SQLAlchemy 1.4 Documentation
SQLAlchemy ORM
- ORM Quick Start
- Object Relational Tutorial (1.x API)
- ORM Mapped Class Configuration
- Relationship Configuration
- Querying Data, Loading Objects
- Using the Session
- Events and Internals
- ORM Extensions
- Asynchronous I/O (asyncio)
- Association Proxy
- Automap
- Baked Queries
- Declarative Extensions
- Mypy / Pep-484 Support for ORM Mappings
- Mutation Tracking
- Ordering List
- Horizontal Sharding
- Hybrid Attributes¶
- Defining Expression Behavior Distinct from Attribute Behavior
- Defining Setters
- Allowing Bulk ORM Update
- Working with Relationships
- Building Custom Comparators
- Reusing Hybrid Properties across Subclasses
- Hybrid Value Objects
- Building Transformers
- API Reference
- Indexable
- Alternate Class Instrumentation
- ORM Examples
Project Versions
- Previous: Horizontal Sharding
- Next: Indexable
- Up: Home
- On this page:
- Hybrid Attributes
- Defining Expression Behavior Distinct from Attribute Behavior
- Defining Setters
- Allowing Bulk ORM Update
- Working with Relationships
- Building Custom Comparators
- Reusing Hybrid Properties across Subclasses
- Hybrid Value Objects
- Building Transformers
- API Reference
Hybrid Attributes¶
Define attributes on ORM-mapped classes that have “hybrid” behavior.
“hybrid” means the attribute has distinct behaviors defined at the class level and at the instance level.
The hybrid
extension provides a special form of
method decorator, is around 50 lines of code and has almost no
dependencies on the rest of SQLAlchemy. It can, in theory, work with
any descriptor-based expression system.
Consider a mapping Interval
, representing integer start
and end
values. We can define higher level functions on mapped classes that produce SQL
expressions at the class level, and Python expression evaluation at the
instance level. Below, each function decorated with hybrid_method
or
hybrid_property
may receive self
as an instance of the class, or
as the class itself:
from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session, aliased
from sqlalchemy.ext.hybrid import hybrid_property, hybrid_method
Base = declarative_base()
class Interval(Base):
__tablename__ = 'interval'
id = Column(Integer, primary_key=True)
start = Column(Integer, nullable=False)
end = Column(Integer, nullable=False)
def __init__(self, start, end):
self.start = start
self.end = end
@hybrid_property
def length(self):
return self.end - self.start
@hybrid_method
def contains(self, point):
return (self.start <= point) & (point <= self.end)
@hybrid_method
def intersects(self, other):
return self.contains(other.start) | self.contains(other.end)
Above, the length
property returns the difference between the
end
and start
attributes. With an instance of Interval
,
this subtraction occurs in Python, using normal Python descriptor
mechanics:
>>> i1 = Interval(5, 10)
>>> i1.length
5
When dealing with the Interval
class itself, the hybrid_property
descriptor evaluates the function body given the Interval
class as
the argument, which when evaluated with SQLAlchemy expression mechanics
(here using the QueryableAttribute.expression
accessor)
returns a new SQL expression:
>>> print(Interval.length.expression)
interval."end" - interval.start
>>> print(Session().query(Interval).filter(Interval.length > 10))
SELECT interval.id AS interval_id, interval.start AS interval_start,
interval."end" AS interval_end
FROM interval
WHERE interval."end" - interval.start > :param_1
ORM methods such as Query.filter_by()
generally use getattr()
to
locate attributes, so can also be used with hybrid attributes:
>>> print(Session().query(Interval).filter_by(length=5))
SELECT interval.id AS interval_id, interval.start AS interval_start,
interval."end" AS interval_end
FROM interval
WHERE interval."end" - interval.start = :param_1
The Interval
class example also illustrates two methods,
contains()
and intersects()
, decorated with
hybrid_method
. This decorator applies the same idea to
methods that hybrid_property
applies to attributes. The
