diff --git a/.gitignore b/.gitignore
index 83e8a08..bece568 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,3 +23,5 @@ parsetab_*
 .tox
 .eggs/
 venv/
+.\#*
+\#*\#
\ No newline at end of file
diff --git a/ming/odm/mapper.py b/ming/odm/mapper.py
index 6e89a22..b79eff0 100644
--- a/ming/odm/mapper.py
+++ b/ming/odm/mapper.py
@@ -1,6 +1,6 @@
 import six
 import warnings
-from copy import copy
+from copy import copy, deepcopy
 
 from ming.base import Object, NoDefault
 from ming.utils import wordwrap
@@ -61,6 +61,16 @@ def __init__(self, mapped_class, collection, session, **kwargs):
             raise TypeError('Unknown kwd args: %r' % kwargs)
         self._instrument_class(properties, include_properties, exclude_properties)
 
+    @classmethod
+    def replace_session(cls, session):
+        for _mapper in cls.all_mappers():
+            _mapper.session = session
+            _mapper.mapped_class.query.session = session
+            _mapper.mapped_class.__mongometa__.session = session
+            _mapper._compiled = False
+            _mapper.compile()
+            _mapper.session.ensure_indexes(_mapper.collection)
+
     def __repr__(self):
         return '<Mapper %s:%s>' % (
             self.mapped_class.__name__, self.collection.m.collection_name)
@@ -74,13 +84,12 @@ def insert(self, obj, state, session, **kwargs):
 
     @_with_hooks('update')
     def update(self, obj, state, session, **kwargs):
-        fields = state.options.get('fields', None)
-        if fields is None:
-            fields = ()
-
         doc = self.collection(state.document, skip_from_bson=True)
-        ret = session.impl.save(doc, *fields, validate=False)
+        ret = session.impl.save(doc, validate=False, state=state)
         state.status = state.clean
+        # Make sure that st.document is never the same as st.original_document
+        # otherwise mutating one mutates the other.
+        state.original_document = deepcopy(doc)
         return ret
 
     @_with_hooks('delete')
@@ -177,9 +186,7 @@ def _from_doc(self, doc, options, validate=True):
 
         # Make sure that st.document is never the same as st.original_document
         # otherwise mutating one mutates the other.
-        # There is no need to deepcopy as nested mutable objects are already
-        # copied by InstrumentedList and InstrumentedObj to instrument them.
-        st.original_document = doc
+        st.original_document = deepcopy(doc)
 
         if validate is False:
             # .create calls this after it already created the document with the
diff --git a/ming/session.py b/ming/session.py
index 3dd33ea..7a829b7 100644
--- a/ming/session.py
+++ b/ming/session.py
@@ -9,11 +9,12 @@
 import six
 
 from .base import Cursor, Object
-from .utils import fixup_index, fix_write_concern
+from .utils import fixup_index, fix_write_concern, doc_to_set
 from . import exc
 
 log = logging.getLogger(__name__)
 
+
 def annotate_doc_failure(func):
     '''Decorator to wrap a session operation so that any pymongo errors raised
     will note the document that caused the failure
@@ -30,7 +31,7 @@ def wrapper(self, doc, *args, **kwargs):
     return update_wrapper(wrapper, func)
 
 
-class Session(object):
+class Session:
     _registry = {}
     _datastores = {}
 
@@ -139,7 +140,8 @@ def find_and_modify(self, cls, query=None, sort=None, new=False, **kw):
 
     def _prep_save(self, doc, validate):
         hook = doc.m.before_save
-        if hook: hook(doc)
+        if hook:
+            hook(doc)
         if validate:
             if doc.m.schema is None:
                 data = dict(doc)
@@ -151,8 +153,12 @@ def _prep_save(self, doc, validate):
         return data
 
     @annotate_doc_failure
-    def save(self, doc, *args, **kwargs):
+    def save(self, doc, *args, state=None, **kwargs):
         data = self._prep_save(doc, kwargs.pop('validate', True))
+        if not args and state is not None and state.original_document:
+            args = tuple(set((k for k, v in
+                              doc_to_set(state.original_document)
+                              ^ doc_to_set(data))))
         if args:
             values = dict((arg, data[arg]) for arg in args)
             result = self._impl(doc).update(
diff --git a/ming/tests/odm/test_declarative.py b/ming/tests/odm/test_declarative.py
index 9cbd9f3..1af5e89 100644
--- a/ming/tests/odm/test_declarative.py
+++ b/ming/tests/odm/test_declarative.py
@@ -1,6 +1,7 @@
 import sys
 from collections import defaultdict
 from unittest import TestCase, SkipTest
+from unittest.mock import MagicMock
 
