frappe-impl-controllers

Document Controllers — Implementation Workflows

Step-by-step workflows for building server-side DocType logic with full Python power. For exact syntax, see frappe-syntax-controllers.

Version: v14/v15/v16 | v15+: Supports auto-generated type annotations

Quick Decision: Controller vs Server Script?

NEED full Python (imports, classes, generators)?     → Controller
NEED external libraries (requests, pandas)?          → Controller
NEED try/except with rollback?                       → Controller
NEED frappe.enqueue() for background jobs?           → Controller
NEED to extend standard ERPNext DocType?             → Controller
Quick validation without custom app?                 → Server Script
Simple auto-fill or notification?                    → Server Script

Rule: ALWAYS use Controllers when you need a custom app. ALWAYS use Server Scripts for no-code prototyping.

Workflow 1: Create a New Controller

Step 1: Create DocType via Frappe UI or bench new-doctype

Step 2: File is auto-generated at:

apps/myapp/myapp/{module}/doctype/{doctype_name}/{doctype_name}.py

Step 3: Implement the controller class:

import frappe
from frappe import _
from frappe.model.document import Document

class MyDocType(Document):
    def validate(self):
        self.validate_dates()
        self.calculate_totals()

    def validate_dates(self):
        if self.from_date and self.to_date and self.from_date > self.to_date:
            frappe.throw(_("From Date cannot be after To Date"))

    def calculate_totals(self):
        self.total = sum(item.amount for item in self.items)

Step 4: Run bench restart (or bench watch for hot-reload in dev)

Naming convention: DocType "Sales Order" → class SalesOrder, file sales_order.py

Workflow 2: Choose the Right Hook

WHAT DO YOU WANT?
├── Validate data / calculate fields before save?
│   └── validate — changes to self ARE saved
│
├── Action AFTER save (emails, linked docs, logs)?
│   └── on_update — changes to self NOT saved (use db_set)
│
├── Only for NEW documents?
│   └── after_insert
│
├── Before/after SUBMIT?
│   ├── Check before submit → before_submit
│   └── Ledger entries after → on_submit
│
├── Before/after CANCEL?
│   ├── Prevent cancel → before_cancel
│   └── Reverse entries → on_cancel
│
├── Before DELETE?
│   └── on_trash (throw to prevent)
│
├── Custom document naming?
│   └── autoname
│
└── Detect ANY change (including db_set)?
    └── on_change

See references/decision-tree.md for all hooks with execution order.

CRITICAL: validate vs on_update

Aspect	validate	on_update
When	Before DB write	After DB write
self.x = y saved?	YES	NO — use db_set
Can abort with throw?	YES	Already saved
get_doc_before_save()	Available	Available
Use for	Validation, calculations	Notifications, linked docs

# WRONG — changes in on_update are NOT saved
def on_update(self):
    self.status = "Completed"  # LOST!

# CORRECT — use db_set
def on_update(self):
    frappe.db.set_value(self.doctype, self.name, "status", "Completed")

Workflow 3: Validation with Error Collection

def validate(self):
    errors = []
    if not self.items:
        errors.append(_("At least one item is required"))
    for item in self.items:
        if item.qty <= 0:
            errors.append(_("Row {0}: Qty must be positive").format(item.idx))
    if self.from_date > self.to_date:
        errors.append(_("From Date cannot be after To Date"))
    if errors:
        frappe.throw("<br>".join(errors))

Workflow 4: Detect Field Changes

def validate(self):
    old = self.get_doc_before_save()
    if old and old.status != self.status:
        self.flags.status_changed = True
        self.status_changed_on = frappe.utils.now()

def on_update(self):
    if self.flags.get('status_changed'):
        self.notify_status_change()

Rule: ALWAYS use self.flags to pass data between hooks. NEVER rely on external state.

Workflow 5: Custom Naming (autoname)

from frappe.model.naming import getseries

def autoname(self):
    # Format: PRJ-CUST-2025-001
    code = (self.customer or "GEN")[:4].upper()
    year = frappe.utils.getdate(self.start_date or frappe.utils.today()).year
    prefix = f"PRJ-{code}-{year}-"
    self.name = getseries(prefix, 3)

Alternative — before_naming:

def before_naming(self):
    if self.is_priority:
        self.naming_series = "PRIORITY-.#####"
    else:
        self.naming_series = "STD-.#####"

Workflow 6: Submittable Document

DRAFT (docstatus=0) → submit() → SUBMITTED (docstatus=1) → cancel() → CANCELLED (docstatus=2)

submit():  validate → before_submit → [DB: docstatus=1] → on_update → on_submit
cancel():  before_cancel → [DB: docstatus=2] → on_cancel

class PurchaseOrder(Document):
    def validate(self):
        self.validate_items()
        self.calculate_totals()

    def before_submit(self):
        # ONLY submit-specific checks here
        if self.total > 100000 and not self.manager_approval:
            frappe.throw(_("Manager approval required for POs over 100,000"))

    def on_submit(self):
        self.update_ordered_qty()
        self.create_purchase_receipt_draft()

    def before_cancel(self):
        if frappe.db.exists("Purchase Invoice",
                {"purchase_order": self.name, "docstatus": 1}):
            frappe.throw(_("Cancel linked invoices first"))

    def on_cancel(self):
        self.reverse_ordered_qty()

Rule: NEVER duplicate validation between validate and before_submit. validate ALWAYS runs before before_submit.

