Hybrid Attributes — SQLAlchemy 0.7.1 Documentation

# SQLAlchemy 0.7.1 Documentation

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Version: 0.7.1 Last Updated: 06/05/2011 13:13:59

# 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 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"

## 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. 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

## 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.

## API Reference

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.

  `__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)

  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.