diff --git a/facho/fe/model/__init__.py b/facho/fe/model/__init__.py
index a2cbe20..f144d32 100644
--- a/facho/fe/model/__init__.py
+++ b/facho/fe/model/__init__.py
@@ -44,6 +44,31 @@ class AccountingSupplierParty(model.Model):
     __name__ = 'AccountingSupplierParty'
 
     party = fields.Many2One(Party)
+
+class InvoicedQuantity(model.Model):
+    __name__  = 'InvoiceQuantity'
+
+    code = fields.Attribute('unitCode', default='NAR')
+
+
+class PriceAmount(model.Model):
+    __name__ = 'PriceAmount'
+
+    currency = fields.Attribute('currencyID', default='COP')
+    
+class Price(model.Model):
+    __name__ = 'Price'
+
+    amount = fields.Many2One(PriceAmount)
+
+    def __default_set__(self, value):
+        self.amount = value
+
+class InvoiceLine(model.Model):
+    __name__ = 'InvoiceLine'
+
+    quantity = fields.Many2One(InvoicedQuantity)
+    price = fields.Many2One(Price)
     
 class Invoice(model.Model):
     __name__ = 'Invoice'
@@ -57,7 +82,8 @@ class Invoice(model.Model):
 
     supplier = fields.Many2One(AccountingSupplierParty)
     customer = fields.Many2One(AccountingCustomerParty)
-    
+    lines = fields.One2Many(InvoiceLine)
+
     def set_issue(self, name, value):
         if not isinstance(value, datetime):
             raise ValueError('expected type datetime')
diff --git a/tests/test_model_invoice.py b/tests/test_model_invoice.py
index 30f5b91..d85c87b 100644
--- a/tests/test_model_invoice.py
+++ b/tests/test_model_invoice.py
@@ -10,6 +10,7 @@ from datetime import datetime
 import pytest
 
 import facho.fe.model as model
+import facho.fe.form as form
 
 def test_simple_invoice():
     invoice = model.Invoice()
@@ -17,3 +18,9 @@ def test_simple_invoice():
     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(5_000)
+
+    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><Price><PriceAmount currencyID="COP">5000.0</PriceAmount></Price></InvoiceLine></Invoice>' == invoice.to_xml()