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ZG�dd��dej �Z G�d d ��d ej �ZG�dd��dej�Z G�d d��de �ZdS�)aEm��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 :mod:`~sqlalchemy.ext.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 :class:`.hybrid_method` or :class:`.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 :class:`.hybrid_property` descriptor evaluates the function body given the ``Interval`` class as the argument, which when evaluated with SQLAlchemy expression mechanics returns a new SQL expression:: >>> 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 :meth:`~.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 :class:`.hybrid_method`. This decorator applies the same idea to methods that :class:`.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" .. _hybrid_distinct_expression: 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 :mod:`~sqlalchemy.ext.hybrid` decorators define the :meth:`.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 :data:`.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 :meth:`.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 .. _hybrid_bulk_update: Allowing Bulk ORM Update ------------------------ A hybrid can define a custom "UPDATE" handler for when using the :meth:`.Query.update` method, allowing the hybrid to be used in the SET clause of the update. Normally, when using a hybrid with :meth:`.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 :meth:`.Query.update` which can affect this, using the :meth:`.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. .. versionadded:: 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 :ref:`mapper_column_property_sql_expressions`, 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 .. _hybrid_custom_comparators: 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 :meth:`.hybrid_property.comparator` decorator introduced in this section **replaces** the use of the :meth:`.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 :class:`.ColumnOperators` interface. A "coercion" operation like lowercasing can be applied to all comparison operations (i.e. ``eq``, ``lt``, ``gt``, etc.) using :meth:`.Operators.operate`:: class CaseInsensitiveComparator(Comparator): def operate(self, op, other): return op(func.lower(self.__clause_element__()), func.lower(other)) .. _hybrid_reuse_subclass: Reusing Hybrid Properties across Subclasses ------------------------------------------- A hybrid can be referred to from a superclass, to allow modifying methods like :meth:`.hybrid_property.getter`, :meth:`.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 :meth:`.hybrid_property.expression` and :meth:`.hybrid_property.comparator` alone as the first reference to the superclass, these names conflict with the same-named accessors on the class- level :class:`.QueryableAttribute` object returned at the class level. To override these methods when referring directly to the parent class descriptor, add the special qualifier :attr:`.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) .. versionadded:: 1.2 Added :meth:`.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): 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. .. seealso:: `Hybrids and Value Agnostic Types <http://techspot.zzzeek.org/2011/10/21/hybrids-and-value-agnostic-types/>`_ - on the techspot.zzzeek.org blog `Value Agnostic Types, Part II <http://techspot.zzzeek.org/2011/10/29/value-agnostic-types-part-ii/>`_ - on the techspot.zzzeek.org blog .. _hybrid_transformers: Building Transformers ---------------------- A *transformer* is an object which can receive a :class:`.Query` object and return a new one. The :class:`.Query` object includes a method :meth:`.with_transformation` that returns a new :class:`.Query` transformed by the given function. We can combine this with the :class:`.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 :class:`.Query` where we :meth:`~.Query.join` twice along ``Node.parent`` to get to the ``grandparent``. We can instead return a transforming callable that we'll combine with the :class:`.Comparator` class to receive any :class:`.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 :meth:`.Operators.operate` method at the base of the :class:`.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 :attr:`.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 :class:`.Query` first to join to ``Node.parent``, then to compare ``parent_alias`` using :attr:`.Operators.eq` against the left and right sides, passing into :class:`.Query.filter`: .. sourcecode:: pycon+sql >>> 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 {stop} 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 :class:`.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) .. sourcecode:: pycon+sql {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 {stop} The "transformer" pattern is an experimental pattern that starts to make usage of some functional programming paradigms. 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