Custom Objects tab: complete relationship resolution (1:1, 1:n, n:m) + menu-scoped tabs + link-table abstraction

[christianbur]

Plugin Version

0.6.x

Proposed functionality

This feature request is a follow-up for the new Custom Objects tab and consolidates the intent from #612 and #578, aligned with maintainer feedback to extend the new tab behavior.

Requested behavior:

1. Relationship coverage in the new tab:

• 1:1 relationships should show reverse links on both sides.
• 1:n relationships should show reverse links on both sides.
• n:m relationships should support explicit link-table/link-type modeling and render logical target objects instead of intermediate technical rows.
• Current problem: if object A references object B, the relationship may be visible on B as an incoming link, but the corresponding reverse view from A back to B is not shown consistently. In practice, A -> B can be visible while B -> A is missing in the tab, so users cannot rely on the UI for symmetric relationship discovery.

2. Explicit link-table indicator:

• Add field-level metadata (example: link_table=true) to declare that a relation is implemented via an intermediate COT.
• When enabled, the tab should resolve and display direct logical endpoints.
• Intermediate link objects should be hidden by default (optional technical drill-down can remain available).

3. Flexible target types for link-table fields:

• For link-table/link-type fields, allow linking to all NetBox object types.
• This is important because COT field type immutability makes later refactoring expensive, and otherwise new target types from Custom Objects or other plugins can only be linked with substantial follow-up effort.
• Needed for cross-domain links (IPAM/DCIM/Virtualization/Tenancy/custom objects).

4. Menu-scoped tabs for linked objects (integration with Add support for custom menus for Custom Object Types (COTs) #610):

• Keep the default Custom Objects tab for COTs without menu_name.
• Create additional linked-object tabs per menu_name.
• If an object is linked from multiple menus, render multiple tabs.

5. Add button on the tab:

• The linked-objects tab should provide an Add button so users can create a new related object directly from the current context.
• The action should preserve the current host object as context, so the new relationship can be created with minimal manual input.

Related issues:

• Extend the "Custom Objects linking to this object" panel to support all relationship types #612 (closed as duplicate/not planned, but requirements remain relevant for the new tab)
• Configurable “linking custom objects” panels #578 (closed after reframing toward the new tab)
• Add support for custom menus for Custom Object Types (COTs) #610 (menu-based separation of tabs/navigation)

Use case

As Custom Object usage grows across multiple domains/projects, operators need predictable and symmetric relationship visibility from detail pages.

This proposal improves:

• Impact analysis (who references what)
• Reverse navigation (from target back to source)
• Usability in n:m models where link objects are technical implementation details
• Scalability of the UI via menu-based tab separation instead of one overloaded tab

Without these capabilities, relationship discovery is incomplete and operational workflows become error-prone.

External dependencies

None.