Fix P0-P2 bugs: sale.order M2M, display_name, DefaultGet

P0: Fix sale.order creation (was completely broken)
- Corrected M2M junction table name from sale_order_line_account_tax_rel
  to account_tax_sale_order_line_rel (ORM sorts alphabetically)
- Added fallback in BeforeCreate if sequence generation fails

P1: Add display_name as magic field on ALL models
- Added to addMagicFields() in pkg/orm/model.go (like Python BaseModel)
- Computed on-the-fly in Read() from recName field, no DB column
- Removed explicit display_name from res.partner (now auto-inherited)

P2: Add DefaultGet hooks for sale.order and purchase.order
- Sets company_id, currency_id, date_order/date_planned from environment
- Follows same pattern as account.move's DefaultGet

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/orm/model.go

@@ -129,6 +129,13 @@ func (m *Model) addMagicFields() {
 		Readonly: true,
 	}))
 
+	// display_name: computed on-the-fly from rec_name, not stored in DB.
+	// Mirrors: odoo/orm/models.py BaseModel.display_name (computed field on ALL models)
+	m.AddField(Char("display_name", FieldOpts{
+		String:  "Display Name",
+		Compute: "_compute_display_name",
+	}))
+
 	if m.logAccess {
 		m.AddField(Many2one("create_uid", "res.users", FieldOpts{
 			String:   "Created by",

pkg/orm/recordset.go

@@ -349,12 +349,17 @@ func (rs *Recordset) Read(fields []string) ([]Values, error) {
 	var storedFields []string    // Fields that come from the DB query
 	var m2mFields []string       // Many2many fields (from junction table)
 	var relatedFields []string   // Related fields (from joined table)
+	wantDisplayName := false     // Whether display_name was requested
 
 	for _, name := range fields {
 		f := m.GetField(name)
 		if f == nil {
 			return nil, fmt.Errorf("orm: field %q not found on %s", name, m.name)
 		}
+		if name == "display_name" && f.Compute != "" && !f.Store {
+			wantDisplayName = true
+			continue
+		}
 		if f.Type == TypeMany2many {
 			m2mFields = append(m2mFields, name)
 		} else if f.Related != "" && !f.Store {
@@ -365,6 +370,25 @@ func (rs *Recordset) Read(fields []string) ([]Values, error) {
 		}
 	}
 
+	// If display_name is requested, ensure the rec_name field is fetched from DB
+	recName := m.recName
+	if wantDisplayName {
+		recNameAlreadyFetched := false
+		for _, sf := range storedFields {
+			if sf == recName {
+				recNameAlreadyFetched = true
+				break
+			}
+		}
+		if !recNameAlreadyFetched {
+			recF := m.GetField(recName)
+			if recF != nil && recF.IsStored() {
+				columns = append(columns, fmt.Sprintf("%q", recF.Column()))
+				storedFields = append(storedFields, recName)
+			}
+		}
+	}
+
 	// Build query
 	var args []interface{}
 	idPlaceholders := make([]string, len(rs.ids))
@@ -426,6 +450,25 @@ func (rs *Recordset) Read(fields []string) ([]Values, error) {
 		}
 	}
 
+	// Post-fetch: compute display_name from the rec_name field value.
+	// Mirrors: odoo/orm/models.py BaseModel._compute_display_name()
+	if wantDisplayName {
+		for _, rec := range results {
+			if nameVal, ok := rec[recName]; ok {
+				switch v := nameVal.(type) {
+				case string:
+					rec["display_name"] = v
+				case nil:
+					rec["display_name"] = ""
+				default:
+					rec["display_name"] = fmt.Sprintf("%v", v)
+				}
+			} else {
+				rec["display_name"] = ""
+			}
+		}
+	}
+
 	// Post-fetch: M2M fields (from junction tables)
 	if len(m2mFields) > 0 && len(rs.ids) > 0 {
 		for _, fname := range m2mFields {