methods return boolean values, and take advantage of the Python |
and &
bitwise operators to produce equivalent instance-level and
SQL expression-level boolean behavior:
>>> i1.contains(6)
True
>>> i1.contains(15)
False
>>> i1.intersects(Interval(7, 18))
True
>>> i1.intersects(Interval(25, 29))
False
>>> print(Session().query(Interval).filter(Interval.contains(15)))
SELECT interval.id AS interval_id, interval.start AS interval_start,
interval."end" AS interval_end
FROM interval
WHERE interval.start <= :start_1 AND interval."end" > :end_1
>>> ia = aliased(Interval)
>>> print(Session().query(Interval, ia).filter(Interval.intersects(ia)))
SELECT interval.id AS interval_id, interval.start AS interval_start,
interval."end" AS interval_end, interval_1.id AS interval_1_id,
interval_1.start AS interval_1_start, interval_1."end" AS interval_1_end
FROM interval, interval AS interval_1
WHERE interval.start <= interval_1.start
AND interval."end" > interval_1.start
OR interval.start <= interval_1."end"
AND interval."end" > interval_1."end"
Defining Expression Behavior Distinct from Attribute Behavior¶
Our usage of the &
and |
bitwise operators above was
fortunate, considering our functions operated on two boolean values to
return a new one. In many cases, the construction of an in-Python
function and a SQLAlchemy SQL expression have enough differences that
two separate Python expressions should be defined. The
hybrid
decorators define the
hybrid_property.expression()
modifier for this purpose. As an
example we’ll define the radius of the interval, which requires the
usage of the absolute value function:
from sqlalchemy import func
class Interval(object):
# ...
@hybrid_property
def radius(self):
return abs(self.length) / 2
@radius.expression
def radius(cls):
return func.abs(cls.length) / 2
Above the Python function abs()
is used for instance-level
operations, the SQL function ABS()
is used via the func
object for class-level expressions:
>>> i1.radius
2
>>> print(Session().query(Interval).filter(Interval.radius > 5))
SELECT interval.id AS interval_id, interval.start AS interval_start,
interval."end" AS interval_end
FROM interval
WHERE abs(interval."end" - interval.start) / :abs_1 > :param_1
Note
When defining an expression for a hybrid property or method, the expression method must retain the name of the original hybrid, else the new hybrid with the additional state will be attached to the class with the non-matching name. To use the example above:
class Interval(object):
# ...
@hybrid_property
def radius(self):
return abs(self.length) / 2
# WRONG - the non-matching name will cause this function to be
# ignored
@radius.expression
def radius_expression(cls):
return func.abs(cls.length) / 2
This is also true for other mutator methods, such as
hybrid_property.update_expression()
. This is the same behavior
as that of the @property
construct that is part of standard Python.
Defining Setters¶
Hybrid properties can also define setter methods. If we wanted
length
above, when set, to modify the endpoint value:
class Interval(object):
# ...
@hybrid_property
def length(self):
return self.end - self.start
@length.setter
def length(self, value):
self.end = self.start + value
The length(self, value)
method is now called upon set:
>>> i1 = Interval(5, 10)
>>> i1.length
5
>>> i1.length = 12
>>> i1.end
17
Allowing Bulk ORM Update¶
A hybrid can define a custom “UPDATE” handler for when using the
Query.update()
method, allowing the hybrid to be used in the
SET clause of the update.
Normally, when using a hybrid with Query.update()
, the SQL
expression is used as the column that’s the target of the SET. If our
Interval
class had a hybrid start_point
that linked to
Interval.start
, this could be substituted directly:
session.query(Interval).update({Interval.start_point: 10})
However, when using a composite hybrid like Interval.length
, this
hybrid represents more than one column. We can set up a handler that will
accommodate a value passed to Query.update()
which can affect
this, using the hybrid_property.update_expression()
decorator.
