/Windows/Python3.8/WPy64-3830/WPy64-3830/python-3.8.3.amd64/Lib/site-packages/sqlalchemy/ext/orderinglist.py

# ext/orderinglist.py

# Copyright (C) 2005-2020 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