# `Diffo.Provider.Extension.InheritedPartyDeclaration`
[🔗](https://github.com/diffo-dev/diffo/blob/v0.9.0/lib/diffo/provider/extension/inherited_party_declaration.ex#L5)

DSL entity for an `inherited_party` declaration inside `parties do` on an Instance resource.

Generates an Ash calculation of the same name as `role` that walks the instance graph to
inherit a party from a reached instance. The calculation is injected by
`TransformInheritedRefs` at compile time — no `PartyRef` edge is created on the consuming
instance itself.

## Fields

- `role` — atom; the name of the generated calculation (and the party slot name from the
  consumer's perspective — the surfaced `PartyRef` carries this role).
- `source_role` — atom; the `PartyRef` role to read on each reached instance
  (e.g. `:provider`). Required. This terminal deref is **not** a `via:` hop — `via:`
  reaches the instance; routing *through* a party (the ref graph) is calc territory
  (see #227).
- `via` — the hop chain to the instance holding the ref, in the unified #213 grammar
  (shared with `inherited_characteristic`): a bare atom is `{:reverse, assignment: alias}`
  shorthand; tuples are `{:forward | :reverse, assignment: alias}` or
  `{:forward | :reverse, relationship: type | [type: t, alias: a]}`. When `nil`, defaults
  to `[role]`. See `Diffo.Provider.Extension.Traversal`.
- `collapse` — `:first` or `:last`; collapses the consumer-ordered result list (the refs
  at `source_role` across reached instances) to that end. When set, the calc returns a
  single party or `nil` rather than a list.

## Example

    parties do
      inherited_party :provider, source_role: :provider
      inherited_party :nni_owner, via: [:uplink], source_role: :owner
      # forward relationship hop + collapse to a single owner
      inherited_party :circuit_owner,
        via: [{:forward, relationship: [alias: :circuit]}],
        source_role: :owner,
        collapse: :first
    end

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*Consult [api-reference.md](api-reference.md) for complete listing*