A handler that works similarly to our setter would be:
class Interval(object):
# ...
@hybrid_property
def length(self):
return self.end - self.start
@length.setter
def length(self, value):
self.end = self.start + value
@length.update_expression
def length(cls, value):
return [
(cls.end, cls.start + value)
]
Above, if we use Interval.length
in an UPDATE expression as:
session.query(Interval).update(
{Interval.length: 25}, synchronize_session='fetch')
We’ll get an UPDATE statement along the lines of:
UPDATE interval SET end=start + :value
In some cases, the default “evaluate” strategy can’t perform the SET expression in Python; while the addition operator we’re using above is supported, for more complex SET expressions it will usually be necessary to use either the “fetch” or False synchronization strategy as illustrated above.
Note
For ORM bulk updates to work with hybrids, the function name of the hybrid must match that of how it is accessed. Something like this wouldn’t work:
class Interval(object):
# ...
def _get(self):
return self.end - self.start
def _set(self, value):
self.end = self.start + value
def _update_expr(cls, value):
return [
(cls.end, cls.start + value)
]
length = hybrid_property(
fget=_get, fset=_set, update_expr=_update_expr
)
The Python descriptor protocol does not provide any reliable way for
a descriptor to know what attribute name it was accessed as, and
the UPDATE scheme currently relies upon being able to access the
attribute from an instance by name in order to perform the instance
synchronization step.
New in version 1.2: added support for bulk updates to hybrid properties.
Working with Relationships¶
There’s no essential difference when creating hybrids that work with related objects as opposed to column-based data. The need for distinct expressions tends to be greater. The two variants we’ll illustrate are the “join-dependent” hybrid, and the “correlated subquery” hybrid.
Join-Dependent Relationship Hybrid¶
Consider the following declarative
mapping which relates a User
to a SavingsAccount
:
from sqlalchemy import Column, Integer, ForeignKey, Numeric, String
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
Base = declarative_base()
class SavingsAccount(Base):
__tablename__ = 'account'
id = Column(Integer, primary_key=True)
user_id = Column(Integer, ForeignKey('user.id'), nullable=False)
balance = Column(Numeric(15, 5))
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(100), nullable=False)
accounts = relationship("SavingsAccount", backref="owner")
@hybrid_property
def balance(self):
if self.accounts:
return self.accounts[0].balance
else:
return None
@balance.setter
def balance(self, value):
if not self.accounts:
account = Account(owner=self)
else:
account = self.accounts[0]
account.balance = value
@balance.expression
def balance(cls):
return SavingsAccount.balance
The above hybrid property balance
works with the first
SavingsAccount
entry in the list of accounts for this user. The
in-Python getter/setter methods can treat accounts
as a Python
list available on self
.
However, at the expression level, it’s expected that the User
class will
be used in an appropriate context such that an appropriate join to
SavingsAccount
will be present:
>>> print(Session().query(User, User.balance).
... join(User.accounts).filter(User.balance > 5000))
SELECT "user".id AS user_id, "user".name AS user_name,
account.balance AS account_balance
FROM "user" JOIN account ON "user".id = account.user_id
WHERE account.balance > :balance_1
Note however, that while the instance level accessors need to worry
about whether self.accounts
is even present, this issue expresses
itself differently at the SQL expression level, where we basically
would use an outer join:
>>> from sqlalchemy import or_
>>> print (Session().query(User, User.balance).outerjoin(User.accounts).
