Loading Columns

    Deferred column loading allows particular columns of a table be loaded only upon direct access, instead of when the entity is queried using . This feature is useful when one wants to avoid loading a large text or binary field into memory when it’s not needed. Individual columns can be lazy loaded by themselves or placed into groups that lazy-load together, using the function to mark them as “deferred”. In the example below, we define a mapping that will load each of .excerpt and .photo in separate, individual-row SELECT statements when each attribute is first referenced on the individual object instance:

    Classical mappings as always place the usage of deferred() in the properties dictionary against the table-bound :

    1. mapper_registry.map_imperatively(Book, book_table, properties={
    2.     'photo':deferred(book_table.c.photo)
    3. })

    Deferred columns can be associated with a “group” name, so that they load together when any of them are first accessed. The example below defines a mapping with a photos deferred group. When one .photo is accessed, all three photos will be loaded in one SELECT statement. The .excerpt will be loaded separately when it is accessed:

    1. class Book(Base):
    2.     __tablename__ = 'book'
    4.     book_id = Column(Integer, primary_key=True)
    5.     title = Column(String(200), nullable=False)
    6.     summary = Column(String(2000))
    7.     excerpt = deferred(Column(Text))
    8.     photo1 = deferred(Column(Binary), group='photos')
    9.     photo2 = deferred(Column(Binary), group='photos')
    10.     photo3 = deferred(Column(Binary), group='photos')

    Columns can be marked as “deferred” or reset to “undeferred” at query time using options which are passed to the Query.options() method; the most basic query options are and undefer():

    1. from sqlalchemy.orm import defer
    2. from sqlalchemy.orm import undefer
    4. query = session.query(Book)
    5. query = query.options(defer('summary'), undefer('excerpt'))
    6. query.all()

    Above, the “summary” column will not load until accessed, and the “excerpt” column will load immediately even if it was mapped as a “deferred” column.

    attributes which are marked with a “group” can be undeferred using undefer_group(), sending in the group name:

    1. from sqlalchemy.orm import undefer_group
    3. query = session.query(Book)
    4. query.options(undefer_group('photos')).all()

    To specify column deferral for a that loads multiple types of entities at once, the deferral options may be specified more explicitly using class-bound attributes, rather than string names:

    1. from sqlalchemy.orm import defer
    3. query = session.query(Book, Author).join(Book.author)
    4. query = query.options(defer(Author.bio))

    Column deferral options may also indicate that they take place along various relationship paths, which are themselves often eagerly loaded with loader options. All relationship-bound loader options support chaining onto additional loader options, which include loading for further levels of relationships, as well as onto column-oriented attributes at that path. Such as, to load Author instances, then joined-eager-load the Author.books collection for each author, then apply deferral options to column-oriented attributes onto each Book entity from that relationship, the loader option can be combined with the load_only() option (described later in this section) to defer all Book columns except those explicitly specified:

    1. from sqlalchemy.orm import joinedload
    3. query = session.query(Author)
    4. query = query.options(
    5.             joinedload(Author.books).load_only(Book.summary, Book.excerpt),
    6.         )

    Option structures as above can also be organized in more complex ways, such as hierarchically using the method, which allows multiple sub-options to be chained to a common parent option at once. Any mixture of string names and class-bound attribute objects may be used:

    1. from sqlalchemy.orm import defer
    2. from sqlalchemy.orm import joinedload
    3. from sqlalchemy.orm import load_only
    5. query = session.query(Author)
    6. query = query.options(
    7.             joinedload(Author.book).options(
    8.                 load_only("summary", "excerpt"),
    9.                 joinedload(Book.citations).options(
    10.                     joinedload(Citation.author),
    11.                     defer(Citation.fulltext)
    12.                 )
    13.         )

    New in version 1.3.6: Added Load.options() to allow easier construction of hierarchies of loader options.

    Another way to apply options to a path is to use the function. This function is used to indicate a particular path within a loader option structure without actually setting any options at that level, so that further sub-options may be applied. The defaultload() function can be used to create the same structure as we did above using as:

    1. query = session.query(Author)
    2. query = query.options(
    3.     joinedload(Author.book).load_only("summary", "excerpt"),
    4.     defaultload(Author.book).joinedload(Book.citations).joinedload(Citation.author),
    5.     defaultload(Author.book).defaultload(Book.citations).defer(Citation.fulltext)
    6. )

    See also

    Relationship Loading with Loader Options - targeted towards relationship loading

    The ORM loader option system supports the concept of “wildcard” loader options, in which a loader option can be passed an asterisk "*" to indicate that a particular option should apply to all applicable attributes of a mapped class. Such as, if we wanted to load the Book class but only the “summary” and “excerpt” columns, we could say:

    Above, the option is applied using a wildcard to all column attributes on the class. Then, the undefer() option is used against the “summary” and “excerpt” fields so that they are the only columns loaded up front. A query for the above entity will include only the “summary” and “excerpt” fields in the SELECT, along with the primary key columns which are always used by the ORM.

