FieldViaAssignedRelationship — read a field from a source instance via AssignmentRelationship

[matt-beanland]

Description

There is no general-purpose way to read a base field from the source instance of an AssignmentRelationship. Resources that need to surface such a field must write bespoke calculation modules by hand, repeating the same traversal logic each time.

The NBN case is the clearest example: an AVC wants to surface the name of the CVC that assigned it an svlan — that name serves as the CVC ID on the AVC. The CVC is the source of an AssignmentRelationship where the AVC is the target. Without a shared calculation module the AVC must contain custom traversal code.

What we need

A calculation module Diffo.Provider.Calculations.FieldViaAssignedRelationship with arguments mirroring inherited_place / inherited_party:

• via: — optional list of alias steps. When provided, filters AssignmentRelationship by target_id = current.id, alias = step. When omitted, traverses all AssignmentRelationship records where target_id = current.id with no alias filter. Multi-hop is supported by chaining steps.
• field: — the field name to read from the resolved source instance (analogous to source_role: on inherited_place).

Both forms return a list of field values, consistent with inherited_place and inherited_party.

# Aliased — field from the source holding the :svlan assignment slot
calculate :cvc_id, :string,
  {Diffo.Provider.Calculations.FieldViaAssignedRelationship, [via: [:svlan], field: :name]}

# Unaliased — field from any instance that assigned something to this one
calculate :assigner_name, :string,
  {Diffo.Provider.Calculations.FieldViaAssignedRelationship, [field: :name]}

Why it matters

Layered service graphs routinely need to surface fields from assigning instances — CVC ID on an AVC, NNI name on a CVC, assigner name on an assigned resource. These are all the same pattern: traverse an assignment by alias (or without), read a field. A shared calculation module eliminates the repeated boilerplate and enables a consistent demo path.

[matt-beanland]

Implemented

Diffo.Provider.Calculations.FieldViaAssignedRelationship is now available for use on any instance resource.

How it works

The calculation traverses AssignmentRelationship from the target side (the assignee) to reach the source instances (the pool owners), then reads a named field from each. Two forms are supported:

Aliased — follows the assignment slot identified by via: alias steps:

calculate :cvc_id, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaAssignedRelationship, [via: [:svlan], field: :name]}

Unaliased — returns the field from every instance that assigned anything to this one:

calculate :assigner_names, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaAssignedRelationship, [field: :name]}

Multi-hop is supported by chaining alias steps in via:, consistent with inherited_place and inherited_party.

Test coverage

Five integration tests in test/provider/extension/field_via_assigned_relationship_test.exs cover: field returned via alias, empty list when no assignment, alias filtering (two sources with different aliases — only the matching one returned), unaliased returning all sources, and empty list for the unaliased form with no assignments.

The AccessService test resource carries example declarations of both forms (assigner_name and assigner_names).