Delegated Types (Recording/Recordable Pattern)

A container model (Recording) unifies many content types (Recordables) that share metadata, access control, and operations.

Core insight: When many content types must comingle and share behavior, create a unified container rather than duplicating metadata or using complex polymorphic queries.

When to Use

digraph decision {
    "How many content types?" [shape=diamond];
    "Do they comingle in UI?" [shape=diamond];
    "Share metadata/operations?" [shape=diamond];
    "Direct associations" [shape=box];
    "Delegated types" [shape=box];
    "Consider merging types" [shape=box];

    "How many content types?" -> "Direct associations" [label="1-2"];
    "How many content types?" -> "Do they comingle in UI?" [label="3-4"];
    "How many content types?" -> "Share metadata/operations?" [label="5+"];
    "Do they comingle in UI?" -> "Consider merging types" [label="rarely"];
    "Do they comingle in UI?" -> "Share metadata/operations?" [label="often"];
    "Share metadata/operations?" -> "Direct associations" [label="no"];
    "Share metadata/operations?" -> "Delegated types" [label="yes"];
}

Use when:

5+ content types (Messages, Documents, Uploads, Comments, etc.)
Types appear together in feeds, timelines, activity streams
Shared metadata: project, creator, timestamps, access control
Common operations: copy, move, archive, search across all types
Single query needed: "all content in project X sorted by date"

Don't use when:

Only 1-3 types (use direct associations)
Types are similar enough to merge (see flattening.md)
Types don't comingle in UI
No shared metadata or operations

Quick Reference

Decision	Answer
1-2 content types	Direct associations
3-4 types, rarely comingle	Consider merging or direct
3-4 types, often comingle	Maybe delegated types
5+ types, shared metadata	Delegated types
Only 1 type left	Flatten (see flattening.md)
Container adds no value	Flatten

Difference from Simple Polymorphic

Aspect	Simple Polymorphic	Delegated Types
Query target	Each concrete type	Recording (container)
Shared metadata	Duplicated in each table	Once in Recording
Pagination	Complex (merge in Ruby)	Natural (query Recording)
Access control	Per-type	On Recording

Sub-Documents

implementing.md - Two-layer pattern (Bucket + Recording), schema, models, querying
flattening.md - When to remove delegated types, PR 445 example, signals

Real-World Usage

Basecamp uses this for: Messages, Documents, Uploads, Comments, Todos - unified project timeline with shared access control and operations.

Fizzy removed it: When only Comments remained after merging EventSummary (see flattening.md).

vanilla-rails-delegated-types