diff --git a/facho/fe/form/__init__.py b/facho/fe/form/__init__.py
index 0aa6318..b12a86a 100644
--- a/facho/fe/form/__init__.py
+++ b/facho/fe/form/__init__.py
@@ -99,7 +99,7 @@ class Amount:
             return self.fromNumber(val)
         if isinstance(val, Amount):
             return val
-        raise TypeError("cant cast to amount")
+        raise TypeError("cant cast %s to amount" % (type(val)))
     
     def __add__(self, rother):
         other = self._cast(rother)
diff --git a/facho/fe/model/__init__.py b/facho/fe/model/__init__.py
index 74aaf01..e1d895c 100644
--- a/facho/fe/model/__init__.py
+++ b/facho/fe/model/__init__.py
@@ -1,6 +1,8 @@
 import facho.model as model
 import facho.model.fields as fields
+import facho.fe.form as form
 from datetime import date, datetime
+from copy import copy
 
 class Name(model.Model):
     __name__ = 'Name'
@@ -48,24 +50,44 @@ class AccountingSupplierParty(model.Model):
 
     party = fields.Many2One(Party)
 
-class InvoicedQuantity(model.Model):
-    __name__  = 'InvoiceQuantity'
+class Quantity(model.Model):
+    __name__  = 'Quantity'
 
     code = fields.Attribute('unitCode', default='NAR')
+    value = fields.Virtual(default=0, update_internal=True)
+
+    def __default_set__(self, value):
+        self.value = value
+        return value
+
+    def __mul__(self, other):
+        return form.Amount(self.value) * other.value
 
 
 class Amount(model.Model):
     __name__ = 'Amount'
 
     currency = fields.Attribute('currencyID', default='COP')
-    
+    value = fields.Virtual(default=form.Amount(0), update_internal=True)
+
+    def __default_set__(self, value):
+        self.value = value
+        return value
+
 class Price(model.Model):
     __name__ = 'Price'
 
     amount = fields.Many2One(Amount, name='PriceAmount')
-
+    value = fields.Virtual(default=form.Amount(0))
+    
     def __default_set__(self, value):
         self.amount = value
+        self.value = value
+        return value
+
+    def __mul__(self, other):
+        return self.value * other.value
+
 
 class Percent(model.Model):
     __name__ = 'Percent'
@@ -102,13 +124,52 @@ class TaxTotal(model.Model):
 
     tax_amount = fields.Many2One(Amount, name='TaxAmount')
     subtotals = fields.One2Many(TaxSubTotal)
+
+
+class AllowanceCharge(model.Model):
+    __name__ = 'AllowanceCharge'
+
+    amount = fields.Many2One(Amount)
+    is_discount = fields.Virtual(default=False)
     
+    def isCharge(self):
+        return self.is_discount == False
+
+    def isDiscount(self):
+        return self.is_discount == True
+
 class InvoiceLine(model.Model):
     __name__ = 'InvoiceLine'
 
-    quantity = fields.Many2One(InvoicedQuantity)
+    quantity = fields.Many2One(Quantity, name='InvoicedQuantity')
     taxtotal = fields.Many2One(TaxTotal)
     price = fields.Many2One(Price)
+    amount = fields.Many2One(Amount, name='LineExtensionAmount')
+    allowance_charge = fields.One2Many(AllowanceCharge)
+
+    @fields.on_change(['price', 'quantity'])
+    def update_amount(self, name, value):
+        charge = form.AmountCollection(self.allowance_charge)\
+            .filter(lambda charge: charge.isCharge())\
+            .map(lambda charge: charge.amount)\
+            .sum()
+
+        discount = form.AmountCollection(self.allowance_charge)\
+            .filter(lambda charge: charge.isDiscount())\
+            .map(lambda charge: charge.amount)\
+            .sum()
+
+        total = self.quantity  * self.price
+        self.amount = total + charge - discount
+
+class LegalMonetaryTotal(model.Model):
+    __name__ = 'LegalMonetaryTotal'
+
+    line_extension_amount = fields.Many2One(Amount, name='LineExtensionAmount', default=form.Amount(0))
+    tax_exclusive_amount = fields.Many2One(Amount, name='TaxExclusiveAmount')
+    tax_inclusive_amount = fields.Many2One(Amount, name='TaxInclusiveAmount')
+    charge_total_amount = fields.Many2One(Amount, name='ChargeTotalAmount')
+    payable_amount = fields.Many2One(Amount, name='PayableAmount')
     
 class Invoice(model.Model):
     __name__ = 'Invoice'
@@ -123,7 +184,13 @@ class Invoice(model.Model):
     supplier = fields.Many2One(AccountingSupplierParty)
     customer = fields.Many2One(AccountingCustomerParty)
     lines = fields.One2Many(InvoiceLine)
+    legal_monetary_total = fields.Many2One(LegalMonetaryTotal)
 
