# SQLAlchemy 0.6.0 Documentation

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Version: 0.6.0 Last Updated: 04/18/2010 19:16:18

# Examples

The SQLAlchemy distribution includes a variety of code examples illustrating a select set of patterns, some typical and some not so typical. All are runnable and can be found in the `/examples` directory of the distribution. Each example contains a README in its `__init__.py` file, each of which are listed below.

Additional SQLAlchemy examples, some user contributed, are available on the wiki at http://www.sqlalchemy.org/trac/wiki/UsageRecipes.

## Adjacency List

Location: /examples/adjacency_list/

An example of a dictionary-of-dictionaries structure mapped using an adjacency list model.

E.g.:

    node = TreeNode('rootnode')
    node.append('node1')
    node.append('node3')
    session.add(node)
    session.commit()

    dump_tree(node)

## Associations

Location: /examples/association/

Examples illustrating the usage of the “association object” pattern, where an intermediary object associates two endpoint objects together.

The first example illustrates a basic association from a User object to a collection or Order objects, each which references a collection of Item objects.

The second example builds upon the first to add the Association Proxy extension.

E.g.:

    # create an order
    order = Order('john smith')

    # append an OrderItem association via the "itemassociations"
    # collection with a custom price.
    order.itemassociations.append(OrderItem(item('MySQL Crowbar'), 10.99))

    # append two more Items via the transparent "items" proxy, which
    # will create OrderItems automatically using the default price.
    order.items.append(item('SA Mug'))
    order.items.append(item('SA Hat'))

## Attribute Instrumentation

Location: /examples/custom_attributes/

Two examples illustrating modifications to SQLAlchemy’s attribute management system.

`listen_for_events.py` illustrates the usage of `AttributeExtension` to intercept attribute events. It additionally illustrates a way to automatically attach these listeners to all class attributes using a `InstrumentationManager`.

`custom_management.py` illustrates much deeper usage of `InstrumentationManager` as well as collection adaptation, to completely change the underlying method used to store state on an object. This example was developed to illustrate techniques which would be used by other third party object instrumentation systems to interact with SQLAlchemy’s event system and is only intended for very intricate framework integrations.

## Beaker Caching

Location: /examples/beaker_caching/

Illustrates how to embed Beaker cache functionality within the Query object, allowing full cache control as well as the ability to pull “lazy loaded” attributes from long term cache as well.

In this demo, the following techniques are illustrated:

- Using custom subclasses of Query
- Basic technique of circumventing Query to pull from a custom cache source instead of the database.
- Rudimental caching with Beaker, using “regions” which allow global control over a fixed set of configurations.
- Using custom MapperOption objects to configure options on a Query, including the ability to invoke the options deep within an object graph when lazy loads occur.

E.g.:

    # query for Person objects, specifying cache
    q = Session.query(Person).options(FromCache("default", "all_people"))

    # specify that each Person's "addresses" collection comes from
    # cache too
    q = q.options(RelationshipCache("default", "by_person", Person.addresses))

    # query
    print q.all()

To run, both SQLAlchemy and Beaker (1.4 or greater) must be installed or on the current PYTHONPATH. The demo will create a local directory for datafiles, insert initial data, and run. Running the demo a second time will utilize the cache files already present, and exactly one SQL statement against two tables will be emitted - the displayed result however will utilize dozens of lazyloads that all pull from cache.

The demo scripts themselves, in order of complexity, are run as follows:

    python examples/beaker_caching/helloworld.py

    python examples/beaker_caching/relationship_caching.py

    python examples/beaker_caching/advanced.py

    python examples/beaker_caching/local_session_caching.py

Listing of files:

> environment.py - Establish data / cache file paths, and configurations, bootstrap fixture data if necessary.

> meta.py - Represent persistence structures which allow the usage of Beaker caching with SQLAlchemy. Introduces a query option called FromCache.

> model.py - The datamodel, which represents Person that has multiple Address objects, each with PostalCode, City, Country

> fixture_data.py - creates demo PostalCode, Address, Person objects in the database.

> helloworld.py - the basic idea.

> relationship_caching.py - Illustrates how to add cache options on relationship endpoints, so that lazyloads load from cache.

> advanced.py - Further examples of how to use FromCache. Combines techniques from the first two scripts.

