`?i-SrSSKJr SSKJr SSKJr SSKJr SSKJr SSKJr SSKJ r SS KJ r S S K J r S S K Jr S S KJr \(a S SKJr S SKJr S/r\ "S5r"SS\\5rg)a7Define attributes on ORM-mapped classes that have "index" attributes for columns with :class:`_types.Indexable` types. "index" means the attribute is associated with an element of an :class:`_types.Indexable` column with the predefined index to access it. The :class:`_types.Indexable` types include types such as :class:`_types.ARRAY`, :class:`_types.JSON` and :class:`_postgresql.HSTORE`. The :mod:`~sqlalchemy.ext.indexable` extension provides :class:`_schema.Column`-like interface for any element of an :class:`_types.Indexable` typed column. In simple cases, it can be treated as a :class:`_schema.Column` - mapped attribute. Synopsis ======== Given ``Person`` as a model with a primary key and JSON data field. While this field may have any number of elements encoded within it, we would like to refer to the element called ``name`` individually as a dedicated attribute which behaves like a standalone column:: from sqlalchemy import Column, JSON, Integer from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.ext.indexable import index_property Base = declarative_base() class Person(Base): __tablename__ = "person" id = Column(Integer, primary_key=True) data = Column(JSON) name = index_property("data", "name") Above, the ``name`` attribute now behaves like a mapped column. We can compose a new ``Person`` and set the value of ``name``:: >>> person = Person(name="Alchemist") The value is now accessible:: >>> person.name 'Alchemist' Behind the scenes, the JSON field was initialized to a new blank dictionary and the field was set:: >>> person.data {'name': 'Alchemist'} The field is mutable in place:: >>> person.name = "Renamed" >>> person.name 'Renamed' >>> person.data {'name': 'Renamed'} When using :class:`.index_property`, the change that we make to the indexable structure is also automatically tracked as history; we no longer need to use :class:`~.mutable.MutableDict` in order to track this change for the unit of work. Deletions work normally as well:: >>> del person.name >>> person.data {} Above, deletion of ``person.name`` deletes the value from the dictionary, but not the dictionary itself. A missing key will produce ``AttributeError``:: >>> person = Person() >>> person.name AttributeError: 'name' Unless you set a default value:: >>> class Person(Base): ... __tablename__ = "person" ... ... id = Column(Integer, primary_key=True) ... data = Column(JSON) ... ... name = index_property("data", "name", default=None) # See default >>> person = Person() >>> print(person.name) None The attributes are also accessible at the class level. Below, we illustrate ``Person.name`` used to generate an indexed SQL criteria:: >>> from sqlalchemy.orm import Session >>> session = Session() >>> query = session.query(Person).filter(Person.name == "Alchemist") The above query is equivalent to:: >>> query = session.query(Person).filter(Person.data["name"] == "Alchemist") Multiple :class:`.index_property` objects can be chained to produce multiple levels of indexing:: from sqlalchemy import Column, JSON, Integer from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.ext.indexable import index_property Base = declarative_base() class Person(Base): __tablename__ = "person" id = Column(Integer, primary_key=True) data = Column(JSON) birthday = index_property("data", "birthday") year = index_property("birthday", "year") month = index_property("birthday", "month") day = index_property("birthday", "day") Above, a query such as:: q = session.query(Person).filter(Person.year == "1980") On a PostgreSQL backend, the above query will render as: .. sourcecode:: sql SELECT person.id, person.data FROM person WHERE person.data -> %(data_1)s -> %(param_1)s = %(param_2)s Default Values ============== :class:`.index_property` includes special behaviors for when the indexed data structure does not exist, and a set operation is called: * For an :class:`.index_property` that is given an integer index value, the default data structure will be a Python list of ``None`` values, at least as long as the index value; the value is then set at its place in the list. This means for an index value of zero, the list will be initialized to ``[None]`` before setting the given value, and for an index value of five, the list will be initialized to ``[None, None, None, None, None]`` before setting the fifth element to the given value. Note that an existing list is **not** extended in place to receive a value. * for an :class:`.index_property` that is given any other kind of index value (e.g. strings usually), a Python dictionary is used as the default data structure. * The default data structure can be set to any Python callable using the :paramref:`.index_property.datatype` parameter, overriding the previous rules. Subclassing =========== :class:`.index_property` can be subclassed, in particular for the common use case of providing coercion of values or SQL expressions as they are accessed. Below is a common recipe for use with a PostgreSQL JSON type, where we want to also include automatic casting plus ``astext()``:: class pg_json_property(index_property): def __init__(self, attr_name, index, cast_type): super(pg_json_property, self).__init__(attr_name, index) self.cast_type = cast_type def expr(self, model): expr = super(pg_json_property, self).expr(model) return expr.astext.cast(self.cast_type) The above subclass can be used with the PostgreSQL-specific version of :class:`_postgresql.JSON`:: from sqlalchemy import Column, Integer from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.dialects.postgresql import JSON Base = declarative_base() class Person(Base): __tablename__ = "person" id = Column(Integer, primary_key=True) data = Column(JSON) age = pg_json_property("data", "age", Integer) The ``age`` attribute at the instance level works as before; however when rendering SQL, PostgreSQL's ``->>`` operator will be used for indexed access, instead of the usual index operator of ``->``:: >>> query = session.query(Person).filter(Person.age < 20) The above query will render: .. sourcecode:: sql SELECT person.id, person.data FROM person WHERE CAST(person.data ->> %(data_1)s AS INTEGER) < %(param_1)s ) annotations)Any)Callable)cast)Optional) TYPE_CHECKING)TypeVar)Union)inspect)hybrid_property) flag_modified)SQLColumnExpression)_HasClauseElementindex_property_Tc^\rSrSrSr\"\\"55r\SSS4S U4Sjjjr S SSjjr SSjr SSjr SS jr SS jrS rU=r$)rzA property generator. The generated property describes an object attribute that corresponds to an :class:`_types.Indexable` column. .. seealso:: :mod:`sqlalchemy.ext.indexable` NTc>^U(a;[TU]URURURUR 5 O&[TU]URSSUR 5 XlTUlX0l[T[5nU=(a UnUbX@l OU(a U4SjUl O [Ul X`l g)aCreate a new :class:`.index_property`. :param attr_name: An attribute name of an `Indexable` typed column, or other attribute that returns an indexable structure. :param index: The index to be used for getting and setting this value. This should be the Python-side index value for integers. :param default: A value which will be returned instead of `AttributeError` when there is not a value at given index. :param datatype: default datatype to use when the field is empty. By default, this is derived from the type of index used; a Python list for an integer index, or a Python dictionary for any other style of index. For a list, the list will be initialized to a list of None values that is at least ``index`` elements long. :param mutable: if False, writes and deletes to the attribute will be disallowed. :param onebased: assume the SQL representation of this value is one-based; that is, the first index in SQL is 1, not zero. 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