diff --git a/facho/fe/model/__init__.py b/facho/fe/model/__init__.py
index 42536d6..a2cbe20 100644
--- a/facho/fe/model/__init__.py
+++ b/facho/fe/model/__init__.py
@@ -49,7 +49,7 @@ class Invoice(model.Model):
     __name__ = 'Invoice'
 
     id = fields.Many2One(ID)
-    issue = fields.Function(setter='set_issue')
+    issue = fields.Virtual(setter='set_issue')
     issue_date = fields.Many2One(Date, name='IssueDate')
     issue_time = fields.Many2One(Time, name='IssueTime')
     
diff --git a/facho/model/fields/__init__.py b/facho/model/fields/__init__.py
index fb85bc9..77d3bef 100644
--- a/facho/model/fields/__init__.py
+++ b/facho/model/fields/__init__.py
@@ -2,5 +2,6 @@ from .attribute import Attribute
 from .many2one import Many2One
 from .model import Model
 from .function import Function
+from .virtual import Virtual
 
-__all__ = [Attribute, Many2One, Model]
+__all__ = [Attribute, Many2One, Model, Virtual]
diff --git a/facho/model/fields/attribute.py b/facho/model/fields/attribute.py
index 2ad9f73..e8fd7a0 100644
--- a/facho/model/fields/attribute.py
+++ b/facho/model/fields/attribute.py
@@ -1,17 +1,18 @@
 from .field import Field
 
 class Attribute(Field):
-    def __init__(self, tag):
+    def __init__(self, tag, default=None):
         self.tag = tag
-  
+        self.value = default
+
     def __get__(self, inst, cls):
         if inst is None:
             return self
 
         assert self.name is not None
-        (tag, value) =  inst._xml_attributes[self.name]
-        return value
+        return self.value
 
     def __set__(self, inst, value):
         assert self.name is not None
+        self.value = value
         inst._xml_attributes[self.name] = (self.tag, value)
diff --git a/facho/model/fields/function.py b/facho/model/fields/function.py
index 9ea2ac3..2294041 100644
--- a/facho/model/fields/function.py
+++ b/facho/model/fields/function.py
@@ -2,29 +2,24 @@ from .field import Field
 from .model import Model
 
 class Function(Field):
-    def __init__(self, getter=None, setter=None, field=None):
+    def __init__(self, field, getter=None):
         self.field = field
         self.getter = getter
-        self.setter = setter
 
     def __get__(self, inst, cls):
         if inst is None:
             return self
         assert self.name is not None
 
-        if self.getter is None and self.field is None:
-            return None
+        # si se indica `field` se adiciona
+        # como campo del modelo, esto es
+        # que se serializa a xml
+        inst._fields[self.name] = self.field
 
-        if self.getter is None and self.field is not None:
-            return Model(self.field)
+        if self.getter is not None:
+            value = self._call(inst, self.getter, self.name, self.field)
 
-        if self.field is None:
-            return self._call(inst, self.getter, self.name)
-        else:
-            obj = Model(self.field)
-            return self._call(inst, self.getter, self.name, obj)
+            if value is not None:
+                self.field.__set__(inst, value)
 
-    def __set__(self, inst, value):
-        if self.setter is None:
-            return super().__set__(self.name, value)
-        self._call(inst, self.setter, self.name, value)
+        return self.field
diff --git a/tests/test_model.py b/tests/test_model.py
index 220a4ec..b42b473 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -231,7 +231,7 @@ def test_field_function_with_attribute():
     class Person(facho.model.Model):
         __name__ = 'Person'
 
-        hash = fields.Function('get_hash', field=fields.Attribute('hash'))
+        hash = fields.Function(fields.Attribute('hash'), getter='get_hash')
 
         def get_hash(self, name, field):
             return 'calculate'
@@ -248,39 +248,23 @@ def test_field_function_with_model():
     class Person(facho.model.Model):
         __name__ = 'Person'
 
-        hash = fields.Function('get_hash', field=Hash)
+        hash = fields.Function(fields.Many2One(Hash), getter='get_hash')
 
         def get_hash(self, name, field):
             field.id = 'calculate'
-            return field
+
         
     person = Person()
     assert person.hash.id == 'calculate'
     assert '<Person/>'
-
-def test_field_function():
-    class Person(facho.model.Model):
-        __name__ = 'Person'
-
-        hash = fields.Function('get_hash')
-
-        def get_hash(self, name):
-            return 'calculate'
-        
-    person = Person()
-    assert person.hash == 'calculate'
-    assert "<Person/>" == person.to_xml()
-    
+   
 
 def test_field_function_setter():
     class Person(facho.model.Model):
         __name__ = 'Person'
 
         hash = fields.Attribute('hash')
-        password = fields.Function('get_hash', setter='set_hash')
-
-        def get_hash(self, name):
-            return None
+        password = fields.Virtual(setter='set_hash')
 
         def set_hash(self, name, value):
             self.hash = "%s+2" % (value)
@@ -294,7 +278,7 @@ def test_field_function_only_setter():
         __name__ = 'Person'
 
         hash = fields.Attribute('hash')
-        password = fields.Function(setter='set_hash')
+        password = fields.Virtual(setter='set_hash')
 
         def set_hash(self, name, value):
             self.hash = "%s+2" % (value)
@@ -303,7 +287,6 @@ def test_field_function_only_setter():
     person.password = 'calculate'
     assert '<Person hash="calculate+2"/>' == person.to_xml()
     
-
 def test_model_set_default_setter():
     class Hash(facho.model.Model):
         __name__ = 'Hash'
@@ -319,3 +302,15 @@ def test_model_set_default_setter():
     person = Person()
     person.hash = 'hola'
     assert '<Person><Hash>hola+3</Hash></Person>' == person.to_xml()
+
+
+def test_field_virtual():
+    class Person(facho.model.Model):
+        __name__ = 'Person'
+
+        age = fields.Virtual()
+
+    person = Person()
+    person.age = 55
+    assert person.age == 55
+    assert "<Person/>" == person.to_xml()