Views support

Problem/Motivation

Primary Entity Reference currently provides only minimal Views integration. While the field works correctly in
entity forms and renderers, Views cannot:

• Create relationships to the referenced entities
• Filter or sort based on the primary reference flag
• Expose only the primary referenced entity in listings
• Match the functionality that core Entity Reference provides in Views

This makes it difficult to use the field in practical reporting and listing scenarios, particularly when a consumer
needs to join on, or specifically display, the primary referenced entity.

Steps to reproduce

• Create a content type that uses a Primary Entity Reference field.
• Attempt to add the field to a View as a relationship or contextual filter.
• Only minimal field display is available; no relationships or primary-only capabilities exist.

Proposed resolution

Implement full Views integration equivalent to core Entity Reference, using modern
service-based Views data providers and auto-wired Hook Attributes.
Instead of a primary_entity_reference.views.inc file or procedural hook_views_data(),
the module will register and expose its Views data through attribute-driven services.

The implementation would include:

• Views data service
  • Provide a views.views_data service for Primary Entity Reference.
  • Use Hook Attributes (e.g., #[ViewsData]) to auto-register Views data.
  • Expose field, filter, sort, and relationship metadata programmatically.
• Field handler plugin extending core ER field handler:
  • Option: Display all referenced entities
  • Option: Display only the primary referenced entity
• Relationship plugins:
  • A standard ER-style relationship matching core behavior.
  • A primary-only relationship applying a primary = 1 join condition.
• Filter plugin:
  • Filtering by referenced entity with optional “primary only” mode.
• Sort plugin:
  • Sorting by weight or by the primary flag.

Using service discovery and Hook Attributes ensures the module follows modern Drupal patterns and avoids the legacy
.views.inc approach entirely.

Remaining tasks

• Implement Views data provider service with Hook Attributes.
• Add field, filter, sort, and relationship plugins.
• Add primary-only relationship logic.
• Add tests covering each Views integration point.
• Document config options and usage examples.

User interface changes

• In the field handler options, add a setting to display:
  • All referenced entities
  • Primary reference only

API changes

• New Views plugins (field, filter, sort, relationship) under Plugin\views\*.
• New views.views_data service using Hook Attributes.

Data model changes

No data model changes. The existing primary column already supports the necessary behavior.