Workflow 7: Override Standard ERPNext Controller

Method A: Full Override (hooks.py)

# hooks.py
override_doctype_class = {
    "Sales Invoice": "myapp.overrides.CustomSalesInvoice"
}

# myapp/overrides.py
from erpnext.accounts.doctype.sales_invoice.sales_invoice import SalesInvoice

class CustomSalesInvoice(SalesInvoice):
    def validate(self):
        super().validate()  # ALWAYS call parent first
        self.custom_validation()

Method B: Event Handler (Safer, no class override)

# hooks.py
doc_events = {
    "Sales Invoice": {
        "validate": "myapp.events.validate_sales_invoice",
    }
}

# myapp/events.py
def validate_sales_invoice(doc, method=None):
    if doc.grand_total < 0:
        frappe.throw(_("Invalid total"))

Method C: extend_doctype_class (v16+)

# hooks.py
extend_doctype_class = {
    "Sales Invoice": "myapp.extends.SalesInvoiceExtend"
}

# myapp/extends.py — Only methods to add/override
class SalesInvoiceExtend:
    def custom_method(self):
        pass

Rule: ALWAYS call super().validate() in override. NEVER skip parent methods — standard ERPNext logic depends on it.

Workflow 8: Whitelisted Methods (Client-Callable)

class Quotation(Document):
    @frappe.whitelist()
    def apply_discount(self, discount_percent):
        if discount_percent < 0 or discount_percent > 100:
            frappe.throw(_("Discount must be 0-100"))
        self.discount_amount = self.total * (discount_percent / 100)
        self.grand_total = self.total - self.discount_amount
        self.save()
        return {"grand_total": self.grand_total}

Client-side call:

frm.call('apply_discount', { discount_percent: 10 }).then(r => {
    frm.reload_doc();
});

Workflow 9: Flags System

# Document-level flags (built-in)
doc.flags.ignore_permissions = True    # Bypass permission checks
doc.flags.ignore_validate = True       # Skip validate() hook
doc.flags.ignore_mandatory = True      # Skip required field check

# Custom flags for inter-hook communication
def validate(self):
    if self.is_urgent:
        self.flags.needs_notification = True

def on_update(self):
    if self.flags.get('needs_notification'):
        self.notify_team()

Workflow 10: Testing Controllers

# tests/test_my_doctype.py
import frappe
from frappe.tests.utils import FrappeTestCase

class TestMyDocType(FrappeTestCase):
    def test_validate_dates(self):
        doc = frappe.get_doc({
            "doctype": "My DocType",
            "from_date": "2025-01-10",
            "to_date": "2025-01-01"  # Before from_date
        })
        self.assertRaises(frappe.ValidationError, doc.insert)

    def test_calculate_totals(self):
        doc = frappe.get_doc({
            "doctype": "My DocType",
            "items": [
                {"item": "A", "qty": 2, "rate": 100},
                {"item": "B", "qty": 3, "rate": 50}
            ]
        })
        doc.insert()
        self.assertEqual(doc.total, 350)

Run: bench run-tests --module myapp.module.doctype.my_doctype.test_my_doctype

Execution Order Reference

INSERT

before_insert → before_naming → autoname → before_validate →
validate → before_save → [DB INSERT] → after_insert →
on_update → on_change

SAVE (existing)

before_validate → validate → before_save → [DB UPDATE] →
on_update → on_change

SUBMIT

validate → before_submit → [DB: docstatus=1] →
on_update → on_submit → on_change

Anti-Pattern Quick Check

Do NOT	Do Instead
self.x = y in on_update	frappe.db.set_value(...)
self.save() in on_update	Causes infinite loop
frappe.db.commit() in hooks	Let framework handle
Heavy ops in validate	Use frappe.enqueue() in on_update
Skip super().validate()	ALWAYS call parent first
frappe.get_doc() in loops	Use frappe.get_cached_doc()
Hardcoded thresholds	Use Settings DocType

See references/anti-patterns.md for complete list.

Related Skills

• frappe-syntax-controllers — Exact hook signatures and API
• frappe-errors-controllers — Error handling patterns
• frappe-impl-serverscripts — When Server Script suffices
• frappe-syntax-hooks — hooks.py configuration
• frappe-core-database — frappe.db.* operations

See references/decision-tree.md for all hooks. See references/workflows.md for extended patterns. See references/examples.md for complete working examples.