Indexable — SQLAlchemy 1.2 Documentation

Release: 1.2.8 | Release Date: May 28, 2018

# SQLAlchemy 1.2 Documentation

# 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 `postgresql.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 opearator 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

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

  Bases: `sqlalchemy.ext.hybrid.hybrid_property`

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

  New in version 1.1.

  See also

  `sqlalchemy.ext.indexable`

  `__init__`(attr_name, index, default=<object object>, datatype=None, mutable=True, onebased=True)

    Create a new `index_property`.

    Parameters: | attr_name – An attribute name of an Indexable typed column, or other attribute that returns an indexable structure. index – The index to be used for getting and setting this value. This should be the Python-side index value for integers. default – A value which will be returned instead of AttributeError when there is not a value at given index. datatype – default datatype to use when the field is empty. By default, this is derived from the type of index used; a Python list for an integer index, or a Python dictionary for any other style of index. For a list, the list will be initialized to a list of None values that is at least `index` elements long. mutable – if False, writes and deletes to the attribute will be disallowed. onebased – assume the SQL representation of this value is one-based; that is, the first index in SQL is 1, not zero.