ERPNext Custom App - Implementation

This skill helps you determine HOW to build and structure Frappe/ERPNext custom apps. For exact syntax, see erpnext-syntax-customapp .

Version: v14/v15/v16 compatible (differences noted)

Main Decision: What Are You Building?

┌─────────────────────────────────────────────────────────────────────────┐ │ WHAT DO YOU WANT TO CREATE? │ ├─────────────────────────────────────────────────────────────────────────┤ │ │ │ ► Completely new Frappe/ERPNext app? │ │ └─► See: NEW APP WORKFLOW │ │ │ │ ► Extend existing ERPNext functionality? │ │ └─► See: EXTENSION DECISION │ │ │ │ ► Migrate data between fields/DocTypes? │ │ └─► See: PATCH vs FIXTURE DECISION │ │ │ │ ► Export configuration for deployment? │ │ └─► See: FIXTURE WORKFLOW │ │ │ │ ► Update existing app to newer Frappe version? │ │ └─► See: VERSION UPGRADE WORKFLOW │ │ │ └─────────────────────────────────────────────────────────────────────────┘

Decision 1: Do You Need a Custom App?

┌─────────────────────────────────────────────────────────────────────────┐ │ DO YOU ACTUALLY NEED A CUSTOM APP? │ ├─────────────────────────────────────────────────────────────────────────┤ │ │ │ What changes do you need? │ │ │ │ ► Add fields to existing DocType? │ │ └─► NO APP NEEDED: Use Custom Field + Property Setter │ │ (Can be exported as fixtures from ANY app) │ │ │ │ ► Simple automation/validation? │ │ └─► NO APP NEEDED: Server Script or Client Script │ │ (Stored in database, no deployment needed) │ │ │ │ ► Complex business logic, new DocTypes, or Python code? │ │ └─► YES, CREATE APP: You need controllers, models, and deployment │ │ │ │ ► Integration with external system? │ │ └─► USUALLY YES: APIs need whitelisted methods, scheduled sync │ │ │ │ ► Custom reports with complex queries? │ │ └─► DEPENDS: Script Report (no app) vs Query Report (app optional) │ │ │ └─────────────────────────────────────────────────────────────────────────┘

Rule: Start with the SIMPLEST solution. Server Scripts + Custom Fields solve 70% of customization needs without a custom app.

Decision 2: Extension Strategy

┌─────────────────────────────────────────────────────────────────────────┐ │ HOW TO EXTEND ERPNext? │ ├─────────────────────────────────────────────────────────────────────────┤ │ │ │ ► Add fields to existing DocType (e.g., Sales Invoice)? │ │ └─► Custom Field (via UI or fixtures) │ │ └─► Property Setter for behavior changes │ │ │ │ ► Modify DocType behavior/logic? │ │ ├─► v16: Use extend_doctype_class hook (PREFERRED) │ │ └─► v14/v15: Use doc_events hooks in hooks.py │ │ │ │ ► Override Jinja template? │ │ └─► Copy template to your app's templates/ folder │ │ └─► Register via jinja.override_template in hooks.py │ │ │ │ ► Add new DocType related to existing? │ │ └─► Create in your app's module │ │ └─► Link via Link field or Dynamic Link │ │ │ │ ► Add new workspace/menu items? │ │ └─► Create Workspace DocType in your app │ │ └─► Or use standard_portal_menu_items hook │ │ │ └─────────────────────────────────────────────────────────────────────────┘

See: references/decision-tree.md for detailed extension patterns.

Decision 3: Patch vs Fixture

┌─────────────────────────────────────────────────────────────────────────┐ │ SHOULD THIS BE A PATCH OR A FIXTURE? │ ├─────────────────────────────────────────────────────────────────────────┤ │ │ │ Is it CONFIGURATION that should be the same everywhere? │ │ (Custom Fields, Roles, Workflows, Property Setters) │ │ └─► USE FIXTURE │ │ │ │ Is it a ONE-TIME data transformation? │ │ (Migrate old field values, cleanup bad data, populate defaults) │ │ └─► USE PATCH │ │ │ │ Does it need to run BEFORE schema changes? │ │ (Backup data from field that will be deleted) │ │ └─► USE PATCH with [pre_model_sync] │ │ │ │ Does it need to run AFTER schema changes? │ │ (Populate newly added field with calculated values) │ │ └─► USE PATCH with [post_model_sync] │ │ │ │ Is it master data / lookup tables? │ │ (Categories, Status options, Configuration records) │ │ └─► USE FIXTURE for initial, PATCH for updates │ │ │ └─────────────────────────────────────────────────────────────────────────┘

See: references/decision-tree.md for patch timing flowchart.

