root / elixir / trunk / elixir / entity.py

'''
This module provides the ``Entity`` base class, as well as its metaclass
``EntityMeta``.
'''

import sys
import types
import warnings

from copy import deepcopy

import sqlalchemy
from sqlalchemy import Table, Column, Integer, desc, ForeignKey, and_, \
                       ForeignKeyConstraint
from sqlalchemy.orm import MapperExtension, mapper, object_session, \
                           EXT_CONTINUE, polymorphic_union, ScopedSession, \
                           ColumnProperty
from sqlalchemy.sql import ColumnCollection

import elixir
from elixir.statements import process_mutators, MUTATORS
from elixir import options
from elixir.properties import Property

DEBUG = False

__doc_all__ = ['Entity', 'EntityMeta']


def session_mapper_factory(scoped_session):
    def session_mapper(cls, *args, **kwargs):
        if kwargs.pop('save_on_init', True):
            old_init = cls.__init__
            def __init__(self, *args, **kwargs):
                old_init(self, *args, **kwargs)
                scoped_session.add(self)
            cls.__init__ = __init__
        cls.query = scoped_session.query_property()
        return mapper(cls, *args, **kwargs)
    return session_mapper


class EntityDescriptor(object):
    '''
    EntityDescriptor describes fields and options needed for table creation.
    '''

    def __init__(self, entity):
        self.entity = entity
        self.parent = None

        bases = []
        for base in entity.__bases__:
            if isinstance(base, EntityMeta):
                if is_entity(base) and not is_abstract_entity(base):
                    if self.parent:
                        raise Exception(
                            '%s entity inherits from several entities, '
                            'and this is not supported.'
                            % self.entity.__name__)
                    else:
                        self.parent = base
                        bases.extend(base._descriptor.bases)
                        self.parent._descriptor.children.append(entity)
                else:
                    bases.append(base)
        self.bases = bases
        if not is_entity(entity) or is_abstract_entity(entity):
            return

        # entity.__module__ is not always reliable (eg in mod_python)
        self.module = sys.modules.get(entity.__module__)

        self.builders = []

        #XXX: use entity.__subclasses__ ?
        self.children = []

        # used for multi-table inheritance
        self.join_condition = None
        self.has_pk = False
        self._pk_col_done = False

        # columns and constraints waiting for a table to exist
        self._columns = ColumnCollection()
        self.constraints = []

        # properties (it is only useful for checking dupe properties at the
        # moment, and when adding properties before the mapper is created,
        # which shouldn't happen).
        self.properties = {}

        #
        self.relationships = []

        # set default value for options
        self.table_args = []

        # base class(es) options_defaults
        options_defaults = self.options_defaults()

        complete_defaults = options.options_defaults.copy()
        complete_defaults.update({
            'metadata': elixir.metadata,
            'session': elixir.session,
            'collection': elixir.entities
        })

        # set default value for other options
        for key in options.valid_options:
            value = options_defaults.get(key, complete_defaults[key])
            if isinstance(value, dict):
                value = value.copy()
            setattr(self, key, value)

        # override options with module-level defaults defined
        for key in ('metadata', 'session', 'collection'):
            attr = '__%s__' % key
            if hasattr(self.module, attr):
                setattr(self, key, getattr(self.module, attr))

    def options_defaults(self):
        base_defaults = {}
        for base in self.bases:
            base_defaults.update(base._descriptor.options_defaults())
        base_defaults.update(getattr(self.entity, 'options_defaults', {}))
        return base_defaults

    def setup_options(self):
        '''
        Setup any values that might depend on the "using_options" class
        mutator. For example, the tablename or the metadata.
        '''
        elixir.metadatas.add(self.metadata)
        if self.collection is not None:
            self.collection.append(self.entity)

        entity = self.entity
        if self.parent:
            if self.inheritance == 'single':
                self.tablename = self.parent._descriptor.tablename

