Add documentation for Model.default_<field name>

issue2594
review367001

doc/ref/models/fields.rst

@@ -146,6 +146,11 @@ field*.
 
 Define how the field must be loaded: ``lazy`` or ``eager``.
 
+Default value
+=============
+
+See :ref:`default value <topics-fields_default_value>`
+
 Field types
 ===========
 

doc/topics/index.rst

@@ -11,6 +11,7 @@ Introduction to all the key parts of trytond:
 
     install
     models/index
+    models/fields_default_value
     domain
     pyson
     views/index

doc/topics/models/fields_default_value.rst

@@ -0,0 +1,34 @@
+.. _topics-fields_default_value:
+
+=======================
+Default value of fields
+=======================
+
+When a record is created, each field, which doesn't have a value specified,
+is set with the default value if exists.
+
+The following method::
+
+    Model.default_<field name>()
+
+Return the default value for ``field name``.
+
+This example defines an ``Item`` model which has a default ``since``::
+
+    import datetime
+
+    from trytond.model import ModelView, ModelSQL, fields
+
+    class Item(ModelSQL, ModelView):
+        "Item"
+        _description = __doc__
+        _name = "item.item"
+        since = fields.Date('since')
+
+        def default_since(self):
+            return datetime.date.today()
+
+    Item()
+
+See also method ``Model.default_get``: :attr:`~trytond.model.Model.default_get`
+