diff --git a/facho/model/__init__.py b/facho/model/__init__.py
index 33cd824..51a9345 100644
--- a/facho/model/__init__.py
+++ b/facho/model/__init__.py
@@ -24,25 +24,29 @@ class ModelBase(object, metaclass=ModelMeta):
         # forzamos registros de campos al modelo
         # al instanciar
         for (key, v) in type(obj).__dict__.items():
-            if isinstance(v, fields.Attribute) or isinstance(v, fields.Many2One):
+            if isinstance(v, fields.Attribute) or isinstance(v, fields.Many2One) or isinstance(v, fields.Function):
                 if hasattr(v, 'default') and v.default is not None:
                     setattr(obj, key, v.default)
 
 
         return obj
 
+    def _set_attribute(self, field, name, value):
+        self._xml_attributes[field] = (name, value)
+
     def __setitem__(self, key, val):
         self._xml_attributes[key] = val
 
     def __getitem__(self, key):
         return self._xml_attributes[key]
 
-    def __before_xml__(self):
-        pass
+    def _get_field(self, name):
+        return self._fields[name]
+
+    def _set_field(self, name, field):
+        field.name = name
+        self._fields[name] = field
 
-    def __default_set__(self, value):
-        return value
-    
     def _set_content(self, value):
         default = self.__default_set__(value)
         if default is not None:
@@ -55,6 +59,9 @@ class ModelBase(object, metaclass=ModelMeta):
                 field.__before_xml__()
 
     def to_xml(self):
+        """
+        Genera xml del modelo y sus relaciones
+        """
         self._hook_before_xml()
 
         tag = self.__name__
@@ -85,5 +92,19 @@ class ModelBase(object, metaclass=ModelMeta):
             return "<%s%s%s>%s</%s%s>" % (ns, tag, attributes, content, ns, tag)
 
 class Model(ModelBase):
-    pass
+    def __before_xml__(self):
+        """
+        Ejecuta antes de generar el xml, este
+        metodo sirve para realizar actualizaciones
+        en los campos en el ultimo momento
+        """
+        pass
+
+    def __default_set__(self, value):
+        """
+        Al asignar un valor al modelo atraves de una relacion (person.relation = '33')
+        se puede personalizar como hacer esta asignacion.
+        """
+        return value
+
         
diff --git a/facho/model/fields/attribute.py b/facho/model/fields/attribute.py
index e89268a..9d8677c 100644
--- a/facho/model/fields/attribute.py
+++ b/facho/model/fields/attribute.py
@@ -1,8 +1,8 @@
 from .field import Field
 
 class Attribute(Field):
-    def __init__(self, tag, default=None):
-        self.tag = tag
+    def __init__(self, name, default=None):
+        self.attribute = name
         self.value = default
         self.default = default
 
@@ -16,4 +16,4 @@ class Attribute(Field):
     def __set__(self, inst, value):
         assert self.name is not None
         self.value = value
-        inst._xml_attributes[self.name] = (self.tag, value)
+        inst._set_attribute(self.name, self.attribute, value)
diff --git a/facho/model/fields/function.py b/facho/model/fields/function.py
index 20ba6e9..58388e5 100644
--- a/facho/model/fields/function.py
+++ b/facho/model/fields/function.py
@@ -2,9 +2,14 @@ from .field import Field
 from .model import Model
 
 class Function(Field):
-    def __init__(self, field, getter=None):
+    """
+    Permite modificar el modelo cuando se intenta,
+    obtener el valor de este campo.
+    """
+    def __init__(self, field, getter=None, default=None):
         self.field = field
         self.getter = getter
+        self.default = default
 
     def __get__(self, inst, cls):
         if inst is None:
@@ -14,8 +19,7 @@ class Function(Field):
         # si se indica `field` se adiciona
         # como campo del modelo, esto es
         # que se serializa a xml
-        self.field.name = self.name
-        inst._fields[self.name] = self.field
+        inst._set_field(self.name, self.field)
 
         if self.getter is not None:
             value = self._call(inst, self.getter, self.name, self.field)
@@ -24,3 +28,7 @@ class Function(Field):
                 self.field.__set__(inst, value)
 
         return self.field
+
+    def __set__(self, inst, value):
+        inst._set_field(self.name, self.field)
+        self.field.__set__(inst, value)
diff --git a/tests/test_model.py b/tests/test_model.py
index c965649..1028803 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -326,6 +326,15 @@ def test_field_inserted_default_attribute():
     person = Person()
     assert '<Person hash="calculate"/>' == person.to_xml()
 
+def test_field_function_inserted_default_attribute():
+    class Person(facho.model.Model):
+        __name__ = 'Person'
+
+        hash = fields.Function(fields.Attribute('hash'), default='calculate')
+
+    person = Person()
+    assert '<Person hash="calculate"/>' == person.to_xml()
+
 def test_field_inserted_default_many2one():
     class ID(facho.model.Model):
         __name__ = 'ID'