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 class-level expression generator.
Consider a table interval as below:
from sqlalchemy import MetaData, Table, Column, Integer
metadata = MetaData()
interval_table = Table('interval', metadata,
Column('id', Integer, primary_key=True),
Column('start', Integer, nullable=False),
Column('end', Integer, nullable=False)
)
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.ext.hybrid import hybrid_property, hybrid_method
from sqlalchemy.orm import mapper, Session, aliased
class Interval(object):
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)
mapper(Interval, interval_table)
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
At the class level, the usual descriptor behavior of returning the descriptor itself is modified by hybrid_property, to instead evaluate the function body given the Interval class as the argument:
>>> print Interval.length
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 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 contains() and intersects() methods are decorated with hybrid_method. This decorator applies the same idea to methods which accept zero or more arguments. The above 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"
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
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
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. 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 = balance
@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, we can’t travel along relationships to column attributes directly since SQLAlchemy is explicit about joins. So here, 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
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.
The example class below allows case-insensitive comparisons on the attribute named word_insensitive:
from sqlalchemy.ext.hybrid import Comparator
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)
A decorator which allows definition of a Python object method with both instance-level and class-level behavior.
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)
Provide a modifying decorator that defines a SQL-expression producing method.
A decorator which allows definition of a Python descriptor with both instance-level and class-level behavior.
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
Provide a modifying decorator that defines a custom comparator producing method.
The return value of the decorated method should be an instance of Comparator.
Provide a modifying decorator that defines a value-deletion method.
Provide a modifying decorator that defines a SQL-expression producing method.
Provide a modifying decorator that defines a value-setter method.
Bases: sqlalchemy.orm.interfaces.PropComparator
A helper class that allows easy construction of custom PropComparator classes for usage with hybrids.