SQLAlchemy 2.0 Documentation
SQLAlchemy ORM
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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.
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 ofNone
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 |
---|---|
A property generator. The generated property describes an object
attribute that corresponds to an |
- class sqlalchemy.ext.indexable.index_property¶
A property generator. The generated property describes an object attribute that corresponds to an
Indexable
column.See also
Members
Class signature
class
sqlalchemy.ext.indexable.index_property
(sqlalchemy.ext.hybrid.hybrid_property
)-
method
sqlalchemy.ext.indexable.index_property.
__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.
-
method
flambé! the dragon and The Alchemist image designs created and generously donated by Rotem Yaari.
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