frappe-syntax-controllers

# Frappe Syntax: Document Controllers

Document Controllers are Python classes that define all server-side logic for a DocType.
EVERY DocType has exactly one controller file. The controller class extends `frappe.model.document.Document`.

## Quick Reference

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

class SalesOrder(Document):
    def autoname(self):
        """Custom naming logic. Sets self.name."""
        self.name = f"SO-{self.customer_code}-{frappe.utils.now_datetime().year}"

    def validate(self):
        """MAIN validation — runs on EVERY save (insert and update).
        Changes to self ARE saved to database."""
        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.
        Use frappe.db.set_value() for post-save field changes."""
        self.notify_linked_docs()

    def on_submit(self):
        """After submit (docstatus 0 -> 1). Create ledger entries here."""
        self.create_gl_entries()

    def on_cancel(self):
        """After cancel (docstatus 1 -> 2). Reverse ledger entries here."""
        self.reverse_gl_entries()

    @frappe.whitelist()
    def recalculate(self):
        """Exposed to client JS via frm.call('recalculate')."""
        self.total = sum(item.amount for item in self.items)
        return {"total": self.total}
```

### File Location and Naming

| DocType Name | Class Name | File Path |
|---|---|---|
| Sales Order | `SalesOrder` | `selling/doctype/sales_order/sales_order.py` |
| My Custom Doc | `MyCustomDoc` | `module/doctype/my_custom_doc/my_custom_doc.py` |

**Rule**: DocType name -> PascalCase class -> snake_case filename. ALWAYS match exactly.

---

## Lifecycle Hook Execution Order

### INSERT (new document)

```
before_insert -> before_naming -> autoname -> before_validate -> validate
-> before_save -> [db_insert] -> after_insert -> on_update -> on_change
```

### SAVE (existing document)

```
before_validate -> validate -> before_save -> [db_update]
-> on_update -> on_change
```

### SUBMIT (docstatus 0 -> 1)

```
before_validate -> validate -> before_submit -> [db_update]
-> on_submit -> on_update -> on_change
```

### CANCEL (docstatus 1 -> 2)

```
before_cancel -> [db_update] -> on_cancel -> on_change
```

### UPDATE AFTER SUBMIT

```
before_update_after_submit -> [db_update]
-> on_update_after_submit -> on_change
```

### DELETE

```
on_trash -> [db_delete] -> after_delete
```

### DISCARD [v15+]

```
before_discard -> [db_set docstatus=2] -> on_discard
```

**Complete hook reference with parameters**: See [lifecycle-methods.md](references/lifecycle-methods.md)

---

## Hook Selection Decision Tree

```
What do you need to do?
|
+-- Validate data or calculate fields?
|   +-- validate (changes to self ARE saved)
|
+-- Action AFTER save (emails, sync, linked docs)?
|   +-- on_update (changes to self are NOT saved)
|
+-- Only for NEW documents?
|   +-- after_insert (runs once on first save only)
|
+-- Custom document name?
|   +-- autoname (set self.name)
|
+-- Before/after SUBMIT?
|   +-- Validate before submit? -> before_submit
|   +-- Create entries after submit? -> on_submit
|
+-- Before/after CANCEL?
|   +-- Check linked docs? -> before_cancel
|   +-- Reverse entries? -> on_cancel
|
+-- Cleanup before delete?
|   +-- on_trash
|
+-- React to ANY value change (including db_set)?
|   +-- on_change (MUST be idempotent)
```

---

## Critical Rules

### 1. Changes after on_update are NOT saved

```python
# WRONG - change is lost after on_update
def on_update(self):
    self.status = "Completed"  # NOT saved to database

# CORRECT - use db_set or frappe.db.set_value
def on_update(self):
    self.db_set("status", "Completed")
```

### 2. NEVER call frappe.db.commit() in controllers

```python
# WRONG - breaks Frappe transaction management
def validate(self):
    frappe.db.commit()  # Can cause partial updates on error

# CORRECT - Frappe commits automatically at end of request
def validate(self):
    self.update_related()  # No commit needed
```

### 3. ALWAYS call super() when overriding

```python
# WRONG - parent validation is skipped entirely
def validate(self):
    self.custom_check()

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

### 4. Use flags for recursion prevention

```python
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()
```

### 5. NEVER put validation logic in on_update

```python
# WRONG - document is already saved when this throws
def on_update(self):
    if self.total < 0:
        frappe.throw("Invalid total")  # Too late!

# CORRECT - validate BEFORE save
def validate(self):
    if self.total < 0:
        frappe.throw("Invalid total")  # Blocks save
```

---

## Document Naming (autoname)

| Method | Example | Result | Version |
|---|---|---|---|
| `field:fieldname` | `field:customer_name` | `ABC Company` | All |
| `naming_series:` | `naming_series:` | `SO-2024-00001` | All |
| Expression | `PRE-.#####` | `PRE-00001` | All |
| Old-style format | `INV-{YYYY}-{####}` | `INV-2024-0001` | Deprecated v16 |
| `hash` / `random` | `hash` | `a1b2c3d4e5` | All |
| `Prompt` | `Prompt` | User enters name | All |
| `autoincrement` | `autoincrement` | `1`, `2`, `3` | All |
| **`UUID`** | `UUID` | `550e8400-e29b-...` | **v16+** |
| Custom method | `autoname()` in controller | Any pattern | All |

### Custom autoname Method

```python
from frappe.model.naming import getseries

class Project(Document):
    def autoname(self):
        prefix = f"P-{self.customer[:3].upper()}-"
        self.name = getseries(prefix, 3)
        # Result: P-ACM-001, P-ACM-002, etc.
```

### UUID Naming [v16+]

Set `autoname = "UUID"` in DocType definit