import frappe
from frappe.model.document import Document

class SalesOrder(Document):
    def validate(self):
        """Main validation - runs on every save."""
        if not self.items:
            frappe.throw(_("Items are required"))
        self.total = sum(item.amount for item in self.items)
    
    def on_update(self):
        """After save - changes to self are NOT saved."""
        self.update_linked_docs()

What do you want to do?
│
├─► Validate or calculate fields?
│   └─► validate
│
├─► Action after save (emails, linked docs)?
│   └─► on_update
│
├─► Only for NEW docs?
│   └─► after_insert
│
├─► On SUBMIT?
│   ├─► Check beforehand? → before_submit
│   └─► Action afterwards? → on_submit
│
├─► On CANCEL?
│   ├─► Check beforehand? → before_cancel
│   └─► Cleanup? → on_cancel
│
├─► Custom document name?
│   └─► autoname
│
└─► Cleanup before delete?
    └─► on_trash

# ❌ WRONG - change is lost
def on_update(self):
    self.status = "Completed"  # NOT saved

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

# ❌ WRONG - Frappe handles commits
def on_update(self):
    frappe.db.commit()  # DON'T DO THIS

# ✅ CORRECT - no commit needed
def on_update(self):
    self.update_related()  # Frappe commits automatically

# ❌ WRONG - parent logic is skipped
def validate(self):
    self.custom_check()

# ✅ CORRECT - parent logic is preserved
def validate(self):
    super().validate()
    self.custom_check()

def on_update(self):
    if self.flags.get('from_linked_doc'):
        return
    
    linked = frappe.get_doc("Linked Doc", self.linked_doc)
    linked.flags.from_linked_doc = True
    linked.save()

{
  "doctype": "DocType",
  "autoname": "UUID"
}

# Frappe automatically generates UUID7
# In naming.py:
if meta.autoname == "UUID":
    doc.name = str(uuid_utils.uuid7())

# UUID names are validated on import
from uuid import UUID
try:
    UUID(doc.name)
except ValueError:
    frappe.throw(_("Invalid UUID: {}").format(doc.name))

from frappe.model.naming import getseries

class Project(Document):
    def autoname(self):
        # Custom naming based on customer
        prefix = f"P-{self.customer}-"
        self.name = getseries(prefix, 3)
        # Result: P-ACME-001, P-ACME-002, etc.

# hooks.py
override_doctype_class = {
    "Sales Order": "custom_app.overrides.CustomSalesOrder"
}

# custom_app/overrides.py
from erpnext.selling.doctype.sales_order.sales_order import SalesOrder

class CustomSalesOrder(SalesOrder):
    def validate(self):
        super().validate()
        self.custom_validation()

# hooks.py
doc_events = {
    "Sales Order": {
        "validate": "custom_app.events.validate_sales_order",
        "on_submit": "custom_app.events.on_submit_sales_order"
    }
}

# custom_app/events.py
def validate_sales_order(doc, method):
    if doc.total > 100000:
        doc.requires_approval = 1

class StockEntry(Document):
    def on_submit(self):
        """After submit - create stock ledger entries."""
        self.update_stock_ledger()
    
    def on_cancel(self):
        """After cancel - reverse the entries."""
        self.reverse_stock_ledger()

class ExternalCustomer(Document):
    @staticmethod
    def get_list(args):
        return external_api.get_customers(args.get("filters"))
    
    @staticmethod
    def get_count(args):
        return external_api.count_customers(args.get("filters"))
    
    @staticmethod
    def get_stats(args):
        return {}

from frappe.model.document import Document

class MyDocType(Document):
    pass

from frappe.utils.nestedset import NestedSet

class Department(NestedSet):
    pass

from erpnext.selling.doctype.sales_order.sales_order import SalesOrder

class CustomSalesOrder(SalesOrder):
    def validate(self):
        super().validate()
        self.custom_validation()

class Person(Document):
    if TYPE_CHECKING:
        from frappe.types import DF
        first_name: DF.Data
        last_name: DF.Data
        birth_date: DF.Date

export_python_type_annotations = True

When to use UUID:
├── Cross-system data synchronization
├── Bulk record creation
├── Global uniqueness required
└── No human-readable name needed

When to use traditional naming:
├── User-facing document references (SO-00001)
├── Sequential numbering required
├── Auditing requires readable names
└── Integration with legacy systems

def on_update(self):
    self.status = "Done"  # Will be lost!

def validate(self):
    frappe.db.commit()  # Breaks transaction!

def validate(self):
    self.my_check()  # Parent validate is skipped