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

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

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. Two variants of we’ll illustrate are the “join-dependent” hybrid, and the “correlated subquery” hybrid.

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

>>> 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

>>> 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

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')

>>> 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.

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):
        return op(func.lower(self.__clause_element__()), func.lower(other))

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):
        if not isinstance(other, CaseInsensitiveWord):
            other = CaseInsensitiveWord(other)
        return op(self.word, other.word)

    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.

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 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):
        def transform(q):
            cls = self.__clause_element__()
            parent_alias = aliased(cls)
            return q.join(parent_alias, cls.parent).\
                        filter(op(parent_alias.parent, other))
        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):
        return op(self.parent_alias.parent, other)

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¶

class sqlalchemy.ext.hybrid.hybrid_method(func, expr=None)¶

Bases: sqlalchemy.orm.base._InspectionAttr

A decorator which allows definition of a Python object method with both instance-level and class-level behavior.

__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)

expression(expr)¶

Provide a modifying decorator that defines a SQL-expression producing method.

class sqlalchemy.ext.hybrid.hybrid_property(fget, fset=None, fdel=None, expr=None)¶

Bases: sqlalchemy.orm.base._InspectionAttr

A decorator which allows definition of a Python descriptor with both instance-level and class-level behavior.

__init__(fget, fset=None, fdel=None, 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

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.

deleter(fdel)¶

Provide a modifying decorator that defines a value-deletion method.

expression(expr)¶

Provide a modifying decorator that defines a SQL-expression producing method.

setter(fset)¶

Provide a modifying decorator that defines a value-setter method.

class sqlalchemy.ext.hybrid.Comparator(expression)¶

Bases: sqlalchemy.orm.interfaces.PropComparator

A helper class that allows easy construction of custom PropComparator classes for usage with hybrids.

sqlalchemy.ext.hybrid.HYBRID_METHOD = symbol('HYBRID_METHOD')¶

sqlalchemy.ext.hybrid.HYBRID_PROPERTY = symbol('HYBRID_PROPERTY')¶