+
−#
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−# Copyright (c) 2008 Andreas Blixt <andreas@blixt.org>
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−# Project homepage: <http://code.google.com/p/blixt/>
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−#
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−# License: MIT license <http://www.opensource.org/licenses/mit-license.php>
+
−#
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−
+
−"""Data-related Google App Engine extensions.
+
−"""
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−
+
−from google.appengine.ext import db
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−
+
−import sys
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−
+
−# Add ModelWithFieldAttributes *before* PolyModel, so that everything does
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−# not become a "ModelWithFieldAttributes" in the Datastore.
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−from soc.models import base
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−
+
−class Error(Exception):
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−    """Base of all exceptions in the blixt.data module."""
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−    pass
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−
+
−class PolyModel(base.ModelWithFieldAttributes):
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−    """An extension to Google App Engine models that improves the support for
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−    inheritance.
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−
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−    Any models extending a model that extends PolyModel will be stored in the
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−    datastore as the same kind as the model that extends PolyModel, but with
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−    their additional properties. When they are retrieved from the datastore,
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−    they will be loaded as the appropiate model class.
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−
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−    The difference from the Model class is that queries will include models that
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−    inherit from the model being queried.
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−    """
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−    inheritance_line = db.StringListProperty()
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−
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−    def __init__(self, parent = None, key_name = None, _app = None, **kwds):
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−        """Creates a new instance of this polymorphic model.
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−
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−        Args:
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−          parent: Parent instance for this instance or None, indicating a top-
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−            level instance.
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−          key_name: Name for new model instance.
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−          _app: Intentionally undocumented.
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−          args: Keyword arguments mapping to properties of model.
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−        """
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−        if self.__class__ == PolyModel or not isinstance(self, PolyModel):
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−            raise Error('Instances of PolyModel must be created through a '
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−                        'subclass of PolyModel.')
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−
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−        line = []
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−        for c in self.__class__.__mro__:
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−            if c == PolyModel:
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−                self.__class__._kind = p
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−                break
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−            line.append('%s.%s' % (c.__module__, c.__name__))
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−            p = c
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−
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−        kwds['inheritance_line'] = line
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−
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−        super(PolyModel, self).__init__(parent = parent, key_name = key_name,
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−                                        _app = _app, **kwds)
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−
+
−    @classmethod
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−    def _kind_type(cls):
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−        """Gets the class highest in the model inheritance hierarchy (the class
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−        that will be used as the datastore kind.)
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−        """
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−        p = cls
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−
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−        for c in cls.__mro__:
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−            # The meta-class 'PropertiedClass' calls kind() which leads here.
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−            # The variable 'PolyModel' is not assigned until after the meta-
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−            # class has finished setting up the class. Therefore, a string
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−            # comparison is used instead of a value comparison.
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−            if c.__name__ == 'PolyModel': break
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−            p = c
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−
+
−        return p
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−
+
−    @classmethod
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−    def all(cls):
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−        """Returns a query over all instances of this model, as well as the
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−        instances of any descendant models, from the datastore.
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−
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−        Returns:
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−          Query that will retrieve all instances from entity collection.
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−        """
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−        qry = super(PolyModel, cls).all()
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−
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−        if cls != cls._kind_type():
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−            full_name = '%s.%s' % (cls.__module__, cls.__name__)
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−            qry.filter('inheritance_line =', full_name)
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−
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−        return qry
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−
+
−    @classmethod
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−    def from_entity(cls, entity):
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−        """Converts the entity representation of this model to an instance.
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−
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−        Converts datastore.Entity instance to an instance of the appropiate
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−        class, which can be cls or any descendant thereof.
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−
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−        Args:
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−          entity: Entity loaded directly from datastore.
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−
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−        Raises:
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−          KindError when cls is incorrect model for entity.
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−        """
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−        if entity.has_key('inheritance_line'):
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−            mod_name, cls_name = entity['inheritance_line'][0].rsplit('.', 1)
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−            __import__(mod_name)
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−            cls = getattr(sys.modules[mod_name], cls_name)
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−        return super(PolyModel, cls).from_entity(entity)
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−
+
−    @classmethod
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−    def kind(cls):
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−        """Returns the datastore kind we use for this model.
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−
+
−        This is the name of the class that is highest up in the inheritance
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−        hierarchy of this model.
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−        """
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−        return cls._kind_type().__name__
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−