A first test using PickledObjectField and a rewritten dbserialize module to store Attributes. No migrations set up yet.

src/utils/picklefield.py

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+#
+#  Copyright (c) 2009-2010 Gintautas Miliauskas
+#
+#   Permission is hereby granted, free of charge, to any person
+#   obtaining a copy of this software and associated documentation
+#   files (the "Software"), to deal in the Software without
+#   restriction, including without limitation the rights to use,
+#   copy, modify, merge, publish, distribute, sublicense, and/or sell
+#   copies of the Software, and to permit persons to whom the
+#   Software is furnished to do so, subject to the following
+#   conditions:
+#
+#   The above copyright notice and this permission notice shall be
+#   included in all copies or substantial portions of the Software.
+#
+#   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+#   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+#   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+#   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+#   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+#   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+#   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+#   OTHER DEALINGS IN THE SOFTWARE.
+
+
+"""Pickle field implementation for Django."""
+from copy import deepcopy
+from base64 import b64encode, b64decode
+from zlib import compress, decompress
+import six
+import django
+from django.db import models
+
+# django 1.5 introduces force_text instead of force_unicode
+try:
+    from django.utils.encoding import force_text
+except ImportError:
+    from django.utils.encoding import force_unicode as force_text
+
+# python 3.x does not have cPickle module
+try:
+    from cPickle import loads, dumps # cpython 2.x
+except ImportError:
+    from pickle import loads, dumps # cpython 3.x, other interpreters
+try:
+    from django.utils import simplejson as json
+except ImportError:
+    import json
+
+DEFAULT_PROTOCOL = 2
+#from picklefield import DEFAULT_PROTOCOL
+#from picklefield.compat import force_text, loads, dumps
+
+
+class PickledObject(str):
+    """
+    A subclass of string so it can be told whether a string is a pickled
+    object or not (if the object is an instance of this class then it must
+    [well, should] be a pickled one).
+
+    Only really useful for passing pre-encoded values to ``default``
+    with ``dbsafe_encode``, not that doing so is necessary. If you
+    remove PickledObject and its references, you won't be able to pass
+    in pre-encoded values anymore, but you can always just pass in the
+    python objects themselves.
+    """
+
+
+class _ObjectWrapper(object):
+    """
+    A class used to wrap object that have properties that may clash with the
+    ORM internals.
+
+    For example, objects with the `prepare_database_save` property such as
+    `django.db.Model` subclasses won't work under certain conditions and the
+    same apply for trying to retrieve any `callable` object.
+    """
+    __slots__ = ('_obj',)
+
+    def __init__(self, obj):
+        self._obj = obj
+
+
+def wrap_conflictual_object(obj):
+    if hasattr(obj, 'prepare_database_save') or callable(obj):
+        obj = _ObjectWrapper(obj)
+    return obj
+
+
+def dbsafe_encode(value, compress_object=False, pickle_protocol=DEFAULT_PROTOCOL):
+    # We use deepcopy() here to avoid a problem with cPickle, where dumps
+    # can generate different character streams for same lookup value if
+    # they are referenced differently.
+    # The reason this is important is because we do all of our lookups as
+    # simple string matches, thus the character streams must be the same
+    # for the lookups to work properly. See tests.py for more information.
+    value = dumps(deepcopy(value), protocol=pickle_protocol)
+    if compress_object:
+        value = compress(value)
+    value = b64encode(value).decode() # decode bytes to str
+    return PickledObject(value)
+
+
+def dbsafe_decode(value, compress_object=False):
+    value = value.encode() # encode str to bytes
+    value = b64decode(value)
+    if compress_object:
+        value = decompress(value)
+    return loads(value)
+
+
+def _get_subfield_superclass():
+    # hardcore trick to support django < 1.3 - there was something wrong with
+    # inheritance and SubfieldBase before django 1.3
+    # see https://github.com/django/django/commit/222c73261650201f5ce99e8dd4b1ce0d30a69eb4
+    if django.VERSION < (1,3):
+        return models.Field
+    return six.with_metaclass(models.SubfieldBase, models.Field)
+
+
+class PickledObjectField(_get_subfield_superclass()):
+    """
+    A field that will accept *any* python object and store it in the
+    database. PickledObjectField will optionally compress its values if
+    declared with the keyword argument ``compress=True``.