        if not self.tablename:
            if self.shortnames:
                self.tablename = entity.__name__.lower()
            else:
                modulename = entity.__module__.replace('.', '_')
                tablename = "%s_%s" % (modulename, entity.__name__)
                self.tablename = tablename.lower()
        elif hasattr(self.tablename, '__call__'):
            self.tablename = self.tablename(entity)

        if not self.identity:
            if 'polymorphic_identity' in self.mapper_options:
                self.identity = self.mapper_options['polymorphic_identity']
            else:
                #TODO: include module name (We could have b.Account inherit
                # from a.Account)
                self.identity = entity.__name__.lower()
        elif 'polymorphic_identity' in self.mapper_options:
            raise Exception('You cannot use the "identity" option and the '
                            'polymorphic_identity mapper option at the same '
                            'time.')
        elif hasattr(self.identity, '__call__'):
            self.identity = self.identity(entity)

        if self.polymorphic:
            if not isinstance(self.polymorphic, basestring):
                self.polymorphic = options.DEFAULT_POLYMORPHIC_COL_NAME

    #---------------------
    # setup phase methods

    def setup_autoload_table(self):
        self.setup_table(True)

    def create_pk_cols(self):
        """
        Create primary_key columns. That is, call the 'create_pk_cols'
        builders then add a primary key to the table if it hasn't already got
        one and needs one.

        This method is "semi-recursive" in some cases: it calls the
        create_keys method on ManyToOne relationships and those in turn call
        create_pk_cols on their target. It shouldn't be possible to have an
        infinite loop since a loop of primary_keys is not a valid situation.
        """
        if self._pk_col_done:
            return

        self.call_builders('create_pk_cols')

        if not self.autoload:
            if self.parent:
                if self.inheritance == 'multi':
                    # Add columns with foreign keys to the parent's primary
                    # key columns
                    parent_desc = self.parent._descriptor
                    tablename = parent_desc.table_fullname
                    join_clauses = []
                    fk_columns = []
                    for pk_col in parent_desc.primary_keys:
                        colname = options.MULTIINHERITANCECOL_NAMEFORMAT % \
                                  {'entity': self.parent.__name__.lower(),
                                   'key': pk_col.key}

                        # It seems like SA ForeignKey is not happy being given
                        # a real column object when said column is not yet
                        # attached to a table
                        pk_col_name = "%s.%s" % (tablename, pk_col.key)
                        col = Column(colname, pk_col.type, primary_key=True)
                        fk_columns.append(col)
                        self.add_column(col)
                        join_clauses.append(col == pk_col)
                    self.join_condition = and_(*join_clauses)
                    self.add_constraint(
                        ForeignKeyConstraint(fk_columns,
                            parent_desc.primary_keys, ondelete='CASCADE'))
                elif self.inheritance == 'concrete':
                    # Copy primary key columns from the parent.
                    for col in self.parent._descriptor.columns:
                        if col.primary_key:
                            self.add_column(col.copy())
            elif not self.has_pk and self.auto_primarykey:
                if isinstance(self.auto_primarykey, basestring):
                    colname = self.auto_primarykey
                else:
                    colname = options.DEFAULT_AUTO_PRIMARYKEY_NAME

                self.add_column(
                    Column(colname, options.DEFAULT_AUTO_PRIMARYKEY_TYPE,
                           primary_key=True))
        self._pk_col_done = True

    def setup_relkeys(self):
        self.call_builders('create_non_pk_cols')

    def before_table(self):
        self.call_builders('before_table')

    def setup_table(self, only_autoloaded=False):
        '''
        Create a SQLAlchemy table-object with all columns that have been
        defined up to this point.
        '''
        if self.entity.table is not None:
            return

        if self.autoload != only_autoloaded:
            return

        kwargs = self.table_options
        if self.autoload:
            args = self.table_args
            kwargs['autoload'] = True
        else:
            if self.parent:
                if self.inheritance == 'single':
                    # we know the parent is setup before the child
                    self.entity.table = self.parent.table

