characteristic's own name (no rename).
collapse — :first or :last; collapses the consumer-ordered result list to that end (List.first/1 / List.last/1). When set, the calc returns a single record or nil rather than a list.

Example

characteristics do
  inherited_characteristic :card, via: [:primary]
  inherited_characteristic :nnis, via: [{:forward, relationship: :contains}]
  inherited_characteristic :cvc,
    via: [{:forward, relationship: [alias: :circuit]}, {:reverse, assignment: :cvlan}],
    read: :cvc, collapse: :first
end

Result shape

Without collapse, a list of entries — one per reached instance carrying a characteristic at the read role (or a list of records for {:array, _} values). With collapse, a single such record or nil. %Diffo.Unknown{} entries appear when a reached struct can't be projected to a loadable resource or declares no characteristic at the role; see Diffo.Provider.Calculations.InheritedCharacteristic for the reason vocabulary.

`{:forward	:reverse, relationship: type}` or
`{:forward	:reverse, relationship: [type: t, alias: a]}`