root / elixir / trunk / elixir / properties.py

'''
This module provides support for defining properties on your entities. It both
provides, the `Property` class which acts as a building block for common
properties such as fields and relationships (for those, please consult the
corresponding modules), but also provides some more specialized properties,
such as `ColumnProperty` and `Synonym`. It also provides the GenericProperty
class which allows you to wrap any SQLAlchemy property, and its DSL-syntax
equivalent: has_property_.

`has_property`
--------------
The ``has_property`` statement allows you to define properties which rely on
their entity's table (and columns) being defined before they can be declared
themselves. The `has_property` statement takes two arguments: first the name of
the property to be defined and second a function (often given as an anonymous
lambda) taking one argument and returning the desired SQLAlchemy property. That
function will be called whenever the entity table is completely defined, and
will be given the .c attribute of the entity as argument (as a way to access
the entity columns).

Here is a quick example of how to use ``has_property``.

.. sourcecode:: python

    class OrderLine(Entity):
        has_field('quantity', Float)
        has_field('unit_price', Float)
        has_property('price',
                     lambda c: column_property(
                         (c.quantity * c.unit_price).label('price')))
'''

from elixir.statements import PropertyStatement
from sqlalchemy.orm import column_property, synonym

__doc_all__ = ['EntityBuilder', 'Property', 'GenericProperty',
               'ColumnProperty']

class EntityBuilder(object):
    '''
    Abstract base class for all entity builders. An Entity builder is a class
    of objects which can be added to an Entity (usually by using special
    properties or statements) to "build" that entity. Building an entity,
    meaning to add columns to its "main" table, create other tables, add
    properties to its mapper, ... To do so an EntityBuilder must override the
    corresponding method(s). This is to ensure the different operations happen
    in the correct order (for example, that the table is fully created before
    the mapper that use it is defined).
    '''
    def create_pk_cols(self):
        pass

    def create_non_pk_cols(self):
        pass

    def before_table(self):
        pass

    def create_tables(self):
        '''
        Subclasses may override this method to create tables.
        '''

    def after_table(self):
        pass

    def create_properties(self):
        '''
        Subclasses may override this method to add properties to the involved
        entity.
        '''

    def before_mapper(self):
        pass

    def after_mapper(self):
        pass

    def finalize(self):
        pass

    # helper methods
    def add_table_column(self, column):
        self.entity._descriptor.add_column(column)

    def add_mapper_property(self, name, prop):
        self.entity._descriptor.add_property(name, prop)

    def add_mapper_extension(self, ext):
        self.entity._descriptor.add_mapper_extension(ext)


class CounterMeta(type):
    '''
    A simple meta class which adds a ``_counter`` attribute to the instances of
    the classes it is used on. This counter is simply incremented for each new
    instance.
    '''
    counter = 0

    def __call__(self, *args, **kwargs):
        instance = type.__call__(self, *args, **kwargs)
        instance._counter = CounterMeta.counter
        CounterMeta.counter += 1
        return instance


class Property(EntityBuilder):
    '''
    Abstract base class for all properties of an Entity.
    '''
    __metaclass__ = CounterMeta

    def __init__(self, *args, **kwargs):
        self.entity = None
        self.name = None

    def attach(self, entity, name):
        """Attach this property to its entity, using 'name' as name.

        Properties will be attached in the order they were declared.
        """
        self.entity = entity
        self.name = name

        # register this property as a builder
        entity._descriptor.builders.append(self)

    def __repr__(self):
        return "Property(%s, %s)" % (self.name, self.entity)


class GenericProperty(Property):
    '''
    Generic catch-all class to wrap an SQLAlchemy property.

    .. sourcecode:: python

        class OrderLine(Entity):
            quantity = Field(Float)
            unit_price = Field(Numeric)
            price = GenericProperty(lambda c: column_property(
                             (c.quantity * c.unit_price).label('price')))
    '''

    def __init__(self, prop, *args, **kwargs):
        super(GenericProperty, self).__init__(*args, **kwargs)
        self.prop = prop
        #XXX: move this to Property?
        self.args = args
        self.kwargs = kwargs

    def create_properties(self):
        if hasattr(self.prop, '__call__'):
            prop_value = self.prop(self.entity.table.c)
        else:
            prop_value = self.prop
        prop_value = self.evaluate_property(prop_value)
        self.add_mapper_property(self.name, prop_value)

    def evaluate_property(self, prop):
        if self.args or self.kwargs:
            raise Exception('superfluous arguments passed to GenericProperty')
        return prop


class ColumnProperty(GenericProperty):
    '''
    A specialized form of the GenericProperty to generate SQLAlchemy
    ``column_property``'s.

    It takes a function (often given as an anonymous lambda) as its first
    argument. Other arguments and keyword arguments are forwarded to the
    column_property construct. That first-argument function must accept exactly
    one argument and must return the desired (scalar-returning) SQLAlchemy
    ClauseElement.

    The function will be called whenever the entity table is completely
    defined, and will be given
    the .c attribute of the table of the entity as argument (as a way to
    access the entity columns). The ColumnProperty will first wrap your
    ClauseElement in an
    "empty" label (ie it will be labelled automatically during queries),
    then wrap that in a column_property.

    .. sourcecode:: python

        class OrderLine(Entity):
            quantity = Field(Float)
            unit_price = Field(Numeric)
            price = ColumnProperty(lambda c: c.quantity * c.unit_price,
                                   deferred=True)

    Please look at the `corresponding SQLAlchemy
    documentation <http://www.sqlalchemy.org/docs/05/mappers.html
    #sql-expressions-as-mapped-attributes>`_ for details.
    '''

    def evaluate_property(self, prop):
        return column_property(prop.label(None), *self.args, **self.kwargs)


class Synonym(GenericProperty):
    '''
    This class represents a synonym property of another property (column, ...)
    of an entity.  As opposed to the `synonym` kwarg to the Field class (which
    share the same goal), this class can be used to define a synonym of a
    property defined in a parent class (of the current class). On the other
    hand, it cannot define a synonym for the purpose of using a standard python
    property in queries. See the Field class for details on that usage.

    .. sourcecode:: python

    class Person(Entity):
        name = Field(String(30))
        primary_email = Field(String(100))
        email_address = Synonym('primary_email')

    class User(Person):
        user_name = Synonym('name')
        password = Field(String(20))
    '''

    def evaluate_property(self, prop):
        return synonym(prop, *self.args, **self.kwargs)

#class Composite(GenericProperty):
#    def __init__(self, prop):
#        super(GenericProperty, self).__init__()
#        self.prop = prop

#    def evaluate_property(self, prop):
#        return composite(prop.label(self.name))

#start = Composite(Point, lambda c: (c.x1, c.y1))

#mapper(Vertex, vertices, properties={
#    'start':composite(Point, vertices.c.x1, vertices.c.y1),
#    'end':composite(Point, vertices.c.x2, vertices.c.y2)
#})


has_property = PropertyStatement(GenericProperty)