# SQLAlchemy 0.9 Documentation

Search:
Release: 0.9.3 | Release Date: February 19, 2014

# ORM 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. Descriptions and source code for all can be found here.

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

## Mapping Recipes

### 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)

Listing of files:

- adjacency_list.py

### Associations

Examples illustrating the usage of the “association object” pattern, where an intermediary class mediates the relationship between two classes that are associated in a many-to-many pattern.

Listing of files:

- basic_association.py - illustrate a many-to-many relationship between an “Order” and a collection of “Item” objects, associating a purchase price with each via an association object called “OrderItem”
- dict_of_sets_with_default.py - an advanced association proxy example which illustrates nesting of association proxies to produce multi-level Python collections, in this case a dictionary with string keys and sets of integers as values, which conceal the underlying mapped classes.
- proxied_association.py - same example as basic_association, adding in usage of `sqlalchemy.ext.associationproxy` to make explicit references to `OrderItem` optional.

### Directed 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()

Listing of files:

- directed_graph.py - a directed graph example.

### Dynamic Relations as Dictionaries

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.

Listing of files:

- dynamic_dict.py

### Generic Associations

Illustrates various methods of associating multiple types of parents with a particular child object.

The examples all use the declarative extension along with declarative mixins. Each one presents the identical use case at the end - two classes, `Customer` and `Supplier`, both subclassing the `HasAddresses` mixin, which ensures that the parent class is provided with an `addresses` collection which contains `Address` objects.

The discriminator_on_association.py and generic_fk.py scripts are modernized versions of recipes presented in the 2007 blog post Polymorphic Associations with SQLAlchemy.

Listing of files:

- discriminator_on_association.py - Illustrates a mixin which provides a generic association using a single target table and a single association table, referred to by all parent tables. The association table contains a “discriminator” column which determines what type of parent object associates to each particular row in the association table.
- generic_fk.py - Illustrates a so-called “generic foreign key”, in a similar fashion to that of popular frameworks such as Django, ROR, etc. This approach bypasses standard referential integrity practices, in that the “foreign key” column is not actually constrained to refer to any particular table; instead, in-application logic is used to determine which table is referenced.
- table_per_association.py - Illustrates a mixin which provides a generic association via a individually generated association tables for each parent class. The associated objects themselves are persisted in a single table shared among all parents.
- table_per_related.py - Illustrates a generic association which persists association objects within individual tables, each one generated to persist those objects on behalf of a particular parent class.

### Large Collections

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.

Listing of files:

- large_collection.py

### Nested Sets

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

Listing of files:

- nested_sets.py - Celko’s “Nested Sets” Tree Structure.

### Relationship Join Conditions

Examples of various `orm.relationship()` configurations, which make use of the `primaryjoin` argument to compose special types of join conditions.

Listing of files:

- cast.py - Illustrate a `relationship()` that joins two columns where those columns are not of the same type, and a CAST must be used on the SQL side in order to match them.
- threeway.py - Illustrate a “three way join” - where a primary table joins to a remote table via an association table, but then the primary table also needs to refer to some columns in the remote table directly.

### XML Persistence

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.

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

Listing of files:

- pickle.py - illustrates a quick and dirty way to persist an XML document expressed using ElementTree and pickle.
- adjacency_list.py - Illustrates an explicit way to persist an XML document expressed using ElementTree.
- optimized_al.py - Uses the same strategy as `adjacency_list.py`, but associates each DOM row with its owning document row, so that a full document of DOM nodes can be loaded using O(1) queries - the construction of the “hierarchy” is performed after the load in a non-recursive fashion and is more efficient.

### Versioning Objects

#### Versioning with a History Table

Illustrates an extension which creates version tables for entities and stores records for each change. 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:

    cd examples/versioning
    nosetests -v

A fragment of example usage, using declarative:

    from history_meta import Versioned, versioned_session

    Base = declarative_base()

    class SomeClass(Versioned, 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

    Session = sessionmaker(bind=engine)
    versioned_session(Session)

    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')]

The `Versioned` mixin is designed to work with declarative. To use the extension with classical 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_

Listing of files:

- history_meta.py - Versioned mixin class and other utilities.
- test_versioning.py - Unit tests illustrating usage of the `history_meta.py` module functions.

#### Versioning using Temporal Rows

Illustrates an extension which versions data by storing new rows for each change; that is, what would normally be an UPDATE becomes an INSERT.

Listing of files:

- versioned_map.py - A variant of the versioned_rows example. Here we store a dictionary of key/value pairs, storing the k/v’s in a “vertical” fashion where each key gets a row. The value is split out into two separate datatypes, string and int - the range of datatype storage can be adjusted for individual needs.
- versioned_rows.py - Illustrates a method to intercept changes on objects, turning an UPDATE statement on a single row into an INSERT statement, so that a new row is inserted with the new data, keeping the old row intact.

### Vertical Attribute Mapping

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()

Listing of files:

- dictlike-polymorphic.py - Mapping a polymorphic-valued vertical table as a dictionary.
- dictlike.py - Mapping a vertical table as a dictionary.

## Inheritance Mapping Recipes

### Basic Inheritance Mappings

Working examples of single-table, joined-table, and concrete-table inheritance as described in datamapping_inheritance.

Listing of files:

- concrete.py - Concrete (table-per-class) inheritance example.
- joined.py - Joined-table (table-per-subclass) inheritance example.
- single.py - Single-table inheritance example.

## Special APIs

### Attribute Instrumentation

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

Listing of files:

- custom_management.py - Illustrates customized class instrumentation, using the `sqlalchemy.ext.instrumentation` extension package.
- listen_for_events.py - Illustrates how to attach events to all instrumented attributes and listen for change events.

### Horizontal 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.

The construction of generic sharding routines is an ambitious approach to the issue of organizing instances among multiple databases. For a more plain-spoken alternative, the “distinct entity” approach is a simple method of assigning objects to different tables (and potentially database nodes) in an explicit way - described on the wiki at EntityName.

Listing of files:

- attribute_shard.py

## Extending the ORM

### Dogpile Caching

Illustrates how to embed dogpile.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.

Changed in version 0.8: The example was modernized to use dogpile.cache, replacing Beaker as the caching library in use.

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 dogpile.cache, 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"))

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

    # query
    print q.all()

To run, both SQLAlchemy and dogpile.cache 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 Python modules so that relative imports work:

    python -m examples.dogpile_caching.helloworld

    python -m examples.dogpile_caching.relationship_caching

    python -m examples.dogpile_caching.advanced

    python -m examples.dogpile_caching.local_session_caching

Listing of files:

- environment.py - Establish data / cache file paths, and configurations, bootstrap fixture data if necessary.
- caching_query.py - Represent functions and classes which allow the usage of Dogpile 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 - Installs some sample data. Here we have a handful of postal codes for a few US/ Canadian cities. Then, 100 Person records are installed, each with a randomly selected postal code.
- helloworld.py - Illustrate how to load some data, and cache the results.
- relationship_caching.py - Illustrates how to add cache options on relationship endpoints, so that lazyloads load from cache.
- advanced.py - Illustrate usage of Query combined with the FromCache option, including front-end loading, cache invalidation and collection caching.
- local_session_caching.py - Grok everything so far ? This example creates a new dogpile.cache backend that will persist data in a dictionary which is local to the current session. remove() the session and the cache is gone.

### PostGIS Integration

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()

Listing of files:

- postgis.py