                    # re-add the entity columns to the parent entity so that
                    # they are added to the parent's table (whether the
                    # parent's table is already setup or not).
                    for col in self._columns:
                        self.parent._descriptor.add_column(col)
                    for constraint in self.constraints:
                        self.parent._descriptor.add_constraint(constraint)
                    return
                elif self.inheritance == 'concrete':
                    #TODO: we should also copy columns from the parent table
                    # if the parent is a base (abstract?) entity (whatever the
                    # inheritance type -> elif will need to be changed)

                    # Copy all non-primary key columns from parent table
                    # (primary key columns have already been copied earlier).
                    for col in self.parent._descriptor.columns:
                        if not col.primary_key:
                            self.add_column(col.copy())

                    for con in self.parent._descriptor.constraints:
                        self.add_constraint(
                            ForeignKeyConstraint(
                                [e.parent.key for e in con.elements],
                                [e.target_fullname for e in con.elements],
                                name=con.name, #TODO: modify it
                                onupdate=con.onupdate, ondelete=con.ondelete,
                                use_alter=con.use_alter))

            if self.polymorphic and \
               self.inheritance in ('single', 'multi') and \
               self.children and not self.parent:
                self.add_column(Column(self.polymorphic,
                                       options.POLYMORPHIC_COL_TYPE))

            if self.version_id_col:
                if not isinstance(self.version_id_col, basestring):
                    self.version_id_col = options.DEFAULT_VERSION_ID_COL_NAME
                self.add_column(Column(self.version_id_col, Integer))

            args = list(self.columns) + self.constraints + self.table_args
        self.entity.table = Table(self.tablename, self.metadata,
                                  *args, **kwargs)
        if DEBUG:
            print self.entity.table.repr2()

    def setup_reltables(self):
        self.call_builders('create_tables')

    def after_table(self):
        self.call_builders('after_table')

    def setup_events(self):
        def make_proxy_method(methods):
            def proxy_method(self, mapper, connection, instance):
                for func in methods:
                    ret = func(instance)
                    # I couldn't commit myself to force people to
                    # systematicaly return EXT_CONTINUE in all their event
                    # methods.
                    # But not doing that diverge to how SQLAlchemy works.
                    # I should try to convince Mike to do EXT_CONTINUE by
                    # default, and stop processing as the special case.
#                    if ret != EXT_CONTINUE:
                    if ret is not None and ret != EXT_CONTINUE:
                        return ret
                return EXT_CONTINUE
            return proxy_method

        # create a list of callbacks for each event
        methods = {}

        all_methods = getmembers(self.entity,
                                 lambda a: isinstance(a, types.MethodType))

        for name, method in all_methods:
            for event in getattr(method, '_elixir_events', []):
                event_methods = methods.setdefault(event, [])
                event_methods.append(method)

        if not methods:
            return

        # transform that list into methods themselves
        for event in methods:
            methods[event] = make_proxy_method(methods[event])

        # create a custom mapper extension class, tailored to our entity
        ext = type('EventMapperExtension', (MapperExtension,), methods)()

        # then, make sure that the entity's mapper has our mapper extension
        self.add_mapper_extension(ext)

    def before_mapper(self):
        self.call_builders('before_mapper')

    def _get_children(self):
        children = self.children[:]
        for child in self.children:
            children.extend(child._descriptor._get_children())
        return children

    def translate_order_by(self, order_by):
        if isinstance(order_by, basestring):
            order_by = [order_by]

        order = []
        for colname in order_by:
            #FIXME: get_column uses self.columns[key] instead of property
            # names. self.columns correspond to the columns of the table if
            # the table was already created and to self._columns otherwise,
            # which is a ColumnCollection indexed on columns.key
            # See ticket #108.
            col = self.get_column(colname.strip('-'))
            if colname.startswith('-'):
                col = desc(col)
            order.append(col)
        return order

    def setup_mapper(self):
        '''
        Initializes and assign a mapper to the entity.
        At this point the mapper will usually have no property as they are
        added later.
        '''
        if self.entity.mapper:
            return