... filter(or_(User.balance < 5000, User.balance == None)))
SELECT "user".id AS user_id, "user".name AS user_name,
account.balance AS account_balance
FROM "user" LEFT OUTER JOIN account ON "user".id = account.user_id
WHERE account.balance < :balance_1 OR account.balance IS NULL
Correlated Subquery Relationship Hybrid¶
We can, of course, forego being dependent on the enclosing query’s usage
of joins in favor of the correlated subquery, which can portably be packed
into a single column expression. A correlated subquery is more portable, but
often performs more poorly at the SQL level. Using the same technique
illustrated at Using column_property,
we can adjust our SavingsAccount
example to aggregate the balances for
all accounts, and use a correlated subquery for the column expression:
from sqlalchemy import Column, Integer, ForeignKey, Numeric, String
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy import select, func
Base = declarative_base()
class SavingsAccount(Base):
__tablename__ = 'account'
id = Column(Integer, primary_key=True)
user_id = Column(Integer, ForeignKey('user.id'), nullable=False)
balance = Column(Numeric(15, 5))
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(100), nullable=False)
accounts = relationship("SavingsAccount", backref="owner")
@hybrid_property
def balance(self):
return sum(acc.balance for acc in self.accounts)
@balance.expression
def balance(cls):
return select(func.sum(SavingsAccount.balance)).\
where(SavingsAccount.user_id==cls.id).\
label('total_balance')
The above recipe will give us the balance
column which renders
a correlated SELECT:
>>> print(s.query(User).filter(User.balance > 400))
SELECT "user".id AS user_id, "user".name AS user_name
FROM "user"
WHERE (SELECT sum(account.balance) AS sum_1
FROM account
WHERE account.user_id = "user".id) > :param_1
Building Custom Comparators¶
The hybrid property also includes a helper that allows construction of custom comparators. A comparator object allows one to customize the behavior of each SQLAlchemy expression operator individually. They are useful when creating custom types that have some highly idiosyncratic behavior on the SQL side.
Note
The hybrid_property.comparator()
decorator introduced
in this section replaces the use of the
hybrid_property.expression()
decorator.
They cannot be used together.
The example class below allows case-insensitive comparisons on the attribute
named word_insensitive
:
from sqlalchemy.ext.hybrid import Comparator, hybrid_property
from sqlalchemy import func, Column, Integer, String
from sqlalchemy.orm import Session
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class CaseInsensitiveComparator(Comparator):
def __eq__(self, other):
return func.lower(self.__clause_element__()) == func.lower(other)
class SearchWord(Base):
__tablename__ = 'searchword'
id = Column(Integer, primary_key=True)
word = Column(String(255), nullable=False)
@hybrid_property
def word_insensitive(self):
return self.word.lower()
@word_insensitive.comparator
def word_insensitive(cls):
return CaseInsensitiveComparator(cls.word)
Above, SQL expressions against word_insensitive
will apply the LOWER()
SQL function to both sides:
>>> print(Session().query(SearchWord).filter_by(word_insensitive="Trucks"))
SELECT searchword.id AS searchword_id, searchword.word AS searchword_word
FROM searchword
WHERE lower(searchword.word) = lower(:lower_1)
The CaseInsensitiveComparator
above implements part of the
ColumnOperators
interface. A “coercion” operation like
lowercasing can be applied to all comparison operations (i.e. eq
,
lt
, gt
, etc.) using Operators.operate()
:
class CaseInsensitiveComparator(Comparator):
def operate(self, op, other, **kwargs):
return op(
func.lower(self.__clause_element__()),
func.lower(other),
**kwargs,
)
Reusing Hybrid Properties across Subclasses¶
A hybrid can be referred to from a superclass, to allow modifying
methods like hybrid_property.getter()
, hybrid_property.setter()
to be used to redefine those methods on a subclass. This is similar to
how the standard Python @property
object works:
class FirstNameOnly(Base):
# ...
first_name = Column(String)
@hybrid_property
def name(self):
return self.first_name
@name.setter
def name(self, value):
self.first_name = value
class FirstNameLastName(FirstNameOnly):
# ...
last_name = Column(String)
@FirstNameOnly.name.getter
def name(self):
return self.first_name + ' ' + self.last_name
@name.setter
def name(self, value):
self.first_name, self.last_name = value.split(' ', 1)
Above, the FirstNameLastName
class refers to the hybrid from
FirstNameOnly.name
to repurpose its getter and setter for the subclass.
When overriding hybrid_property.expression()
and
hybrid_property.comparator()
alone as the first reference to the
superclass, these names conflict with the same-named accessors on the class-
level QueryableAttribute
object returned at the class level. To
override these methods when referring directly to the parent class descriptor,
add the special qualifier hybrid_property.overrides
, which will de-
reference the instrumented attribute back to the hybrid object:
class FirstNameLastName(FirstNameOnly):
# ...
last_name = Column(String)
@FirstNameOnly.name.overrides.expression
def name(cls):
return func.concat(cls.first_name, ' ', cls.last_name)
New in version 1.2: Added hybrid_property.getter()
as well as the
ability to redefine accessors per-subclass.
Hybrid Value Objects¶
Note in our previous example, if we were to compare the word_insensitive
attribute of a SearchWord
instance to a plain Python string, the plain
Python string would not be coerced to lower case - the
CaseInsensitiveComparator
we built, being returned by
@word_insensitive.comparator
, only applies to the SQL side.
A more comprehensive form of the custom comparator is to construct a Hybrid
Value Object. This technique applies the target value or expression to a value
object which is then returned by the accessor in all cases. The value object
allows control of all operations upon the value as well as how compared values
are treated, both on the SQL expression side as well as the Python value side.
Replacing the previous CaseInsensitiveComparator
class with a new
CaseInsensitiveWord
class:
class CaseInsensitiveWord(Comparator):
"Hybrid value representing a lower case representation of a word."
def __init__(self, word):
if isinstance(word, basestring):
self.word = word.lower()
elif isinstance(word, CaseInsensitiveWord):
self.word = word.word
else:
self.word = func.lower(word)
def operate(self, op, other, **kwargs):
if not isinstance(other, CaseInsensitiveWord):
other = CaseInsensitiveWord(other)
return op(self.word, other.word, **kwargs)
def __clause_element__(self):
return self.word
def __str__(self):
return self.word
key = 'word'
"Label to apply to Query tuple results"
Above, the CaseInsensitiveWord
object represents self.word
, which may
be a SQL function, or may be a Python native. By overriding operate()
and
__clause_element__()
to work in terms of self.word
, all comparison
operations will work against the “converted” form of word
, whether it be
SQL side or Python side. Our SearchWord
class can now deliver the
CaseInsensitiveWord
object unconditionally from a single hybrid call:
class SearchWord(Base):
__tablename__ = 'searchword'
id = Column(Integer, primary_key=True)
word = Column(String(255), nullable=False)
@hybrid_property
def word_insensitive(self):
return CaseInsensitiveWord(self.word)
The word_insensitive
attribute now has case-insensitive comparison behavior
universally, including SQL expression vs. Python expression (note the Python
value is converted to lower case on the Python side here):
>>> print(Session().query(SearchWord).filter_by(word_insensitive="Trucks"))
SELECT searchword.id AS searchword_id, searchword.word AS searchword_word
FROM searchword
WHERE lower(searchword.word) = :lower_1
SQL expression versus SQL expression:
>>> sw1 = aliased(SearchWord)
>>> sw2 = aliased(SearchWord)
>>> print(Session().query(
... sw1.word_insensitive,
... sw2.word_insensitive).\
... filter(
... sw1.word_insensitive > sw2.word_insensitive
... ))
SELECT lower(searchword_1.word) AS lower_1,
lower(searchword_2.word) AS lower_2
FROM searchword AS searchword_1, searchword AS searchword_2
WHERE lower(searchword_1.word) > lower(searchword_2.word)
Python only expression:
>>> ws1 = SearchWord(word="SomeWord")
>>> ws1.word_insensitive == "sOmEwOrD"
True
>>> ws1.word_insensitive == "XOmEwOrX"
False
>>> print(ws1.word_insensitive)
someword
The Hybrid Value pattern is very useful for any kind of value that may have multiple representations, such as timestamps, time deltas, units of measurement, currencies and encrypted passwords.
See also
Hybrids and Value Agnostic Types - on the techspot.zzzeek.org blog
Value Agnostic Types, Part II - on the techspot.zzzeek.org blog
Building Transformers¶
A transformer is an object which can receive a Query
object and
return a new one. The Query
object includes a method
with_transformation()
that returns a new Query
transformed by
the given function.
We can combine this with the Comparator
class to produce one type
of recipe which can both set up the FROM clause of a query as well as assign
filtering criterion.
Consider a mapped class Node
, which assembles using adjacency list into a
hierarchical tree pattern:
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Node(Base):
__tablename__ = 'node'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('node.id'))
parent = relationship("Node", remote_side=id)
Suppose we wanted to add an accessor grandparent
. This would return the
parent
of Node.parent
. When we have an instance of Node
, this is
simple:
from sqlalchemy.ext.hybrid import hybrid_property
class Node(Base):
# ...
@hybrid_property
def grandparent(self):
return self.parent.parent
For the expression, things are not so clear. We’d need to construct a
Query
where we Query.join()
twice along
Node.parent
to get to the grandparent
. We can instead return a
transforming callable that we’ll combine with the Comparator
class to
receive any Query
object, and return a new one that’s joined to
the Node.parent
attribute and filtered based on the given criterion:
from sqlalchemy.ext.hybrid import Comparator
class GrandparentTransformer(Comparator):
def operate(self, op, other, **kwargs):
def transform(q):
cls = self.__clause_element__()
parent_alias = aliased(cls)
return q.join(parent_alias, cls.parent).filter(
op(parent_alias.parent, other, **kwargs)
)
return transform
Base = declarative_base()
class Node(Base):
__tablename__ = 'node'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('node.id'))
parent = relationship("Node", remote_side=id)
@hybrid_property
def grandparent(self):
return self.parent.parent
@grandparent.comparator
def grandparent(cls):
return GrandparentTransformer(cls)
The GrandparentTransformer
overrides the core Operators.operate()
method at the base of the Comparator
hierarchy to return a query-
transforming callable, which then runs the given comparison operation in a
particular context. Such as, in the example above, the operate
method is
called, given the Operators.eq
callable as well as the right side of
the comparison Node(id=5)
. A function transform
is then returned which
will transform a Query
first to join to Node.parent
,
then to
compare parent_alias
using Operators.eq
against the left and right
sides, passing into Query.filter()
:
>>> from sqlalchemy.orm import Session
>>> session = Session()
sql>>> session.query(Node).\
... with_transformation(Node.grandparent==Node(id=5)).\
... all()
SELECT node.id AS node_id, node.parent_id AS node_parent_id
FROM node JOIN node AS node_1 ON node_1.id = node.parent_id
WHERE :param_1 = node_1.parent_id
We can modify the pattern to be more verbose but flexible by separating the
“join” step from the “filter” step. The tricky part here is ensuring that
successive instances of GrandparentTransformer
use the same
AliasedClass
object against Node
. Below we use a simple
memoizing approach that associates a GrandparentTransformer
with each
class:
class Node(Base):
# ...
@grandparent.comparator
def grandparent(cls):
# memoize a GrandparentTransformer
# per class
if '_gp' not in cls.__dict__:
cls._gp = GrandparentTransformer(cls)
return cls._gp
class GrandparentTransformer(Comparator):
def __init__(self, cls):
self.parent_alias = aliased(cls)
@property
def join(self):
def go(q):
return q.join(self.parent_alias, Node.parent)
return go
def operate(self, op, other, **kwargs):
return op(self.parent_alias.parent, other, **kwargs)
sql>>> session.query(Node).\
... with_transformation(Node.grandparent.join).\
... filter(Node.grandparent==Node(id=5))
SELECT node.id AS node_id, node.parent_id AS node_parent_id
FROM node JOIN node AS node_1 ON node_1.id = node.parent_id
WHERE :param_1 = node_1.parent_id
The “transformer” pattern is an experimental pattern that starts to make usage of some functional programming paradigms. While it’s only recommended for advanced and/or patient developers, there’s probably a whole lot of amazing things it can be used for.