> local_session_caching.py - Grok everything so far ? This example creates a new Beaker container that will persist data in a dictionary which is local to the current session. remove() the session and the cache is gone.

## Derived Attributes

Location: /examples/derived_attributes/

Illustrates a clever technique using Python descriptors to create custom attributes representing SQL expressions when used at the class level, and Python expressions when used at the instance level. In some cases this technique replaces the need to configure the attribute in the mapping, instead relying upon ordinary Python behavior to create custom expression components.

E.g.:

    class BaseInterval(object):
        @hybrid
        def contains(self,point):
            return (self.start <= point) & (point < self.end)

## Directed Graphs

Location: /examples/graphs/

An example of persistence for a directed graph structure. The graph is stored as a collection of edges, each referencing both a “lower” and an “upper” node in a table of nodes. Basic persistence and querying for lower- and upper- neighbors are illustrated:

    n2 = Node(2)
    n5 = Node(5)
    n2.add_neighbor(n5)
    print n2.higher_neighbors()

## Dynamic Relations as Dictionaries

Location: /examples/dynamic_dict/

Illustrates how to place a dictionary-like facade on top of a “dynamic” relation, so that dictionary operations (assuming simple string keys) can operate upon a large collection without loading the full collection at once.

## Horizontal Sharding

Location: /examples/sharding

a basic example of using the SQLAlchemy Sharding API. Sharding refers to horizontally scaling data across multiple databases.

The basic components of a “sharded” mapping are:

- multiple databases, each assigned a ‘shard id’
- a function which can return a single shard id, given an instance to be saved; this is called “shard_chooser”
- a function which can return a list of shard ids which apply to a particular instance identifier; this is called “id_chooser”. If it returns all shard ids, all shards will be searched.
- a function which can return a list of shard ids to try, given a particular Query (“query_chooser”). If it returns all shard ids, all shards will be queried and the results joined together.

In this example, four sqlite databases will store information about weather data on a database-per-continent basis. We provide example shard_chooser, id_chooser and query_chooser functions. The query_chooser illustrates inspection of the SQL expression element in order to attempt to determine a single shard being requested.

## Inheritance Mappings

Location: /examples/inheritance/

Working examples of single-table, joined-table, and concrete-table inheritance as described in Mapping Class Inheritance Hierarchies.

## Large Collections

Location: /examples/large_collection/

Large collection example.

Illustrates the options to use with `relationship()` when the list of related objects is very large, including:

- “dynamic” relationships which query slices of data as accessed
- how to use ON DELETE CASCADE in conjunction with `passive_deletes=True` to greatly improve the performance of related collection deletion.

## Nested Sets

Location: /examples/nested_sets/

Illustrates a rudimentary way to implement the “nested sets” pattern for hierarchical data using the SQLAlchemy ORM.

## Polymorphic Associations

Location: /examples/poly_assoc/

Illustrates polymorphic associations, a method of associating a particular child object with many different types of parent object.

This example is based off the original blog post at http://techspot.zzzeek.org/?p=13 and illustrates three techniques:

- `poly_assoc.py` - imitates the non-foreign-key schema used by Ruby on Rails’ Active Record.
- `poly_assoc_fk.py` - Adds a polymorphic association table so that referential integrity can be maintained.
- `poly_assoc_generic.py` - further automates the approach of `poly_assoc_fk.py` to also generate the association table definitions automatically.

## PostGIS Integration

Location: /examples/postgis

A naive example illustrating techniques to help embed PostGIS functionality.

This example was originally developed in the hopes that it would be extrapolated into a comprehensive PostGIS integration layer. We are pleased to announce that this has come to fruition as GeoAlchemy.

The example illustrates:

- a DDL extension which allows CREATE/DROP to work in conjunction with AddGeometryColumn/DropGeometryColumn
- a Geometry type, as well as a few subtypes, which convert result row values to a GIS-aware object, and also integrates with the DDL extension.
- a GIS-aware object which stores a raw geometry value and provides a factory for functions such as AsText().
- an ORM comparator which can override standard column methods on mapped objects to produce GIS operators.
- an attribute event listener that intercepts strings and converts to GeomFromText().
- a standalone operator example.

The implementation is limited to only public, well known and simple to use extension points.