+
+    Does not actually encode and compress ``None`` objects (although you
+    can still do lookups using None). This way, it is still possible to
+    use the ``isnull`` lookup type correctly.
+    """
+    __metaclass__ = models.SubfieldBase  # for django < 1.3
+
+    def __init__(self, *args, **kwargs):
+        self.compress = kwargs.pop('compress', False)
+        self.protocol = kwargs.pop('protocol', DEFAULT_PROTOCOL)
+        kwargs.setdefault('editable', False)
+        super(PickledObjectField, self).__init__(*args, **kwargs)
+
+    def get_default(self):
+        """
+        Returns the default value for this field.
+
+        The default implementation on models.Field calls force_unicode
+        on the default, which means you can't set arbitrary Python
+        objects as the default. To fix this, we just return the value
+        without calling force_unicode on it. Note that if you set a
+        callable as a default, the field will still call it. It will
+        *not* try to pickle and encode it.
+
+        """
+        if self.has_default():
+            if callable(self.default):
+                return self.default()
+            return self.default
+        # If the field doesn't have a default, then we punt to models.Field.
+        return super(PickledObjectField, self).get_default()
+
+    def to_python(self, value):
+        """
+        B64decode and unpickle the object, optionally decompressing it.
+
+        If an error is raised in de-pickling and we're sure the value is
+        a definite pickle, the error is allowed to propagate. If we
+        aren't sure if the value is a pickle or not, then we catch the
+        error and return the original value instead.
+
+        """
+        if value is not None:
+            try:
+                value = dbsafe_decode(value, self.compress)
+            except:
+                # If the value is a definite pickle; and an error is raised in
+                # de-pickling it should be allowed to propogate.
+                if isinstance(value, PickledObject):
+                    raise
+            else:
+                if isinstance(value, _ObjectWrapper):
+                    return value._obj
+        return value
+
+    def pre_save(self, model_instance, add):
+        value = super(PickledObjectField, self).pre_save(model_instance, add)
+        return wrap_conflictual_object(value)
+
+    def get_db_prep_value(self, value, connection=None, prepared=False):
+        """
+        Pickle and b64encode the object, optionally compressing it.
+
+        The pickling protocol is specified explicitly (by default 2),
+        rather than as -1 or HIGHEST_PROTOCOL, because we don't want the
+        protocol to change over time. If it did, ``exact`` and ``in``
+        lookups would likely fail, since pickle would now be generating
+        a different string.
+
+        """
+        if value is not None and not isinstance(value, PickledObject):
+            # We call force_text here explicitly, so that the encoded string
+            # isn't rejected by the postgresql_psycopg2 backend. Alternatively,
+            # we could have just registered PickledObject with the psycopg
+            # marshaller (telling it to store it like it would a string), but
+            # since both of these methods result in the same value being stored,
+            # doing things this way is much easier.
+            value = force_text(dbsafe_encode(value, self.compress, self.protocol))
+        return value
+
+    def value_to_string(self, obj):
+        value = self._get_val_from_obj(obj)
+        return self.get_db_prep_value(value)
+
+    def get_internal_type(self):
+        return 'TextField'
+
+    def get_db_prep_lookup(self, lookup_type, value, connection=None, prepared=False):
+        if lookup_type not in ['exact', 'in', 'isnull']:
+            raise TypeError('Lookup type %s is not supported.' % lookup_type)
+        # The Field model already calls get_db_prep_value before doing the
+        # actual lookup, so all we need to do is limit the lookup types.
+        try:
+            return super(PickledObjectField, self).get_db_prep_lookup(
+                lookup_type, value, connection=connection, prepared=prepared)
+        except TypeError:
+            # Try not to break on older versions of Django, where the
+            # `connection` and `prepared` parameters are not available.
+            return super(PickledObjectField, self).get_db_prep_lookup(
+                lookup_type, value)
+
+
+# South support; see http://south.aeracode.org/docs/tutorial/part4.html#simple-inheritance
+try:
+    from south.modelsinspector import add_introspection_rules
+except ImportError:
+    pass
+else:
+    add_introspection_rules([], [r"^src\.utils\.picklefield\.PickledObjectField"])