        # for now we don't support the "abstract" parent class in a concrete
        # inheritance scenario as demonstrated in
        # sqlalchemy/test/orm/inheritance/concrete.py
        # this should be added along other
        kwargs = {}
        if self.order_by:
            kwargs['order_by'] = self.translate_order_by(self.order_by)

        if self.version_id_col:
            kwargs['version_id_col'] = self.get_column(self.version_id_col)

        if self.inheritance in ('single', 'concrete', 'multi'):
            if self.parent and \
               (self.inheritance != 'concrete' or self.polymorphic):
                # non-polymorphic concrete doesn't need this
                kwargs['inherits'] = self.parent.mapper

            if self.inheritance == 'multi' and self.parent:
                kwargs['inherit_condition'] = self.join_condition

            if self.polymorphic:
                if self.children:
                    if self.inheritance == 'concrete':
                        keys = [(self.identity, self.entity.table)]
                        keys.extend([(child._descriptor.identity, child.table)
                                     for child in self._get_children()])
                        # Having the same alias name for an entity and one of
                        # its child (which is a parent itself) shouldn't cause
                        # any problem because the join shouldn't be used at
                        # the same time. But in reality, some versions of SA
                        # do misbehave on this. Since it doesn't hurt to have
                        # different names anyway, here they go.
                        pjoin = polymorphic_union(
                                    dict(keys), self.polymorphic,
                                    'pjoin_%s' % self.identity)

                        kwargs['with_polymorphic'] = ('*', pjoin)
                        kwargs['polymorphic_on'] = \
                            getattr(pjoin.c, self.polymorphic)
                    elif not self.parent:
                        kwargs['polymorphic_on'] = \
                            self.get_column(self.polymorphic)

                if self.children or self.parent:
                    kwargs['polymorphic_identity'] = self.identity

                if self.parent and self.inheritance == 'concrete':
                    kwargs['concrete'] = True

        if self.parent and self.inheritance == 'single':
            args = []
        else:
            args = [self.entity.table]

        # let user-defined kwargs override Elixir-generated ones, though that's
        # not very usefull since most of them expect Column instances.
        kwargs.update(self.mapper_options)

        #TODO: document this!
        if 'primary_key' in kwargs:
            cols = self.entity.table.c
            kwargs['primary_key'] = [getattr(cols, colname) for
                colname in kwargs['primary_key']]

        # do the mapping
        if self.session is None:
            self.entity.mapper = mapper(self.entity, *args, **kwargs)
        elif isinstance(self.session, ScopedSession):
            session_mapper = session_mapper_factory(self.session)
            self.entity.mapper = session_mapper(self.entity, *args, **kwargs)
        else:
            raise Exception("Failed to map entity '%s' with its table or "
                            "selectable. You can only bind an Entity to a "
                            "ScopedSession object or None for manual session "
                            "management."
                            % self.entity.__name__)

    def after_mapper(self):
        self.call_builders('after_mapper')

    def setup_properties(self):
        self.call_builders('create_properties')

    def finalize(self):
        self.call_builders('finalize')
        self.entity._setup_done = True

    #----------------
    # helper methods

    def call_builders(self, what):
        for builder in self.builders:
            if hasattr(builder, what):
                getattr(builder, what)()

    def add_column(self, col, check_duplicate=None):
        '''when check_duplicate is None, the value of the allowcoloverride
        option of the entity is used.
        '''
        if check_duplicate is None:
            check_duplicate = not self.allowcoloverride

        if col.key in self._columns:
            if check_duplicate:
                raise Exception("Column '%s' already exist in '%s' ! " %
                                (col.key, self.entity.__name__))
            else:
                del self._columns[col.key]
        # are indexed on col.key
        self._columns.add(col)

        if col.primary_key:
            self.has_pk = True

        table = self.entity.table
        if table is not None:
            if check_duplicate and col.key in table.columns.keys():
                raise Exception("Column '%s' already exist in table '%s' ! " %
                                (col.key, table.name))
            table.append_column(col)
            if DEBUG:
                print "table.append_column(%s)" % col

    def add_constraint(self, constraint):
        self.constraints.append(constraint)

        table = self.entity.table
        if table is not None:
            table.append_constraint(constraint)

    def add_property(self, name, property, check_duplicate=True):
        if check_duplicate and name in self.properties:
            raise Exception("property '%s' already exist in '%s' ! " %
                            (name, self.entity.__name__))
        self.properties[name] = property

