FieldViaAliasedRelationship — read a field from a target instance via an aliased DefinedSimpleRelationship

[matt-beanland]

Description

There is no general-purpose way to read a base field from the target instance of a DefinedSimpleRelationship filtered by alias. Resources that need to surface such a field must write bespoke calculation modules, repeating the same forward-traversal logic each time.

This is the forward-relationship counterpart to FieldViaAssignedRelationship (#156), which traverses AssignmentRelationship in the reverse direction. Together they cover the two primary aliased traversal forms in the diffo graph.

What we need

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

• alias: — the alias to filter on, matching DefinedSimpleRelationship records where source_id = current.id, alias = step. Optional — when omitted, traverses all outgoing DefinedSimpleRelationship records.
• field: — the field name to read from the resolved target instance.

Returns a list of field values.

# Aliased — field from the target of the :provides relationship
calculate :provider_name, :string,
  {Diffo.Provider.Calculations.FieldViaAliasedRelationship, [alias: :provides, field: :name]}

# Unaliased — field from all directly related targets
calculate :related_names, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaAliasedRelationship, [field: :name]}

Why it matters

Both aliased traversal forms — via assignment and via named relationship — are needed for the demo. FieldViaAssignedRelationship covers the reverse (consumer reading from provider); FieldViaAliasedRelationship covers the forward (provider reading from its targets). Both are required this release.

[matt-beanland]

Implemented

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

How it works

The calculation traverses DefinedSimpleRelationship in the forward direction (source → target), optionally filtered by alias, then reads a named field from each resolved target instance.

Aliased — returns the field from the target of the named relationship slot:

calculate :provider_name, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaAliasedRelationship, [alias: :provides, field: :name]}

Unaliased — returns the field from all directly related targets:

calculate :related_names, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaAliasedRelationship, [field: :name]}

This is the forward-relationship counterpart to FieldViaAssignedRelationship (#156), which traverses AssignmentRelationship in reverse. Together they cover both aliased traversal directions in the diffo graph.

Test coverage

Five integration tests in test/provider/extension/field_via_aliased_relationship_test.exs cover: field returned via alias, empty list when no match, alias filtering (two targets with different aliases — only the matching one returned), unaliased returning all targets, and empty list with no relationships.

The ShelfInstance test resource carries example declarations of both forms (linked_target_name and all_linked_names).

[matt-beanland]

On review have decided to make this take arguments type and/or alias with nil wild.
Also will rename a FieldViaRelationship

[matt-beanland]

Update — renamed to FieldViaRelationship

FieldViaAliasedRelationship has been superseded by the more general Diffo.Provider.Calculations.FieldViaRelationship.

What changed

The original module only accepted alias:. The replacement accepts any combination of type: and alias: — both optional, both nil means wildcard on that dimension. This handles the common case where a source has many forward relationships of different types and aliases and you need to narrow by one or both.

# alias only
calculate :linked_name, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaRelationship, [alias: :link, field: :name]}

# type + alias
calculate :assigned_linked_name, {:array, :string},
  {Diffo.Provider.Calculations.FieldViaRelationship, [type: :assignedTo, alias: :link, field: :name]}

The unaliased-only form (source_id alone) was dropped — querying all forward relationships without any filter returns a mix of unrelated things from one source, which is almost never useful.

The ShelfInstance test resource carries examples of both forms, and the test file is test/provider/extension/field_via_relationship_test.exs.