    A similar function is available with less verbosity by using the option. This is a so-called exclusionary option which will apply deferred behavior to all column attributes except those that are named:

    1. from sqlalchemy.orm import load_only
    3. session.query(Book).options(load_only("summary", "excerpt"))

    Wildcard and Exclusionary Options with Multiple-Entity Queries

    Wildcard options and exclusionary options such as may only be applied to a single entity at a time within a Query. To suit the less common case where a is returning multiple primary entities at once, a special calling style may be required in order to apply a wildcard or exclusionary option, which is to use the Load object to indicate the starting entity for a deferral option. Such as, if we were loading Book and Author at once, the will raise an informative error if we try to apply load_only() to both at once. Using looks like:

    1. from sqlalchemy.orm import Load
    3. query = session.query(Book, Author).join(Book.author)
    4. query = query.options(
    5.             Load(Book).load_only("summary", "excerpt")
    6.         )

    Above, Load is used in conjunction with the exclusionary option so that the deferral of all other columns only takes place for the Book class and not the Author class. Again, the Query object should raise an informative error message when the above calling style is actually required that describes those cases where explicit use of is needed.

    New in version 1.4.

    The deferred() loader option and the corresponding loader strategy also support the concept of “raiseload”, which is a loader strategy that will raise if the attribute is accessed such that it would need to emit a SQL query in order to be loaded. This behavior is the column-based equivalent of the raiseload() feature for relationship loading, discussed at . Using the defer.raiseload parameter on the option, an exception is raised if the attribute is accessed:

    1. book = session.query(Book).options(defer(Book.summary, raiseload=True)).first()
    3. # would raise an exception
    4. book.summary

    Deferred “raiseload” can be configured at the mapper level via deferred.raiseload on , so that an explicit undefer() is required in order for the attribute to be usable:

    1. class Book(Base):
    2.     __tablename__ = 'book'
    4.     book_id = Column(Integer, primary_key=True)
    5.     title = Column(String(200), nullable=False)
    6.     summary = deferred(Column(String(2000)), raiseload=True)
    7.     excerpt = deferred(Column(Text), raiseload=True)
    9. book_w_excerpt = session.query(Book).options(undefer(Book.excerpt)).first()

    function sqlalchemy.orm.``defer(key, \addl_attrs, **kw*)

    Indicate that the given column-oriented attribute should be deferred, e.g. not loaded until accessed.

    This function is part of the interface and supports both method-chained and standalone operation.

    e.g.:

    1. from sqlalchemy.orm import defer
    3. session.query(MyClass).options(
    4.                     defer("attribute_one"),
    5.                     defer("attribute_two"))
    7. session.query(MyClass).options(
    8.                     defer(MyClass.attribute_one),
    9.                     defer(MyClass.attribute_two))

    To specify a deferred load of an attribute on a related class, the path can be specified one token at a time, specifying the loading style for each link along the chain. To leave the loading style for a link unchanged, use defaultload():

    1. session.query(MyClass).options(defaultload("someattr").defer("some_column"))

    A object that is present on a certain path can have Load.defer() called multiple times, each will operate on the same parent entity:

    1. session.query(MyClass).options(
    2.                 defaultload("someattr").
    3.                     defer("some_column").
    4.                     defer("some_other_column").
    5.                     defer("another_column")
    6.     )

      • key – Attribute to be deferred.

      • raiseload –

        raise if the column value is to be loaded from emitting SQL. Used to prevent unwanted SQL from being emitted.

        New in version 1.4.

        See also

        Raiseload for Deferred Columns

      • *addl_attrs –

        This option supports the old 0.8 style of specifying a path as a series of attributes, which is now superseded by the method-chained style.

    See also

    undefer()

    function sqlalchemy.orm.``deferred(\columns, **kw*)

    Indicate a column-based mapped attribute that by default will not load unless accessed.

    • Parameters

      • *columns – columns to be mapped. This is typically a single object, however a collection is supported in order to support multiple columns mapped under the same attribute.

      • raiseload –

        boolean, if True, indicates an exception should be raised if the load operation is to take place.

        New in version 1.4.

        See also

        Raiseload for Deferred Columns

      • **kw – additional keyword arguments passed to .

    See also

    Deferred Column Loading

    function sqlalchemy.orm.``query_expression(default_expr=<sqlalchemy.sql.elements.Null object>)

    Indicate an attribute that populates from a query-time SQL expression.

    New in version 1.2.

