# Custom Data Patterns

> Custom Data anti-patterns and corrections for Canvas plugins - unnecessary compatibility checks, misuse of AttributeHubs vs CustomModels

- Skill: `canvas-medical/custom-data-patterns` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add canvas-medical/custom-data-patterns`
- Raw SKILL.md: https://api.skillmd.com/api/skills/canvas-medical/custom-data-patterns/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: canvas-medical (https://skillmd.com/u/canvas-medical)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/canvas-medical/custom-data-patterns

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# Custom Data Patterns

This skill identifies common anti-patterns in how Canvas plugins use Custom Data (CustomModels and AttributeHubs) and provides corrections. These patterns have been observed in real customer plugins.

## When to Use This Skill

Invoke this skill **proactively** as soon as Custom Data is involved — don't wait for a review phase. Use it when:
- The plugin spec calls for persistent data storage (CustomModels or AttributeHubs)
- You are about to write or edit model definitions that subclass `CustomModel`
- You are about to write or edit code that uses `AttributeHub`
- You are adding relationships (ForeignKey, OneToOneField, ManyToManyField) to SDK models
- You are setting up the plugin directory structure and it will include CustomModels
- Reviewing existing plugin code that uses Custom Data

## Quick Reference

Reference the `custom_data_context.txt` file for detailed anti-patterns with BAD/GOOD examples.

> **Custom-data models are keyed on `dbid`, not `id`.** Any query that references `"id"` on a `CustomModel` — `Count("id")`, `order_by("id")`, `values("id")`, `F("id")` — raises `FieldError: Cannot resolve keyword 'id' into field`. Use `dbid` (the primary key on the SDK base model). Tests that mock the queryset (`patch(...objects...)`) never resolve the real field name, so this bug passes CI and 500s against a live DB. See the **database-performance** skill (§"Canvas Execution Limits") for detection and the write-amplification/idempotency patterns that also apply to custom-data-backed sync.