Ordering List

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 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 OrderingList object automates this task, managing the position attribute on all Bullet objects in the collection. It is constructed using the 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 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 relationship() against the target ordering attribute, so that the ordering is correct when first loaded.

Warning

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.

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 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.

API Reference

Object Name	Description
count_from_0(index, collection)	Numbering function: consecutive integers starting at 0.
count_from_1(index, collection)	Numbering function: consecutive integers starting at 1.
count_from_n_factory(start)	Numbering function: consecutive integers starting at arbitrary start.
ordering_list(attr[, count_from], **kw)	Prepares an OrderingList factory for use in mapper definitions.
OrderingList	A custom list that manages position information for its children.

function sqlalchemy.ext.orderinglist.ordering_list(attr, count_from=None, **kw)

Prepares an 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'))

Parameters:

• attr – Name of the mapped attribute to use for storage and retrieval of ordering information

• 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 OrderingList constructor.

function sqlalchemy.ext.orderinglist.count_from_0(index, collection)

Numbering function: consecutive integers starting at 0.

function sqlalchemy.ext.orderinglist.count_from_1(index, collection)

Numbering function: consecutive integers starting at 1.

function sqlalchemy.ext.orderinglist.count_from_n_factory(start)

Numbering function: consecutive integers starting at arbitrary start.

class sqlalchemy.ext.orderinglist.OrderingList(ordering_attr=None, ordering_func=None, reorder_on_append=False)

A custom list that manages position information for its children.

The OrderingList object is normally set up using the ordering_list() factory function, used in conjunction with the relationship() function.

Members

__init__(), append(), insert(), pop(), remove(), reorder()

Class signature

class sqlalchemy.ext.orderinglist.OrderingList (builtins.list)

method sqlalchemy.ext.orderinglist.OrderingList.__init__(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.

Parameters:

• ordering_attr – Name of the attribute that stores the object’s order in the relationship.

• 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.

• 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.

method sqlalchemy.ext.orderinglist.OrderingList.append(entity)

Append object to the end of the list.

method sqlalchemy.ext.orderinglist.OrderingList.insert(index, entity)

Insert object before index.

method sqlalchemy.ext.orderinglist.OrderingList.pop(index=-1)

Remove and return item at index (default last).

Raises IndexError if list is empty or index is out of range.

method sqlalchemy.ext.orderinglist.OrderingList.remove(entity)

Remove first occurrence of value.

Raises ValueError if the value is not present.

method sqlalchemy.ext.orderinglist.OrderingList.reorder()

Synchronize ordering for the entire collection.

Sweeps through the list and ensures that each object has accurate ordering information set.