#FIXME: something like this is needed to propagate the relationships from
# parent entities to their children in a concrete inheritance scenario. But
# this doesn't work because of the backref matching code. In most case
# (test_concrete.py) it doesn't even happen at all.
#        if self.children and self.inheritance == 'concrete':
#            for child in self.children:
#                child._descriptor.add_property(name, property)

        mapper = self.entity.mapper
        if mapper:
            mapper.add_property(name, property)
            if DEBUG:
                print "mapper.add_property('%s', %s)" % (name, repr(property))

    def add_mapper_extension(self, extension):
        extensions = self.mapper_options.get('extension', [])
        if not isinstance(extensions, list):
            extensions = [extensions]
        extensions.append(extension)
        self.mapper_options['extension'] = extensions

    def get_column(self, key, check_missing=True):
        #TODO: this needs to work whether the table is already setup or not
        #TODO: support SA table/autoloaded entity
        try:
            return self.columns[key]
        except KeyError:
            if check_missing:
                raise Exception("No column named '%s' found in the table of "
                                "the '%s' entity!"
                                % (key, self.entity.__name__))

    def get_inverse_relation(self, rel, check_reverse=True):
        '''
        Return the inverse relation of rel, if any, None otherwise.
        '''

        matching_rel = None
        for other_rel in self.relationships:
            if rel.is_inverse(other_rel):
                if matching_rel is None:
                    matching_rel = other_rel
                else:
                    raise Exception(
                            "Several relations match as inverse of the '%s' "
                            "relation in entity '%s'. You should specify "
                            "inverse relations manually by using the inverse "
                            "keyword."
                            % (rel.name, rel.entity.__name__))
        # When a matching inverse is found, we check that it has only
        # one relation matching as its own inverse. We don't need the result
        # of the method though. But we do need to be careful not to start an
        # infinite recursive loop.
        if matching_rel and check_reverse:
            rel.entity._descriptor.get_inverse_relation(matching_rel, False)

        return matching_rel

    def find_relationship(self, name):
        for rel in self.relationships:
            if rel.name == name:
                return rel
        if self.parent:
            return self.parent._descriptor.find_relationship(name)
        else:
            return None

    #------------------------
    # some useful properties

    @property
    def table_fullname(self):
        '''
        Complete name of the table for the related entity.
        Includes the schema name if there is one specified.
        '''
        schema = self.table_options.get('schema', None)
        if schema is not None:
            return "%s.%s" % (schema, self.tablename)
        else:
            return self.tablename

    @property
    def columns(self):
        if self.entity.table is not None:
            return self.entity.table.columns
        else:
            #FIXME: depending on the type of inheritance, we should also
            # return the parent entity's columns (for example for order_by
            # using a column defined in the parent.
            return self._columns

    @property
    def primary_keys(self):
        """
        Returns the list of primary key columns of the entity.

        This property isn't valid before the "create_pk_cols" phase.
        """
        if self.autoload:
            return [col for col in self.entity.table.primary_key.columns]
        else:
            if self.parent and self.inheritance == 'single':
                return self.parent._descriptor.primary_keys
            else:
                return [col for col in self.columns if col.primary_key]

    @property
    def table(self):
        if self.entity.table is not None:
            return self.entity.table
        else:
            return FakeTable(self)

    @property
    def primary_key_properties(self):
        """
        Returns the list of (mapper) properties corresponding to the primary
        key columns of the table of the entity.