E.g.:

    print session.query(Road).filter(Road.road_geom.intersects(r1.road_geom)).all()

## Versioned Objects

Location: /examples/versioning

Illustrates an extension which creates version tables for entities and stores records for each change. The same idea as Elixir’s versioned extension, but more efficient (uses attribute API to get history) and handles class inheritance. The given extensions generate an anonymous “history” class which represents historical versions of the target object.

Usage is illustrated via a unit test module `test_versioning.py`, which can be run via nose:

    nosetests -w examples/versioning/

A fragment of example usage, using declarative:

    from history_meta import VersionedMeta, VersionedListener

    Base = declarative_base(metaclass=VersionedMeta, bind=engine)
    Session = sessionmaker(extension=VersionedListener())

    class SomeClass(Base):
        __tablename__ = 'sometable'

        id = Column(Integer, primary_key=True)
        name = Column(String(50))

        def __eq__(self, other):
            assert type(other) is SomeClass and other.id == self.id

    sess = Session()
    sc = SomeClass(name='sc1')
    sess.add(sc)
    sess.commit()

    sc.name = 'sc1modified'
    sess.commit()

    assert sc.version == 2

    SomeClassHistory = SomeClass.__history_mapper__.class_

    assert sess.query(SomeClassHistory).\
                filter(SomeClassHistory.version == 1).\
                all() \
                == [SomeClassHistory(version=1, name='sc1')]

To apply `VersionedMeta` to a subset of classes (probably more typical), the metaclass can be applied on a per-class basis:

    from history_meta import VersionedMeta, VersionedListener

    Base = declarative_base(bind=engine)

    class SomeClass(Base):
        __tablename__ = 'sometable'

        # ...

    class SomeVersionedClass(Base):
        __metaclass__ = VersionedMeta
        __tablename__ = 'someothertable'

        # ...

The `VersionedMeta` is a declarative metaclass - to use the extension with plain mappers, the `_history_mapper` function can be applied:

    from history_meta import _history_mapper

    m = mapper(SomeClass, sometable)
    _history_mapper(m)

    SomeHistoryClass = SomeClass.__history_mapper__.class_

## Vertical Attribute Mapping

Location: /examples/vertical

Illustrates “vertical table” mappings.

A “vertical table” refers to a technique where individual attributes of an object are stored as distinct rows in a table. The “vertical table” technique is used to persist objects which can have a varied set of attributes, at the expense of simple query control and brevity. It is commonly found in content/document management systems in order to represent user-created structures flexibly.

Two variants on the approach are given. In the second, each row references a “datatype” which contains information about the type of information stored in the attribute, such as integer, string, or date.

Example:

    shrew = Animal(u'shrew')
    shrew[u'cuteness'] = 5
    shrew[u'weasel-like'] = False
    shrew[u'poisonous'] = True

    session.add(shrew)
    session.flush()

    q = (session.query(Animal).
         filter(Animal.facts.any(
           and_(AnimalFact.key == u'weasel-like',
                AnimalFact.value == True))))
    print 'weasel-like animals', q.all()

## XML Persistence

Location: /examples/elementtree/

Illustrates three strategies for persisting and querying XML documents as represented by ElementTree in a relational database. The techniques do not apply any mappings to the ElementTree objects directly, so are compatible with the native cElementTree as well as lxml, and can be adapted to suit any kind of DOM representation system. Querying along xpath-like strings is illustrated as well.

In order of complexity:

- `pickle.py` - Quick and dirty, serialize the whole DOM into a BLOB column. While the example is very brief, it has very limited functionality.
- `adjacency_list.py` - Each DOM node is stored in an individual table row, with attributes represented in a separate table. The nodes are associated in a hierarchy using an adjacency list structure. A query function is introduced which can search for nodes along any path with a given structure of attributes, basically a (very narrow) subset of xpath.
- `optimized_al.py` - Uses the same strategy as `adjacency_list.py`, but adds a `MapperExtension` which optimizes how the hierarchical structure is loaded, such that the full set of DOM nodes are loaded within a single table result set, and are organized hierarchically as they are received during a load.

E.g.:

    # parse an XML file and persist in the database
    doc = ElementTree.parse("test.xml")
    session.add(Document(file, doc))
    session.commit()

    # locate documents with a certain path/attribute structure
    for document in find_document('/somefile/header/field2[@attr=foo]'):
        # dump the XML
        print document