diff --git a/facho/model/__init__.py b/facho/model/__init__.py
index 1f6c14b..fe8cc66 100644
--- a/facho/model/__init__.py
+++ b/facho/model/__init__.py
@@ -28,7 +28,18 @@ class ModelBase(object, metaclass=ModelMeta):
     def __getitem__(self, key):
         return self._xml_attributes[key]
 
+    def __before_xml__(self):
+        pass
+
+    def _hook_before_xml(self):
+        self.__before_xml__()
+        for field in self._fields.values():
+            if hasattr(field, '__before_xml__'):
+                field.__before_xml__()
+
     def to_xml(self):
+        self._hook_before_xml()
+
         tag = self.__name__
         ns = ''
         if self._namespace_prefix is not None:
diff --git a/facho/model/fields/__init__.py b/facho/model/fields/__init__.py
index f368aaf..fb85bc9 100644
--- a/facho/model/fields/__init__.py
+++ b/facho/model/fields/__init__.py
@@ -1,5 +1,6 @@
 from .attribute import Attribute
 from .many2one import Many2One
 from .model import Model
+from .function import Function
 
 __all__ = [Attribute, Many2One, Model]
diff --git a/facho/model/fields/field.py b/facho/model/fields/field.py
index 251223e..e9c09ec 100644
--- a/facho/model/fields/field.py
+++ b/facho/model/fields/field.py
@@ -19,3 +19,9 @@ class Field:
         if name not in namespaces:
             raise KeyError("namespace %s not found" % (name))
         inst._namespace_prefix = name
+
+    def _call(self, inst, method, *args):
+        call = getattr(inst, method or '', None)
+
+        if callable(call):
+            return call(*args)
diff --git a/tests/test_model.py b/tests/test_model.py
index 8aeb7e1..9d27d35 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -165,3 +165,63 @@ def test_field_model_with_namespace():
     person = Person()
     person.id = 33
     assert '<Person xmlns:facho="http://lib.facho.cyou"><facho:ID>33</facho:ID></Person>' == person.to_xml()
+
+def test_field_hook_before_xml():
+    class Hash(facho.model.Model):
+        __name__ = 'Hash'
+        
+    class Person(facho.model.Model):
+        __name__ = 'Person'
+
+        hash = fields.Model(Hash)
+
+        def __before_xml__(self):
+            self.hash = "calculate"
+            
+    person = Person()
+    assert "<Person><Hash>calculate</Hash></Person>" == person.to_xml()
+
+
+def test_field_function_with_attribute():
+    class Person(facho.model.Model):
+        __name__ = 'Person'
+
+        hash = fields.Function('get_hash', field=fields.Attribute('hash'))
+
+        def get_hash(self, name, field):
+            return 'calculate'
+        
+    person = Person()
+    assert '<Person hash="calculate"/>'
+
+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
+
+        def set_hash(self, name, value):
+            self.hash = "%s+2" % (value)
+
+    person = Person()
+    person.password = 'calculate'
+    assert '<Person hash="calculate+2"/>' == person.to_xml()
+