        This property caches its value, so it shouldn't be called before the
        entity is fully set up.
        """
        if not hasattr(self, '_pk_props'):
            col_to_prop = {}
            mapper = self.entity.mapper
            for prop in mapper.iterate_properties:
                if isinstance(prop, ColumnProperty):
                    for col in prop.columns:
                        #XXX: Why is this extra loop necessary? What is this
                        #     "proxy_set" supposed to mean?
                        for col in col.proxy_set:
                            col_to_prop[col] = prop
            pk_cols = [c for c in mapper.mapped_table.c if c.primary_key]
            self._pk_props = [col_to_prop[c] for c in pk_cols]
        return self._pk_props

class FakePK(object):
    def __init__(self, descriptor):
        self.descriptor = descriptor

    @property
    def columns(self):
        return self.descriptor.primary_keys

class FakeTable(object):
    def __init__(self, descriptor):
        self.descriptor = descriptor
        self.primary_key = FakePK(descriptor)

    @property
    def columns(self):
        return self.descriptor.columns

    @property
    def fullname(self):
        '''
        Complete name of the table for the related entity.
        Includes the schema name if there is one specified.
        '''
        schema = self.descriptor.table_options.get('schema', None)
        if schema is not None:
            return "%s.%s" % (schema, self.descriptor.tablename)
        else:
            return self.descriptor.tablename


def is_entity(cls):
    """
    Scan the bases classes of `cls` to see if any is an instance of
    EntityMeta. If we don't find any, it means it is either an unrelated class
    or an entity base class (like the 'Entity' class).
    """
    for base in cls.__bases__:
        if isinstance(base, EntityMeta):
            return True
    return False


# Note that we don't use inspect.getmembers because of
# http://bugs.python.org/issue1785
# See also http://elixir.ematia.de/trac/changeset/262
def getmembers(object, predicate=None):
    base_props = []
    for key in dir(object):
        try:
            value = getattr(object, key)
        except AttributeError:
            continue
        if not predicate or predicate(value):
            base_props.append((key, value))
    return base_props

def is_abstract_entity(dict_or_cls):
    if not isinstance(dict_or_cls, dict):
        dict_or_cls = dict_or_cls.__dict__
    for mutator, args, kwargs in dict_or_cls.get(MUTATORS, []):
        if 'abstract' in kwargs:
            return kwargs['abstract']

    return False

def instrument_class(cls):
    """
    Instrument a class as an Entity. This is usually done automatically through
    the EntityMeta metaclass.
    """
    # Create the entity descriptor
    desc = cls._descriptor = EntityDescriptor(cls)

    # Process mutators
    # We *do* want mutators to be processed for base/abstract classes
    # (so that statements like using_options_defaults work).
    process_mutators(cls)

    # We do not want to do any more processing for base/abstract classes
    # (Entity et al.).
    if not is_entity(cls) or is_abstract_entity(cls):
        return

    cls.table = None
    cls.mapper = None

    # Copy the properties ('Property' instances) of the entity base class(es).
    # We use getmembers (instead of __dict__) so that we also get the
    # properties from the parents of the base class if any.
    base_props = []
    for base in cls.__bases__:
        if isinstance(base, EntityMeta) and \
           (not is_entity(base) or is_abstract_entity(base)):
            base_props += [(name, deepcopy(attr)) for name, attr in
                           getmembers(base, lambda a: isinstance(a, Property))]

    # Process attributes (using the assignment syntax), looking for
    # 'Property' instances and attaching them to this entity.
    properties = [(name, attr) for name, attr in cls.__dict__.iteritems()
                               if isinstance(attr, Property)]
    sorted_props = sorted(base_props + properties,
                          key=lambda i: i[1]._counter)
    for name, prop in sorted_props:
        prop.attach(cls, name)