API Reference¶
Object Name | Description |
---|---|
A helper class that allows easy construction of custom
|
|
A decorator which allows definition of a Python object method with both instance-level and class-level behavior. |
|
Symbol indicating an |
|
A decorator which allows definition of a Python descriptor with both instance-level and class-level behavior. |
|
- class sqlalchemy.ext.hybrid.hybrid_method(func, expr=None)¶
A decorator which allows definition of a Python object method with both instance-level and class-level behavior.
Members
Class signature
class
sqlalchemy.ext.hybrid.hybrid_method
(sqlalchemy.orm.base.InspectionAttrInfo
)-
method
sqlalchemy.ext.hybrid.hybrid_method.
__init__(func, expr=None)¶ Create a new
hybrid_method
.Usage is typically via decorator:
from sqlalchemy.ext.hybrid import hybrid_method class SomeClass(object): @hybrid_method def value(self, x, y): return self._value + x + y @value.expression def value(self, x, y): return func.some_function(self._value, x, y)
-
method
sqlalchemy.ext.hybrid.hybrid_method.
expression(expr)¶ Provide a modifying decorator that defines a SQL-expression producing method.
-
attribute
sqlalchemy.ext.hybrid.hybrid_method.
extension_type = symbol('HYBRID_METHOD')¶ The extension type, if any. Defaults to
NOT_EXTENSION
-
attribute
sqlalchemy.ext.hybrid.hybrid_method.
is_attribute = True¶ True if this object is a Python descriptor.
This can refer to one of many types. Usually a
QueryableAttribute
which handles attributes events on behalf of aMapperProperty
. But can also be an extension type such asAssociationProxy
orhybrid_property
. TheInspectionAttr.extension_type
will refer to a constant identifying the specific subtype.See also
-
method
- class sqlalchemy.ext.hybrid.hybrid_property(fget, fset=None, fdel=None, expr=None, custom_comparator=None, update_expr=None)¶
A decorator which allows definition of a Python descriptor with both instance-level and class-level behavior.
Members
__init__(), comparator(), deleter(), expression(), extension_type, getter(), is_attribute, overrides, setter(), update_expression()
Class signature
class
sqlalchemy.ext.hybrid.hybrid_property
(sqlalchemy.orm.base.InspectionAttrInfo
)-
method
sqlalchemy.ext.hybrid.hybrid_property.
__init__(fget, fset=None, fdel=None, expr=None, custom_comparator=None, update_expr=None)¶ Create a new
hybrid_property
.Usage is typically via decorator:
from sqlalchemy.ext.hybrid import hybrid_property class SomeClass(object): @hybrid_property def value(self): return self._value @value.setter def value(self, value): self._value = value
-
method
sqlalchemy.ext.hybrid.hybrid_property.
comparator(comparator)¶ Provide a modifying decorator that defines a custom comparator producing method.
The return value of the decorated method should be an instance of
Comparator
.Note
The
hybrid_property.comparator()
decorator replaces the use of thehybrid_property.expression()
decorator. They cannot be used together.When a hybrid is invoked at the class level, the
Comparator
object given here is wrapped inside of a specializedQueryableAttribute
, which is the same kind of object used by the ORM to represent other mapped attributes. The reason for this is so that other class-level attributes such as docstrings and a reference to the hybrid itself may be maintained within the structure that’s returned, without any modifications to the original comparator object passed in.Note
When referring to a hybrid property from an owning class (e.g.
SomeClass.some_hybrid
), an instance ofQueryableAttribute
is returned, representing the expression or comparator object as this hybrid object. However, that object itself has accessors calledexpression
andcomparator
; so when attempting to override these decorators on a subclass, it may be necessary to qualify it using thehybrid_property.overrides
modifier first. See that modifier for details.
-
method
sqlalchemy.ext.hybrid.hybrid_property.
deleter(fdel)¶ Provide a modifying decorator that defines a deletion method.
-
method
sqlalchemy.ext.hybrid.hybrid_property.
expression(expr)¶ Provide a modifying decorator that defines a SQL-expression producing method.
