new method on orm class populate_defaults(), which runs at the beginning of save(), since populating them in __init__() is problematic

README.rst

@@ -51,7 +51,7 @@ Usage Example
     doc1 = MyTable(rr, {'animal': 'elephant', 'size': 'large'})
     doc1.save()
 
-    doc1_copy = MyTable.get(rr, doc1.id)
+    doc1_copy = MyTable.load(rr, doc1.id)
     doc1_copy.food = 'bread'
     doc1_copy.save()
 

doublethink/orm.py

@@ -273,6 +273,14 @@ class Document(dict, object):
         '''
         return getattr(self, self.pk_field)
 
+    def populate_defaults(self):
+        '''
+        This method is called by `save()` before persisting the document to
+        the database. Subclasses should override it to populate default values
+        if appropriate.
+        '''
+        pass
+
     def save(self):
         '''
         Persist changes to rethinkdb. Updates only the fields that have
@@ -285,6 +293,7 @@ class Document(dict, object):
         touched.
         '''
         should_insert = False
+        self.populate_defaults()
         try:
             self[self.pk_field]  # raises KeyError if missing
             if self._updates:

setup.py

@@ -3,7 +3,7 @@ import codecs
 
 setuptools.setup(
     name='doublethink',
-    version='0.2.0.dev69',
+    version='0.2.0.dev70',
     packages=['doublethink'],
     classifiers=[
         'Programming Language :: Python :: 2.7',

tests/test_orm.py

@@ -279,4 +279,27 @@ def test_orm_pk(rr):
     assert e['blah'] == 'toot'
     assert e == e_copy
 
+def test_default_values(rr):
+    class Person(doublethink.Document):
+        def populate_defaults(self):
+            if not "age" in self:
+                self.age = 0  # born today
+
+    Person.table_ensure(rr)
+    p = Person(rr, {})
+    assert not "age" in p
+    assert p.age is None
+    p.save()
+    assert p.age == 0
+    assert p.id
+
+    p.age = 50
+    p.save()
+
+    q = Person.load(rr, p.id)
+    assert q.age == 50
+    q.save()
+    assert q.age == 50
+    q.refresh()
+    assert q.age == 50