+    @fields.on_change(['lines'])
+    def update_legal_monetary_total(self, name, value):
+        for line in self.lines:
+            self.legal_monetary_total.line_extension_amount.value += line.amount.value
+            
     def set_issue(self, name, value):
         if not isinstance(value, datetime):
             raise ValueError('expected type datetime')
diff --git a/facho/model/__init__.py b/facho/model/__init__.py
index 57f7489..0c2f73c 100644
--- a/facho/model/__init__.py
+++ b/facho/model/__init__.py
@@ -19,7 +19,7 @@ class ModelBase(object, metaclass=ModelMeta):
         obj = super().__new__(cls, *args, **kwargs)
         obj._xml_attributes = {}
         obj._fields = {}
-        obj._text = ""
+        obj._value = None
         obj._namespace_prefix = None
         obj._on_change_fields = defaultdict(list)
         obj._order_fields = []
@@ -72,7 +72,7 @@ class ModelBase(object, metaclass=ModelMeta):
     def _set_content(self, value):
         default = self.__default_set__(value)
         if default is not None:
-            self._text = str(default)
+            self._value = default
 
     def _hook_before_xml(self):
         self.__before_xml__()
@@ -116,7 +116,9 @@ class ModelBase(object, metaclass=ModelMeta):
                 content += value.to_xml()
             elif isinstance(value, str):
                 content += value
-        content += self._text
+
+        if self._value is not None:
+            content += str(self._value)
 
         if content == "":
             return "<%s%s%s/>" % (ns, tag, attributes)
diff --git a/facho/model/fields/one2many.py b/facho/model/fields/one2many.py
index 112a92f..aeb6547 100644
--- a/facho/model/fields/one2many.py
+++ b/facho/model/fields/one2many.py
@@ -17,9 +17,10 @@ class _RelationProxy():
     def __setattr__(self, attr, value):
         # TODO(bit4bit) hacemos proxy al sistema de notificacion de cambios
         # algo burdo, se usa __dict__ para saltarnos el __getattr__ y generar un fallo por recursion
+        response = setattr(self._obj, attr, value)
         for fun in self.__dict__['_inst']._on_change_fields[self.__dict__['_attribute']]:
             fun(self.__dict__['_inst'], self.__dict__['_attribute'], value)
-        return setattr(self._obj, attr, value)
+        return response
 
 class _Relation():
     def __init__(self, creator, inst, attribute):
diff --git a/tests/test_model_invoice.py b/tests/test_model_invoice.py
index a28df1c..6472fd0 100644
--- a/tests/test_model_invoice.py
+++ b/tests/test_model_invoice.py
@@ -20,9 +20,21 @@ def test_simple_invoice():
     invoice.customer.party.id = '800199436'
 
     line = invoice.lines.create()
-    line.quantity = form.Quantity(1, '94')
+    line.quantity = 1
     line.price = form.Amount(5_000)
     subtotal = line.taxtotal.subtotals.create()
     subtotal.percent = 19.0
+    assert '<Invoice><ID>323200000129</ID><IssueDate>2019-01-16T10:53:10-05:00</IssueDate><IssueTime>10:5310-05:00</IssueTime><AccountingSupplierParty><Party><ID>700085371</ID></Party></AccountingSupplierParty><AccountingCustomerParty><Party><ID>800199436</ID></Party></AccountingCustomerParty><InvoiceLine><InvoicedQuantity unitCode="NAR">1</InvoicedQuantity><TaxTotal><TaxSubTotal><TaxCategory><Percent>19.0</Percent><TaxScheme><ID>01</ID><Name>IVA</Name></TaxScheme></TaxCategory></TaxSubTotal></TaxTotal><Price><PriceAmount currencyID="COP">5000.0</PriceAmount>5000.0</Price><LineExtensionAmount currencyID="COP">5000.0</LineExtensionAmount></InvoiceLine><LegalMonetaryTotal><LineExtensionAmount currencyID="COP">35000.0</LineExtensionAmount></LegalMonetaryTotal></Invoice>' == invoice.to_xml()
 
-    assert '<Invoice><ID>323200000129</ID><IssueDate>2019-01-16T10:53:10-05:00</IssueDate><IssueTime>10:5310-05:00</IssueTime><AccountingSupplierParty><Party><ID>700085371</ID></Party></AccountingSupplierParty><AccountingCustomerParty><Party><ID>800199436</ID></Party></AccountingCustomerParty><InvoiceLine><InvoiceQuantity unitCode="NAR">1.0</InvoiceQuantity><TaxTotal><TaxSubTotal><TaxCategory><Percent>19.0</Percent><TaxScheme><ID>01</ID><Name>IVA</Name></TaxScheme></TaxCategory></TaxSubTotal></TaxTotal><Price><PriceAmount currencyID="COP">5000.0</PriceAmount></Price></InvoiceLine></Invoice>' == invoice.to_xml()
+def _test_simple_invoice_cufe():
+    invoice = model.Invoice()
+    invoice.id = '323200000129'
+    invoice.issue = datetime.strptime('2019-01-16 10:53:10-05:00', '%Y-%m-%d %H:%M:%S%z')
+    invoice.supplier.party.id = '700085371'
+    invoice.customer.party.id = '800199436'
+
+    line = invoice.lines.create()
+    line.quantity = form.Quantity(1, '94')
+    line.price = form.Amount(1_500_000)
+    subtotal = line.taxtotal.subtotals.create()
+    subtotal.percent = 19.0