Indexable

Indexable

Define attributes on ORM-mapped classes that have “index” attributes for columns with Indexable types.

“index” means the attribute is associated with an element of an Indexable column with the predefined index to access it. The Indexable types include types such as ARRAY, JSON and HSTORE.

The indexable extension provides Column-like interface for any element of an Indexable typed column. In simple cases, it can be treated as a Column - mapped attribute.

New in version 1.1.

Synopsis

Given Person as a model with a primary key and JSON data field. While this field may have any number of elements encoded within it, we would like to refer to the element called name individually as a dedicated attribute which behaves like a standalone column:

from sqlalchemy import Column, JSON, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.indexable import index_property
Base = declarative_base()
class Person(Base):
    __tablename__ = 'person'
    id = Column(Integer, primary_key=True)
    data = Column(JSON)
    name = index_property('data', 'name')

Above, the name attribute now behaves like a mapped column. We can compose a new Person and set the value of name:

>>> person = Person(name='Alchemist')

The value is now accessible:

>>> person.name
'Alchemist'

Behind the scenes, the JSON field was initialized to a new blank dictionary and the field was set:

>>> person.data
{"name": "Alchemist'}

The field is mutable in place:

>>> person.name = 'Renamed'
>>> person.name
'Renamed'
>>> person.data
{'name': 'Renamed'}

When using index_property, the change that we make to the indexable structure is also automatically tracked as history; we no longer need to use MutableDict in order to track this change for the unit of work.

Deletions work normally as well:

>>> del person.name
>>> person.data
{}

Above, deletion of person.name deletes the value from the dictionary, but not the dictionary itself.

A missing key will produce AttributeError:

>>> person = Person()
>>> person.name
...
AttributeError: 'name'

Unless you set a default value:

>>> class Person(Base):
>>>     __tablename__ = 'person'
>>>
>>>     id = Column(Integer, primary_key=True)
>>>     data = Column(JSON)
>>>
>>>     name = index_property('data', 'name', default=None)  # See default
>>> person = Person()
>>> print(person.name)
None

The attributes are also accessible at the class level. Below, we illustrate Person.name used to generate an indexed SQL criteria:

>>> from sqlalchemy.orm import Session
>>> session = Session()
>>> query = session.query(Person).filter(Person.name == 'Alchemist')

The above query is equivalent to:

>>> query = session.query(Person).filter(Person.data['name'] == 'Alchemist')

Multiple index_property objects can be chained to produce multiple levels of indexing:

from sqlalchemy import Column, JSON, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.indexable import index_property
Base = declarative_base()
class Person(Base):
    __tablename__ = 'person'
    id = Column(Integer, primary_key=True)
    data = Column(JSON)
    birthday = index_property('data', 'birthday')
    year = index_property('birthday', 'year')
    month = index_property('birthday', 'month')
    day = index_property('birthday', 'day')

Above, a query such as:

q = session.query(Person).filter(Person.year == '1980')

On a PostgreSQL backend, the above query will render as:

SELECT person.id, person.data
FROM person
WHERE person.data -> %(data_1)s -> %(param_1)s = %(param_2)s

Default Values

index_property includes special behaviors for when the indexed data structure does not exist, and a set operation is called:

For an index_property that is given an integer index value, the default data structure will be a Python list of None values, at least as long as the index value; the value is then set at its place in the list. This means for an index value of zero, the list will be initialized to [None] before setting the given value, and for an index value of five, the list will be initialized to [None, None, None, None, None] before setting the fifth element to the given value. Note that an existing list is not extended in place to receive a value.
for an index_property that is given any other kind of index value (e.g. strings usually), a Python dictionary is used as the default data structure.
The default data structure can be set to any Python callable using the index_property.datatype parameter, overriding the previous rules.

Subclassing

index_property can be subclassed, in particular for the common use case of providing coercion of values or SQL expressions as they are accessed. Below is a common recipe for use with a PostgreSQL JSON type, where we want to also include automatic casting plus astext():

class pg_json_property(index_property):
    def __init__(self, attr_name, index, cast_type):
        super(pg_json_property, self).__init__(attr_name, index)
        self.cast_type = cast_type
    def expr(self, model):
        expr = super(pg_json_property, self).expr(model)
        return expr.astext.cast(self.cast_type)

The above subclass can be used with the PostgreSQL-specific version of JSON:

from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.dialects.postgresql import JSON
Base = declarative_base()
class Person(Base):
    __tablename__ = 'person'
    id = Column(Integer, primary_key=True)
    data = Column(JSON)
    age = pg_json_property('data', 'age', Integer)

The age attribute at the instance level works as before; however when rendering SQL, PostgreSQL’s ->> operator will be used for indexed access, instead of the usual index operator of ->:

>>> query = session.query(Person).filter(Person.age < 20)

The above query will render:

SELECT person.id, person.data
FROM person
WHERE CAST(person.data ->> %(data_1)s AS INTEGER) < %(param_1)s

API Reference

Object Name	Description
`index_property`	A property generator. The generated property describes an object attribute that corresponds to an `Indexable` column.

class sqlalchemy.ext.indexable.``index_property(attr_name, index, default=<object object>, datatype=None, mutable=True, onebased=True)

A property generator. The generated property describes an object attribute that corresponds to an Indexable column.

New in version 1.1.