When a hybrid is invoked at the class level, the SQL expression given here is wrapped inside of a specialized
QueryableAttribute
, which is the same kind of object used by the ORM to represent other mapped attributes. The reason for this is so that other class-level attributes such as docstrings and a reference to the hybrid itself may be maintained within the structure that’s returned, without any modifications to the original SQL expression passed in.Note
When referring to a hybrid property from an owning class (e.g.
SomeClass.some_hybrid
), an instance ofQueryableAttribute
is returned, representing the expression or comparator object as well as this hybrid object. However, that object itself has accessors calledexpression
andcomparator
; so when attempting to override these decorators on a subclass, it may be necessary to qualify it using thehybrid_property.overrides
modifier first. See that modifier for details.
-
attribute
sqlalchemy.ext.hybrid.hybrid_property.
extension_type = symbol('HYBRID_PROPERTY')¶ The extension type, if any. Defaults to
NOT_EXTENSION
-
method
sqlalchemy.ext.hybrid.hybrid_property.
getter(fget)¶ Provide a modifying decorator that defines a getter method.
New in version 1.2.
-
attribute
sqlalchemy.ext.hybrid.hybrid_property.
is_attribute = True¶ True if this object is a Python descriptor.
This can refer to one of many types. Usually a
QueryableAttribute
which handles attributes events on behalf of aMapperProperty
. But can also be an extension type such asAssociationProxy
orhybrid_property
. TheInspectionAttr.extension_type
will refer to a constant identifying the specific subtype.See also
-
attribute
sqlalchemy.ext.hybrid.hybrid_property.
overrides¶ Prefix for a method that is overriding an existing attribute.
The
hybrid_property.overrides
accessor just returns this hybrid object, which when called at the class level from a parent class, will de-reference the “instrumented attribute” normally returned at this level, and allow modifying decorators likehybrid_property.expression()
andhybrid_property.comparator()
to be used without conflicting with the same-named attributes normally present on theQueryableAttribute
:class SuperClass(object): # ... @hybrid_property def foobar(self): return self._foobar class SubClass(SuperClass): # ... @SuperClass.foobar.overrides.expression def foobar(cls): return func.subfoobar(self._foobar)
New in version 1.2.
-
method
sqlalchemy.ext.hybrid.hybrid_property.
setter(fset)¶ Provide a modifying decorator that defines a setter method.
-
method
sqlalchemy.ext.hybrid.hybrid_property.
update_expression(meth)¶ Provide a modifying decorator that defines an UPDATE tuple producing method.
The method accepts a single value, which is the value to be rendered into the SET clause of an UPDATE statement. The method should then process this value into individual column expressions that fit into the ultimate SET clause, and return them as a sequence of 2-tuples. Each tuple contains a column expression as the key and a value to be rendered.
E.g.:
class Person(Base): # ... first_name = Column(String) last_name = Column(String) @hybrid_property def fullname(self): return first_name + " " + last_name @fullname.update_expression def fullname(cls, value): fname, lname = value.split(" ", 1) return [ (cls.first_name, fname), (cls.last_name, lname) ]
New in version 1.2.
-
method
- class sqlalchemy.ext.hybrid.Comparator(expression)¶
A helper class that allows easy construction of custom
PropComparator
classes for usage with hybrids.Class signature
class
sqlalchemy.ext.hybrid.Comparator
(sqlalchemy.orm.PropComparator
)
- sqlalchemy.ext.hybrid.HYBRID_METHOD = symbol('HYBRID_METHOD')¶
Symbol indicating an
InspectionAttr
that’s of typehybrid_method
.Is assigned to the
InspectionAttr.extension_type
attribute.See also
Mapper.all_orm_attributes
- sqlalchemy.ext.hybrid.HYBRID_PROPERTY = symbol('HYBRID_PROPERTY')¶
- Symbol indicating an
InspectionAttr
that’s of type
hybrid_method
.
Is assigned to the
InspectionAttr.extension_type
attribute.See also
Mapper.all_orm_attributes
- Symbol indicating an
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