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# ext/orderinglist.py
# Copyright (C) 2005-2021 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php
"""A custom list that manages index/position information for contained
elements.
:author: Jason Kirtland
``orderinglist`` is a helper for mutable ordered relationships. It will
intercept list operations performed on a :func:`_orm.relationship`-managed
collection and
automatically synchronize changes in list position onto a target scalar
attribute.
Example: A ``slide`` table, where each row refers to zero or more entries
in a related ``bullet`` table. The bullets within a slide are
displayed in order based on the value of the ``position`` column in the
``bullet`` table. As entries are reordered in memory, the value of the
``position`` attribute should be updated to reflect the new sort order::
Base = declarative_base()
class Slide(Base):
__tablename__ = 'slide'
id = Column(Integer, primary_key=True)
name = Column(String)
bullets = relationship("Bullet", order_by="Bullet.position")
class Bullet(Base):
__tablename__ = 'bullet'
id = Column(Integer, primary_key=True)
slide_id = Column(Integer, ForeignKey('slide.id'))
position = Column(Integer)
text = Column(String)
The standard relationship mapping will produce a list-like attribute on each
``Slide`` containing all related ``Bullet`` objects,
but coping with changes in ordering is not handled automatically.
When appending a ``Bullet`` into ``Slide.bullets``, the ``Bullet.position``
attribute will remain unset until manually assigned. When the ``Bullet``
is inserted into the middle of the list, the following ``Bullet`` objects
will also need to be renumbered.
The :class:`.OrderingList` object automates this task, managing the
``position`` attribute on all ``Bullet`` objects in the collection. It is
constructed using the :func:`.ordering_list` factory::
from sqlalchemy.ext.orderinglist import ordering_list
Base = declarative_base()
class Slide(Base):
__tablename__ = 'slide'
id = Column(Integer, primary_key=True)
name = Column(String)
bullets = relationship("Bullet", order_by="Bullet.position",
collection_class=ordering_list('position'))
class Bullet(Base):
__tablename__ = 'bullet'
id = Column(Integer, primary_key=True)
slide_id = Column(Integer, ForeignKey('slide.id'))
position = Column(Integer)
text = Column(String)
With the above mapping the ``Bullet.position`` attribute is managed::
s = Slide()
s.bullets.append(Bullet())
s.bullets.append(Bullet())
s.bullets[1].position
>>> 1
s.bullets.insert(1, Bullet())
s.bullets[2].position
>>> 2
The :class:`.OrderingList` construct only works with **changes** to a
collection, and not the initial load from the database, and requires that the
list be sorted when loaded. Therefore, be sure to specify ``order_by`` on the
:func:`_orm.relationship` against the target ordering attribute, so that the
ordering is correct when first loaded.
.. warning::
:class:`.OrderingList` only provides limited functionality when a primary
key column or unique column is the target of the sort. Operations
that are unsupported or are problematic include:
* two entries must trade values. This is not supported directly in the
case of a primary key or unique constraint because it means at least
one row would need to be temporarily removed first, or changed to
a third, neutral value while the switch occurs.
* an entry must be deleted in order to make room for a new entry.
SQLAlchemy's unit of work performs all INSERTs before DELETEs within a
single flush. In the case of a primary key, it will trade
an INSERT/DELETE of the same primary key for an UPDATE statement in order
to lessen the impact of this limitation, however this does not take place
for a UNIQUE column.
A future feature will allow the "DELETE before INSERT" behavior to be
possible, alleviating this limitation, though this feature will require
explicit configuration at the mapper level for sets of columns that
are to be handled in this way.
:func:`.ordering_list` takes the name of the related object's ordering
attribute as an argument. By default, the zero-based integer index of the
object's position in the :func:`.ordering_list` is synchronized with the
ordering attribute: index 0 will get position 0, index 1 position 1, etc. To
start numbering at 1 or some other integer, provide ``count_from=1``.
"""
from .. import util
from ..orm.collections import collection
from ..orm.collections import collection_adapter
__all__ = ["ordering_list"]
def ordering_list(attr, count_from=None, **kw):
"""Prepares an :class:`OrderingList` factory for use in mapper definitions.
Returns an object suitable for use as an argument to a Mapper
relationship's ``collection_class`` option. e.g.::
from sqlalchemy.ext.orderinglist import ordering_list
class Slide(Base):
__tablename__ = 'slide'
id = Column(Integer, primary_key=True)
name = Column(String)
bullets = relationship("Bullet", order_by="Bullet.position",
collection_class=ordering_list('position'))
:param attr:
Name of the mapped attribute to use for storage and retrieval of
ordering information
:param count_from:
Set up an integer-based ordering, starting at ``count_from``. For
example, ``ordering_list('pos', count_from=1)`` would create a 1-based
list in SQL, storing the value in the 'pos' column. Ignored if
``ordering_func`` is supplied.
Additional arguments are passed to the :class:`.OrderingList` constructor.
"""
kw = _unsugar_count_from(count_from=count_from, **kw)
return lambda: OrderingList(attr, **kw)
# Ordering utility functions
def count_from_0(index, collection):
"""Numbering function: consecutive integers starting at 0."""
return index
def count_from_1(index, collection):
"""Numbering function: consecutive integers starting at 1."""
return index + 1
def count_from_n_factory(start):
"""Numbering function: consecutive integers starting at arbitrary start."""
def f(index, collection):
return index + start
try:
f.__name__ = "count_from_%i" % start
except TypeError:
pass
return f
def _unsugar_count_from(**kw):
"""Builds counting functions from keyword arguments.
