interfaces.py

SQLAlchemy. 经典的Python ORM框架。学习必看。

# interfaces.py
# Copyright (C) 2005, 2006, 2007, 2008 Michael Bayer [e-mail]
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Semi-private implementation objects which form the basis
of ORM-mapped attributes, query options and mapper extension.

Defines the [sqlalchemy.orm.interfaces#MapperExtension] class,
which can be end-user subclassed to add event-based functionality
to mappers.  The remainder of this module is generally private to the
ORM.
"""

from itertools import chain
from sqlalchemy import exceptions, logging
from sqlalchemy.sql import expression
class_mapper = None

__all__ = ['EXT_CONTINUE', 'EXT_STOP', 'EXT_PASS', 'MapperExtension',
           'MapperProperty', 'PropComparator', 'StrategizedProperty',
           'build_path', 'MapperOption',
           'ExtensionOption', 'PropertyOption',
           'AttributeExtension', 'StrategizedOption', 'LoaderStrategy' ]

EXT_CONTINUE = EXT_PASS = object()
EXT_STOP = object()

class MapperExtension(object):
    """Base implementation for customizing Mapper behavior.

    For each method in MapperExtension, returning a result of EXT_CONTINUE
    will allow processing to continue to the next MapperExtension in line or
    use the default functionality if there are no other extensions.

    Returning EXT_STOP will halt processing of further extensions handling
    that method.  Some methods such as ``load`` have other return
    requirements, see the individual documentation for details.  Other than
    these exception cases, any return value other than EXT_CONTINUE or
    EXT_STOP will be interpreted as equivalent to EXT_STOP.

    EXT_PASS is a synonym for EXT_CONTINUE and is provided for backward
    compatibility.
    """

    def instrument_class(self, mapper, class_):
        return EXT_CONTINUE

    def init_instance(self, mapper, class_, oldinit, instance, args, kwargs):
        return EXT_CONTINUE

    def init_failed(self, mapper, class_, oldinit, instance, args, kwargs):
        return EXT_CONTINUE

    def get_session(self):
        """Retrieve a contextual Session instance with which to
        register a new object.

        Note: this is not called if a session is provided with the
        `__init__` params (i.e. `_sa_session`).
        """

        return EXT_CONTINUE

    def load(self, query, *args, **kwargs):
        """Override the `load` method of the Query object.

        The return value of this method is used as the result of
        ``query.load()`` if the value is anything other than EXT_CONTINUE.
        """

        return EXT_CONTINUE

    def get(self, query, *args, **kwargs):
        """Override the `get` method of the Query object.

        The return value of this method is used as the result of
        ``query.get()`` if the value is anything other than EXT_CONTINUE.
        """

        return EXT_CONTINUE

    def get_by(self, query, *args, **kwargs):
        """Override the `get_by` method of the Query object.

        The return value of this method is used as the result of
        ``query.get_by()`` if the value is anything other than
        EXT_CONTINUE.

        DEPRECATED.
        """

        return EXT_CONTINUE

    def select_by(self, query, *args, **kwargs):
        """Override the `select_by` method of the Query object.

        The return value of this method is used as the result of
        ``query.select_by()`` if the value is anything other than
        EXT_CONTINUE.

        DEPRECATED.
        """

        return EXT_CONTINUE

    def select(self, query, *args, **kwargs):
        """Override the `select` method of the Query object.

        The return value of this method is used as the result of
        ``query.select()`` if the value is anything other than
        EXT_CONTINUE.

        DEPRECATED.
        """

        return EXT_CONTINUE


    def translate_row(self, mapper, context, row):
        """Perform pre-processing on the given result row and return a
        new row instance.

        This is called as the very first step in the ``_instance()``
        method.
        """

        return EXT_CONTINUE

    def create_instance(self, mapper, selectcontext, row, class_):
        """Receive a row when a new object instance is about to be
        created from that row.

        The method can choose to create the instance itself, or it can return
        EXT_CONTINUE to indicate normal object creation should take place.

        mapper
          The mapper doing the operation

        selectcontext
          SelectionContext corresponding to the instances() call

        row
          The result row from the database

        class\_
          The class we are mapping.

        return value
          A new object instance, or EXT_CONTINUE
        """

        return EXT_CONTINUE

    def append_result(self, mapper, selectcontext, row, instance, result, **flags):
        """Receive an object instance before that instance is appended
        to a result list.

        If this method returns EXT_CONTINUE, result appending will proceed
        normally.  if this method returns any other value or None,
        result appending will not proceed for this instance, giving
        this extension an opportunity to do the appending itself, if
        desired.

        mapper
          The mapper doing the operation.

        selectcontext
          SelectionContext corresponding to the instances() call.

        row
          The result row from the database.

        instance
          The object instance to be appended to the result.

        result
          List to which results are being appended.

        \**flags
          extra information about the row, same as criterion in
          `create_row_processor()` method of [sqlalchemy.orm.interfaces#MapperProperty]
        """

        return EXT_CONTINUE

    def populate_instance(self, mapper, selectcontext, row, instance, **flags):
        """Receive a newly-created instance before that instance has
        its attributes populated.