 from ming import schema as S
 from ming import create_datastore
@@ -811,3 +812,60 @@ def test_hook_base(self):
                          [
                              {'_id': doc._id, 'a': doc.a}
                          ])
+
+
+class TestReplacingSession(TestCase):
+
+    def setUp(self):
+        Mapper._mapper_by_classname.clear()
+        self.datastore = create_datastore('mim:///test_db')
+        self.session = ODMSession(bind=self.datastore)
+        class Basic(MappedClass):
+            class __mongometa__:
+                name = 'hook'
+                session = self.session
+            _id = FieldProperty(S.ObjectId)
+            a = FieldProperty(int)
+        Mapper.compile_all()
+        self.Basic = Basic
+        self.session.remove(self.Basic)
+
+    def test_hook_base(self):
+        assert id(self.Basic.query.session) == id(self.session)
+        session2 = MagicMock()
+        new_session = ODMSession(bind=session2)
+        Mapper.replace_session(new_session)
+        assert id(self.Basic.query.session) == id(new_session)
+        assert id(self.session) != id(new_session)
+
+class TestBeforeSave(TestCase):
+
+    def setUp(self):
+        Mapper._mapper_by_classname.clear()
+        self.datastore = create_datastore('mim:///test_db')
+        self.session = ODMSession(bind=self.datastore)
+        class Basic(MappedClass):
+            class __mongometa__:
+                name = 'hook'
+                session = self.session
+                def before_save(instance):
+                    instance.a = 9
+
+            _id = FieldProperty(S.ObjectId)
+            a = FieldProperty(int)
+        Mapper.compile_all()
+        self.Basic = Basic
+        self.session.remove(self.Basic)
+
+    def test_hook_base(self):
+        doc = self.Basic()
+        doc.a = 5
+        self.session.flush()  # first insert
+        self.session.close()
+        doc = self.Basic.query.get(doc._id)
+        assert doc.a == 9, doc.a
+        doc.a = 6 
+        self.session.flush()  # then save
+        self.session.close()
+        doc = self.Basic.query.get(doc._id)
+        assert doc.a == 9, doc.a
diff --git a/ming/tests/odm/test_mapper.py b/ming/tests/odm/test_mapper.py
index e8055a7..0c202da 100644
--- a/ming/tests/odm/test_mapper.py
+++ b/ming/tests/odm/test_mapper.py
@@ -277,6 +277,23 @@ def test_group(self, pymongo_group):
         self.Basic.query.group()
         assert pymongo_group.called
 
+    def test_multiple_update_flushes(self):
+        initial_doc = self.Basic()
+        initial_doc.a = 1
+        self.session.flush()
+        self.session.close()
+
+        doc_updating = self.Basic.query.get(_id=initial_doc._id)
+        doc_updating.a = 2
+        self.session.flush()
+        doc_updating.a = 1  # back to "initial" value
+        doc_updating.e = 'foo'  # change something else too
+        self.session.flush()
+        self.session.close()
+
+        doc_after_updates = self.Basic.query.get(_id=doc_updating._id)
+        assert doc_after_updates.a == 1
+
 
 class TestRelation(TestCase):
     def setUp(self):
diff --git a/ming/tests/test_session.py b/ming/tests/test_session.py
index 4631e25..80fe412 100644
--- a/ming/tests/test_session.py
+++ b/ming/tests/test_session.py
@@ -81,7 +81,7 @@ def test_base_session(self):
         doc = self.TestDocNoSchema({'_id':5, 'a':5})
         sess.save(doc)
         impl.save.assert_called_with(dict(_id=5, a=5))
-        doc = self.TestDocNoSchema({'_id':5, 'a':5})
+        doc = self.TestDocNoSchema({'_id':5, 'a':5, 'b': 6})
         sess.save(doc, 'a')
         impl.update.assert_called_with(dict(_id=5), {'$set':dict(a=5)})
         doc = self.TestDocNoSchema({'_id':5, 'a':5})
diff --git a/ming/utils.py b/ming/utils.py
index cabf3c6..8c66f7d 100644
--- a/ming/utils.py
+++ b/ming/utils.py
@@ -145,3 +145,16 @@ def fix_write_concern(kwargs):
         warnings.warn('safe option is now deprecated', DeprecationWarning)
         kwargs['w'] = int(kwargs.pop('safe'))
     return kwargs
+
+
+def to_hashable(v):
+    if isinstance(v, list):
+        return tuple((to_hashable(sv) for sv in v))
+    elif isinstance(v, dict):
+        return tuple(((to_hashable(k), to_hashable(sv))
+                      for k, sv in sorted(v.items())))
+    return v
+
+
+def doc_to_set(doc):
+    return set((k, to_hashable(v)) for k, v in doc.copy().items())
diff --git a/setup.cfg b/setup.cfg
index ebde9bc..1be30d6 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,3 +1,6 @@
+[pylint]
+# disabling protected-access because of mongodb _id property
+disable = protected-access
 
 [nosetests]
 detailed-errors=1