    # setup misc options here (like tablename etc.)
    desc.setup_options()


class EntityMeta(type):
    """
    Entity meta class.
    You should only use it directly if you want to define your own base class
    for your entities (ie you don't want to use the provided 'Entity' class).
    """

    def __init__(cls, name, bases, dict_):
        instrument_class(cls)

    def __setattr__(cls, key, value):
        if isinstance(value, Property):
            if hasattr(cls, '_setup_done'):
                raise Exception('Cannot set attribute on a class after '
                                'setup_all')
            else:
                value.attach(cls, key)
        else:
            type.__setattr__(cls, key, value)


def setup_entities(entities):
    '''Setup all entities in the list passed as argument'''

    for entity in entities:
        # delete all Elixir properties so that it doesn't interfere with
        # SQLAlchemy. At this point they should have be converted to
        # builders.
        for name, attr in entity.__dict__.items():
            if isinstance(attr, Property):
                delattr(entity, name)

    for method_name in (
            'setup_autoload_table', 'create_pk_cols', 'setup_relkeys',
            'before_table', 'setup_table', 'setup_reltables', 'after_table',
            'setup_events',
            'before_mapper', 'setup_mapper', 'after_mapper',
            'setup_properties',
            'finalize'):
#        if DEBUG:
#            print "=" * 40
#            print method_name
#            print "=" * 40
        for entity in entities:
#            print entity.__name__, "...",
            if hasattr(entity, '_setup_done'):
#                print "already done"
                continue
            method = getattr(entity._descriptor, method_name)
            method()
#            print "ok"


def cleanup_entities(entities):
    """
    Try to revert back the list of entities passed as argument to the state
    they had just before their setup phase.

    As of now, this function is *not* functional in that it doesn't revert to
    the exact same state the entities were before setup. For example, the
    properties do not work yet as those would need to be regenerated (since the
    columns they are based on are regenerated too -- and as such the
    corresponding joins are not correct) but this doesn't happen because of
    the way relationship setup is designed to be called only once (especially
    the backref stuff in create_properties).
    """
    for entity in entities:
        desc = entity._descriptor

        if hasattr(entity, '_setup_done'):
            del entity._setup_done

        entity.table = None
        entity.mapper = None

        desc._pk_col_done = False
        desc.has_pk = False
        desc._columns = ColumnCollection()
        desc.constraints = []
        desc.properties = {}

class EntityBase(object):
    """
    This class holds all methods of the "Entity" base class, but does not act
    as a base class itself (it does not use the EntityMeta metaclass), but
    rather as a parent class for Entity. This is meant so that people who want
    to provide their own base class but don't want to loose or copy-paste all
    the methods of Entity can do so by inheriting from EntityBase:

    .. sourcecode:: python

        class MyBase(EntityBase):
            __metaclass__ = EntityMeta

            def myCustomMethod(self):
                # do something great
    """

    def __init__(self, **kwargs):
        self.set(**kwargs)

    def set(self, **kwargs):
        for key, value in kwargs.iteritems():
            setattr(self, key, value)

    @classmethod
    def update_or_create(cls, data, surrogate=True):
        pk_props = cls._descriptor.primary_key_properties

        # if all pk are present and not None
        if not [1 for p in pk_props if data.get(p.key) is None]:
            pk_tuple = tuple([data[prop.key] for prop in pk_props])
            record = cls.query.get(pk_tuple)
            if record is None:
                if surrogate:
                    raise Exception("Cannot create surrogate with pk")
                else:
                    record = cls()
        else:
            if surrogate:
                record = cls()
            else:
                raise Exception("Cannot create non surrogate without pk")
        record.from_dict(data)
        return record

    def from_dict(self, data):
        """
        Update a mapped class with data from a JSON-style nested dict/list
        structure.
        """
        # surrogate can be guessed from autoincrement/sequence but I guess
        # that's not 100% reliable, so we'll need an override

        mapper = sqlalchemy.orm.object_mapper(self)

        for key, value in data.iteritems():
            if isinstance(value, dict):
                dbvalue = getattr(self, key)
                rel_class = mapper.get_property(key).mapper.class_
                pk_props = rel_class._descriptor.primary_key_properties