Decision 4: Module Organization

┌─────────────────────────────────────────────────────────────────────────┐ │ HOW MANY MODULES DO YOU NEED? │ ├─────────────────────────────────────────────────────────────────────────┤ │ │ │ Small app (1-5 DocTypes, single purpose)? │ │ └─► ONE MODULE with app name │ │ Example: my_app/my_app/ (module "My App") │ │ │ │ Medium app (6-15 DocTypes, multiple areas)? │ │ └─► 2-4 MODULES by functional area │ │ Example: core/, settings/, integrations/ │ │ │ │ Large app (15+ DocTypes, complex domain)? │ │ └─► MODULES by business domain │ │ Example: inventory/, sales/, purchasing/, settings/ │ │ │ │ Multi-tenant or vertical solution? │ │ └─► Consider MULTIPLE APPS instead │ │ Base app + vertical-specific apps │ │ │ └─────────────────────────────────────────────────────────────────────────┘

Rule: Each DocType belongs to EXACTLY one module. Choose module = where would a user look for this DocType?

Quick Implementation Workflows

New App Workflow

1. Create app structure → bench new-app my_app
2. Configure pyproject → Edit pyproject.toml (v15+) or setup.py (v14)
3. Define modules → Edit modules.txt
4. Create DocTypes → bench --site mysite new-doctype MyDocType
5. Write controllers → my_app/doctype/my_doctype/my_doctype.py
6. Configure hooks → hooks.py for integration
7. Export fixtures → bench --site mysite export-fixtures
8. Test installation → bench --site testsite install-app my_app

See: references/workflows.md for detailed steps.

Patch Workflow

1. Plan the migration → What data moves where?
2. Choose timing → [pre_model_sync] or [post_model_sync]
3. Write patch file → myapp/patches/v1_0/description.py
4. Add to patches.txt → Under correct section
5. Test locally → bench --site testsite migrate
6. Handle errors → Add rollback logic if needed
7. Test on copy of prod → ALWAYS before production

See: references/workflows.md for patch patterns.

Fixture Workflow

1. Configure hooks.py → Define fixtures list with filters
2. Make changes via UI → Custom Fields, Property Setters, etc.
3. Export fixtures → bench --site mysite export-fixtures --app my_app
4. Verify JSON files → Check my_app/fixtures/*.json
5. Commit to version control
6. Test import → bench --site newsite migrate

See: references/workflows.md for fixture strategies.

Version-Specific Considerations

Aspect v14 v15 v16

Build config setup.py pyproject.toml pyproject.toml

DocType extension doc_events doc_events extend_doctype_class preferred

Python minimum 3.10 3.10 3.11

INI patches ✅ ✅ ✅

Fixtures format JSON JSON JSON

v16 Breaking Changes

• extend_doctype_class hook: Cleaner DocType extension pattern

• Data masking: Field-level privacy configuration available

• UUID naming: New naming rule option for DocTypes

• Chrome PDF: wkhtmltopdf deprecated for PDF generation

Critical Implementation Rules

✅ ALWAYS

• Start with bench new-app

• Never create structure manually

• Define version in init.py

• Build will fail without it

• Test patches on database copy - Never run untested patches on production

• Use batch processing - Any patch touching 1000+ records needs batching

• Filter fixtures - Never export all records of a DocType

• Version your patches - Use v1_0, v2_0 directories for organization

❌ NEVER

• Put frappe/erpnext in pyproject dependencies - They're not on PyPI

• Include transactional data in fixtures - Only configuration!

• Hardcode site-specific values - Use hooks or settings DocTypes

• Skip frappe.db.commit() in large patches - Memory will explode

• Delete fields without backup patch - Data loss is irreversible

• Modify core ERPNext files - Always use hooks or override patterns

Reference Files

File Contents

references/decision-tree.md

Complete decision flowcharts

references/workflows.md

Step-by-step implementation guides

references/examples.md

Complete working examples

references/anti-patterns.md

Common mistakes to avoid

See Also

• erpnext-syntax-customapp

• Exact syntax reference

• erpnext-syntax-hooks

• Hooks configuration

• erpnext-impl-hooks

• Hook implementation patterns

• erpnext-database

• Database operations for patches