        The normal population of attributes is according to each
        attribute's corresponding MapperProperty (which includes
        column-based attributes as well as relationships to other
        classes).  If this method returns EXT_CONTINUE, instance
        population will proceed normally.  If any other value or None
        is returned, instance population will not proceed, giving this
        extension an opportunity to populate the instance itself, if
        desired.
        """

        return EXT_CONTINUE

    def before_insert(self, mapper, connection, instance):
        """Receive an object instance before that instance is INSERTed
        into its table.

        This is a good place to set up primary key values and such
        that aren't handled otherwise.

        Column-based attributes can be modified within this method which will
        result in the new value being inserted.  However *no* changes to the overall
        flush plan can be made; this means any collection modification or
        save() operations which occur within this method will not take effect
        until the next flush call.

        """

        return EXT_CONTINUE

    def after_insert(self, mapper, connection, instance):
        """Receive an object instance after that instance is INSERTed."""

        return EXT_CONTINUE

    def before_update(self, mapper, connection, instance):
        """Receive an object instance before that instance is UPDATEed.

        Note that this method is called for all instances that are marked as
        "dirty", even those which have no net changes to their column-based
        attributes.  An object is marked as dirty when any of its column-based
        attributes have a "set attribute" operation called or when any of its
        collections are modified.  If, at update time, no column-based attributes
        have any net changes, no UPDATE statement will be issued.  This means
        that an instance being sent to before_update is *not* a guarantee that
        an UPDATE statement will be issued (although you can affect the outcome
        here).

        To detect if the column-based attributes on the object have net changes,
        and will therefore generate an UPDATE statement, use
        ``object_session(instance).is_modified(instance, include_collections=False)``.

        Column-based attributes can be modified within this method which will
        result in their being updated.  However *no* changes to the overall
        flush plan can be made; this means any collection modification or
        save() operations which occur within this method will not take effect
        until the next flush call.

        """

        return EXT_CONTINUE

    def after_update(self, mapper, connection, instance):
        """Receive an object instance after that instance is UPDATEed."""

        return EXT_CONTINUE

    def before_delete(self, mapper, connection, instance):
        """Receive an object instance before that instance is DELETEed.

        Note that *no* changes to the overall
        flush plan can be made here; this means any collection modification,
        save() or delete() operations which occur within this method will
        not take effect until the next flush call.

        """

        return EXT_CONTINUE

    def after_delete(self, mapper, connection, instance):
        """Receive an object instance after that instance is DELETEed."""

        return EXT_CONTINUE

class MapperProperty(object):
    """Manage the relationship of a ``Mapper`` to a single class
    attribute, as well as that attribute as it appears on individual
    instances of the class, including attribute instrumentation,
    attribute access, loading behavior, and dependency calculations.
    """

    def setup(self, querycontext, **kwargs):
        """Called by Query for the purposes of constructing a SQL statement.

        Each MapperProperty associated with the target mapper processes the
        statement referenced by the query context, adding columns and/or
        criterion as appropriate.
        """

        pass

    def create_row_processor(self, selectcontext, mapper, row):
        """Return a 3-tuple consiting of two row processing functions and an instance post-processing function.

        Input arguments are the query.SelectionContext and the *first*
        applicable row of a result set obtained within
        query.Query.instances(), called only the first time a particular
        mapper's populate_instance() method is invoked for the overall result.

        The settings contained within the SelectionContext as well as the
        columns present in the row (which will be the same columns present in
        all rows) are used to determine the presence and behavior of the
        returned callables.  The callables will then be used to process all
        rows and to post-process all instances, respectively.

        Callables are of the following form::

            def new_execute(instance, row, **flags):
                # process incoming instance and given row.  the instance is
                # "new" and was just created upon receipt of this row.
                # flags is a dictionary containing at least the following
                # attributes:
                #   isnew - indicates if the instance was newly created as a
                #           result of reading this row
                #   instancekey - identity key of the instance
                # optional attribute:
                #   ispostselect - indicates if this row resulted from a
                #                  'post' select of additional tables/columns

            def existing_execute(instance, row, **flags):
                # process incoming instance and given row.  the instance is
                # "existing" and was created based on a previous row.

            def post_execute(instance, **flags):
                # process instance after all result rows have been processed.
                # this function should be used to issue additional selections
                # in order to eagerly load additional properties.

            return (new_execute, existing_execute, post_execute)

        Either of the three tuples can be ``None`` in which case no function
        is called.
        """

        raise NotImplementedError()

    def cascade_iterator(self, type, object, recursive=None, halt_on=None):
        """Iterate through instances related to the given instance along
        a particular 'cascade' path, starting with this MapperProperty.

        See PropertyLoader for the related instance implementation.
        """

        return iter([])

    def get_criterion(self, query, key, value):
        """Return a ``WHERE`` clause suitable for this
        ``MapperProperty`` corresponding to the given key/value pair,
        where the key is a column or object property name, and value
        is a value to be matched.  This is only picked up by