    See also

    function sqlalchemy.orm.``load_only(\attrs*)

    Indicate that for a particular entity, only the given list of column-based attribute names should be loaded; all others will be deferred.

    This function is part of the Load interface and supports both method-chained and standalone operation.

    Example - given a class User, load only the name and fullname attributes:

    Example - given a relationship User.addresses -> Address, specify subquery loading for the User.addresses collection, but on each Address object load only the email_address attribute:

    1. session.query(User).options(
    2.         subqueryload("addresses").load_only("email_address")
    3. )

    For a that has multiple entities, the lead entity can be specifically referred to using the Load constructor:

    1.    session.query(User, Address).join(User.addresses).options(
    2.                Load(User).load_only("name", "fullname"),
    3.                Load(Address).load_only("email_address")
    4.            )
    6. .. note:: This method will still load a :class:`_schema.Column` even
    7.    if the column property is defined with ``deferred=True``
    8.    for the :func:`.column_property` function.

    New in version 0.9.0.

    function sqlalchemy.orm.``undefer(key, \addl_attrs*)

    The column being undeferred is typically set up on the mapping as a attribute.

    This function is part of the Load interface and supports both method-chained and standalone operation.

    Examples:

    1. # undefer two columns
    2. session.query(MyClass).options(undefer("col1"), undefer("col2"))
    4. # undefer all columns specific to a single class using Load + *
    5. session.query(MyClass, MyOtherClass).options(
    6.     Load(MyClass).undefer("*"))
    8. # undefer a column on a related object
    9. session.query(MyClass).options(
    10.     defaultload(MyClass.items).undefer('text'))

    • Parameters

      • key – Attribute to be undeferred.

      • *addl_attrs –

        This option supports the old 0.8 style of specifying a path as a series of attributes, which is now superseded by the method-chained style.

        Deprecated since version 0.9: The *addl_attrs on is deprecated and will be removed in a future release. Please use method chaining in conjunction with defaultload() to indicate a path.

    See also

    Deferred Column Loading

    undefer_group()

    function sqlalchemy.orm.``undefer_group(name)

    Indicate that columns within the given deferred group name should be undeferred.

    The columns being undeferred are set up on the mapping as attributes and include a “group” name.

    E.g:

    1. session.query(MyClass).options(undefer_group("large_attrs"))

    To undefer a group of attributes on a related entity, the path can be spelled out using relationship loader options, such as defaultload():

    1. session.query(MyClass).options(
    2.     defaultload("someattr").undefer_group("large_attrs"))

    Changed in version 0.9.0: is now specific to a particular entity load path.

    See also

    Deferred Column Loading

    undefer()

    function sqlalchemy.orm.``with_expression(key, expression)

    Apply an ad-hoc SQL expression to a “deferred expression” attribute.

    This option is used in conjunction with the mapper-level construct that indicates an attribute which should be the target of an ad-hoc SQL expression.

    E.g.:

    1. sess.query(SomeClass).options(
    2.     with_expression(SomeClass.x_y_expr, SomeClass.x + SomeClass.y)
    3. )

    New in version 1.2.

    • Parameters

      • key – Attribute to be undeferred.

      • expr – SQL expression to be applied to the attribute.

    Note

    the target attribute is populated only if the target object is not currently loaded in the current Session unless the method is used. Please refer to Query-time SQL expressions as mapped attributes for complete usage details.

    See also

    Column Bundles

    The may be used to query for groups of columns under one namespace.

    The bundle allows columns to be grouped together:

    1. from sqlalchemy.orm import Bundle
    3. bn = Bundle('mybundle', MyClass.data1, MyClass.data2)
    4. for row in session.query(bn).filter(bn.c.data1 == 'd1'):
    5.     print(row.mybundle.data1, row.mybundle.data2)

    The bundle can be subclassed to provide custom behaviors when results are fetched. The method Bundle.create_row_processor() is given the statement object and a set of “row processor” functions at query execution time; these processor functions when given a result row will return the individual attribute value, which can then be adapted into any kind of return data structure. Below illustrates replacing the usual return structure with a straight Python dictionary:

    1. from sqlalchemy.orm import Bundle
    3. class DictBundle(Bundle):
    4.     def create_row_processor(self, query, procs, labels):
    5.         """Override create_row_processor to return values as dictionaries"""
    6.         def proc(row):
    7.             return dict(
    8.                         zip(labels, (proc(row) for proc in procs))
    9.                     )
    10.         return proc

    Note

    The Bundle construct only applies to column expressions. It does not apply to ORM attributes mapped using .

    Changed in version 1.0: The proc() callable passed to the create_row_processor() method of custom Bundle classes now accepts only a single “row” argument.

    A result from the above bundle will return dictionary values: