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``.
| Class | |
A custom list that manages position information for its children. |
| Function | count |
Numbering function: consecutive integers starting at 0. |
| Function | count |
Numbering function: consecutive integers starting at 1. |
| Function | count |
Numbering function: consecutive integers starting at arbitrary start. |
| Function | ordering |
Prepares an :class:`OrderingList` factory for use in mapper definitions. |
| Type Alias | |
Undocumented |
| Function | _reconstitute |
Reconstitute an :class:`.OrderingList`. |
| Function | _unsugar |
Builds counting functions from keyword arguments. |
| Type Variable | _T |
Undocumented |
str, count_from: Optional[ = None, ordering_func: Optional[ = None, reorder_on_append: bool = False) -> Callable[:
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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.
Reconstitute an :class:`.OrderingList`. This is the adjoint to :meth:`.OrderingList.__reduce__`. It is used for unpickling :class:`.OrderingList` objects.