                # If the data doesn't contain any pk, and the relationship
                # already has a value, update that record.
                if not [1 for p in pk_props if p.key in data] and \
                   dbvalue is not None:
                    dbvalue.from_dict(value)
                else:
                    record = rel_class.update_or_create(value)
                    setattr(self, key, record)
            elif isinstance(value, list) and \
                 value and isinstance(value[0], dict):

                rel_class = mapper.get_property(key).mapper.class_
                new_attr_value = []
                for row in value:
                    if not isinstance(row, dict):
                        raise Exception(
                                'Cannot send mixed (dict/non dict) data '
                                'to list relationships in from_dict data.')
                    record = rel_class.update_or_create(row)
                    new_attr_value.append(record)
                setattr(self, key, new_attr_value)
            else:
                setattr(self, key, value)

    def to_dict(self, deep={}, exclude=[]):
        """Generate a JSON-style nested dict/list structure from an object."""
        col_prop_names = [p.key for p in self.mapper.iterate_properties \
                                      if isinstance(p, ColumnProperty)]
        data = dict([(name, getattr(self, name))
                     for name in col_prop_names if name not in exclude])
        for rname, rdeep in deep.iteritems():
            dbdata = getattr(self, rname)
            #FIXME: use attribute names (ie coltoprop) instead of column names
            fks = self.mapper.get_property(rname).remote_side
            exclude = [c.name for c in fks]
            if dbdata is None:
                data[rname] = None
            elif isinstance(dbdata, list):
                data[rname] = [o.to_dict(rdeep, exclude) for o in dbdata]
            else:
                data[rname] = dbdata.to_dict(rdeep, exclude)
        return data

    # session methods
    def flush(self, *args, **kwargs):
        return object_session(self).flush([self], *args, **kwargs)

    def delete(self, *args, **kwargs):
        return object_session(self).delete(self, *args, **kwargs)

    def expire(self, *args, **kwargs):
        return object_session(self).expire(self, *args, **kwargs)

    def refresh(self, *args, **kwargs):
        return object_session(self).refresh(self, *args, **kwargs)

    def expunge(self, *args, **kwargs):
        return object_session(self).expunge(self, *args, **kwargs)

    # This bunch of session methods, along with all the query methods below
    # only make sense when using a global/scoped/contextual session.
    @property
    def _global_session(self):
        return self._descriptor.session.registry()

    #FIXME: remove all deprecated methods, possibly all of these
    def merge(self, *args, **kwargs):
        return self._global_session.merge(self, *args, **kwargs)

    def save(self, *args, **kwargs):
        return self._global_session.save(self, *args, **kwargs)

    def update(self, *args, **kwargs):
        return self._global_session.update(self, *args, **kwargs)

    # only exist in SA < 0.5
    # IMO, the replacement (session.add) doesn't sound good enough to be added
    # here. For example: "o = Order(); o.add()" is not very telling. It's
    # better to leave it as "session.add(o)"
    def save_or_update(self, *args, **kwargs):
        return self._global_session.save_or_update(self, *args, **kwargs)

    # query methods
    @classmethod
    def get_by(cls, *args, **kwargs):
        """
        Returns the first instance of this class matching the given criteria.
        This is equivalent to:
        session.query(MyClass).filter_by(...).first()
        """
        return cls.query.filter_by(*args, **kwargs).first()

    @classmethod
    def get(cls, *args, **kwargs):
        """
        Return the instance of this class based on the given identifier,
        or None if not found. This is equivalent to:
        session.query(MyClass).get(...)
        """
        return cls.query.get(*args, **kwargs)


class Entity(EntityBase):
    '''
    The base class for all entities

    All Elixir model objects should inherit from this class. Statements can
    appear within the body of the definition of an entity to define its
    fields, relationships, and other options.

    Here is an example:

    .. sourcecode:: python

        class Person(Entity):
            name = Field(Unicode(128))
            birthdate = Field(DateTime, default=datetime.now)

    Please note, that if you don't specify any primary keys, Elixir will
    automatically create one called ``id``.

    For further information, please refer to the provided examples or
    tutorial.
    '''
    __metaclass__ = EntityMeta