Keyword argument filter, prepares a simple ``ordering_func`` from a
``count_from`` argument, otherwise passes ``ordering_func`` on unchanged.
"""
count_from = kw.pop("count_from", None)
if kw.get("ordering_func", None) is None and count_from is not None:
if count_from == 0:
kw["ordering_func"] = count_from_0
elif count_from == 1:
kw["ordering_func"] = count_from_1
else:
kw["ordering_func"] = count_from_n_factory(count_from)
return kw
class OrderingList(list):
"""A custom list that manages position information for its children.
The :class:`.OrderingList` object is normally set up using the
:func:`.ordering_list` factory function, used in conjunction with
the :func:`_orm.relationship` function.
"""
def __init__(
self, ordering_attr=None, ordering_func=None, reorder_on_append=False
):
"""A custom list that manages position information for its children.
``OrderingList`` is a ``collection_class`` list implementation that
syncs position in a Python list with a position attribute on the
mapped objects.
This implementation relies on the list starting in the proper order,
so be **sure** to put an ``order_by`` on your relationship.
:param ordering_attr:
Name of the attribute that stores the object's order in the
relationship.
:param ordering_func: Optional. A function that maps the position in
the Python list to a value to store in the
``ordering_attr``. Values returned are usually (but need not be!)
integers.
An ``ordering_func`` is called with two positional parameters: the
index of the element in the list, and the list itself.
If omitted, Python list indexes are used for the attribute values.
Two basic pre-built numbering functions are provided in this module:
``count_from_0`` and ``count_from_1``. For more exotic examples
like stepped numbering, alphabetical and Fibonacci numbering, see
the unit tests.
:param reorder_on_append:
Default False. When appending an object with an existing (non-None)
ordering value, that value will be left untouched unless
``reorder_on_append`` is true. This is an optimization to avoid a
variety of dangerous unexpected database writes.
SQLAlchemy will add instances to the list via append() when your
object loads. If for some reason the result set from the database
skips a step in the ordering (say, row '1' is missing but you get
'2', '3', and '4'), reorder_on_append=True would immediately
renumber the items to '1', '2', '3'. If you have multiple sessions
making changes, any of whom happen to load this collection even in
passing, all of the sessions would try to "clean up" the numbering
in their commits, possibly causing all but one to fail with a
concurrent modification error.
Recommend leaving this with the default of False, and just call
``reorder()`` if you're doing ``append()`` operations with
previously ordered instances or when doing some housekeeping after
manual sql operations.
"""
self.ordering_attr = ordering_attr
if ordering_func is None:
ordering_func = count_from_0
self.ordering_func = ordering_func
self.reorder_on_append = reorder_on_append
# More complex serialization schemes (multi column, e.g.) are possible by
# subclassing and reimplementing these two methods.
def _get_order_value(self, entity):
return getattr(entity, self.ordering_attr)
def _set_order_value(self, entity, value):
setattr(entity, self.ordering_attr, value)
def reorder(self):
"""Synchronize ordering for the entire collection.
Sweeps through the list and ensures that each object has accurate
ordering information set.
"""
for index, entity in enumerate(self):
self._order_entity(index, entity, True)
# As of 0.5, _reorder is no longer semi-private
_reorder = reorder
def _order_entity(self, index, entity, reorder=True):
have = self._get_order_value(entity)
# Don't disturb existing ordering if reorder is False
if have is not None and not reorder:
return
should_be = self.ordering_func(index, self)
if have != should_be:
self._set_order_value(entity, should_be)
def append(self, entity):
super(OrderingList, self).append(entity)
self._order_entity(len(self) - 1, entity, self.reorder_on_append)
def _raw_append(self, entity):
"""Append without any ordering behavior."""
super(OrderingList, self).append(entity)
_raw_append = collection.adds(1)(_raw_append)
def insert(self, index, entity):
super(OrderingList, self).insert(index, entity)
self._reorder()
def remove(self, entity):
super(OrderingList, self).remove(entity)
adapter = collection_adapter(self)
if adapter and adapter._referenced_by_owner:
self._reorder()
def pop(self, index=-1):
entity = super(OrderingList, self).pop(index)
self._reorder()
return entity
def __setitem__(self, index, entity):
if isinstance(index, slice):
step = index.step or 1
start = index.start or 0
if start < 0:
start += len(self)
stop = index.stop or len(self)
if stop < 0:
stop += len(self)
for i in range(start, stop, step):
self.__setitem__(i, entity[i])
else:
self._order_entity(index, entity, True)
super(OrderingList, self).__setitem__(index, entity)
def __delitem__(self, index):
super(OrderingList, self).__delitem__(index)
self._reorder()
def __setslice__(self, start, end, values):
super(OrderingList, self).__setslice__(start, end, values)
self._reorder()
def __delslice__(self, start, end):
super(OrderingList, self).__delslice__(start, end)
self._reorder()
def __reduce__(self):
return _reconstitute, (self.__class__, self.__dict__, list(self))
for func_name, func in list(locals().items()):
if (
util.callable(func)
and func.__name__ == func_name
and not func.__doc__
and hasattr(list, func_name)
):
func.__doc__ = getattr(list, func_name).__doc__
del func_name, func
def _reconstitute(cls, dict_, items):
"""Reconstitute an :class:`.OrderingList`.
This is the adjoint to :meth:`.OrderingList.__reduce__`. It is used for
unpickling :class:`.OrderingList` objects.
"""
obj = cls.__new__(cls)
obj.__dict__.update(dict_)
list.extend(obj, items)
return obj
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