Class Mapping — SQLAlchemy 0.5.0 Documentation

# SQLAlchemy 0.5.0 Documentation

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Version: 0.5.0 Last Updated: 01/06/2009 13:41:02

# Class Mapping

## Defining Mappings

Python classes are mapped to the database using the `mapper()` function.

`sqlalchemy.orm.` `mapper`(class_, local_table=None, *args, **params)

  Return a new `Mapper` object.

  class_
    The class to be mapped.
  local_table
    The table to which the class is mapped, or None if this mapper inherits from another mapper using concrete table inheritance.
  always_refresh
    If True, all query operations for this mapped class will overwrite all data within object instances that already exist within the session, erasing any in-memory changes with whatever information was loaded from the database. Usage of this flag is highly discouraged; as an alternative, see the method populate_existing() on `Query`.
  allow_column_override
    If True, allows the usage of a `relation()` which has the same name as a column in the mapped table. The table column will no longer be mapped.
  allow_null_pks
    Indicates that composite primary keys where one or more (but not all) columns contain NULL is a valid primary key. Primary keys which contain NULL values usually indicate that a result row does not contain an entity and should be skipped.
  batch
    Indicates that save operations of multiple entities can be batched together for efficiency. setting to False indicates that an instance will be fully saved before saving the next instance, which includes inserting/updating all table rows corresponding to the entity as well as calling all `MapperExtension` methods corresponding to the save operation.
  column_prefix
    A string which will be prepended to the key name of all Columns when creating column-based properties from the given Table. Does not affect explicitly specified column-based properties
  concrete
    If True, indicates this mapper should use concrete table inheritance with its parent mapper.
  extension
    A `MapperExtension` instance or list of `MapperExtension` instances which will be applied to all operations by this `Mapper`.
  inherits
    Another `Mapper` for which this `Mapper` will have an inheritance relationship with.
  inherit_condition
    For joined table inheritance, a SQL expression (constructed `ClauseElement`) which will define how the two tables are joined; defaults to a natural join between the two tables.
  inherit_foreign_keys
    when inherit_condition is used and the condition contains no ForeignKey columns, specify the “foreign” columns of the join condition in this list. else leave as None.
  order_by
    A single `Column` or list of `Columns` for which selection operations should use as the default ordering for entities. Defaults to the OID/ROWID of the table if any, or the first primary key column of the table.
  non_primary
    Construct a `Mapper` that will define only the selection of instances, not their persistence. Any number of non_primary mappers may be created for a particular class.
  polymorphic_on
    Used with mappers in an inheritance relationship, a `Column` which will identify the class/mapper combination to be used with a particular row. Requires the `polymorphic_identity` value to be set for all mappers in the inheritance hierarchy. The column specified by `polymorphic_on` is usually a column that resides directly within the base mapper’s mapped table; alternatively, it may be a column that is only present within the <selectable> portion of the `with_polymorphic` argument.
  _polymorphic_map
    Used internally to propagate the full map of polymorphic identifiers to surrogate mappers.
  polymorphic_identity
    A value which will be stored in the Column denoted by polymorphic_on, corresponding to the class identity of this mapper.
  polymorphic_fetch
    Deprecated. Unloaded columns load as deferred in all cases; loading can be controlled using the “with_polymorphic” option.
  properties
    A dictionary mapping the string names of object attributes to `MapperProperty` instances, which define the persistence behavior of that attribute. Note that the columns in the mapped table are automatically converted into `ColumnProperty` instances based on the key property of each `Column` (although they can be overridden using this dictionary).
  include_properties
    An inclusive list of properties to map. Columns present in the mapped table but not present in this list will not be automatically converted into properties.
  exclude_properties
    A list of properties not to map. Columns present in the mapped table and present in this list will not be automatically converted into properties. Note that neither this option nor include_properties will allow an end-run around Python inheritance. If mapped class `B` inherits from mapped class `A`, no combination of includes or excludes will allow `B` to have fewer properties than its superclass, `A`.
  primary_key
    A list of `Column` objects which define the primary key to be used against this mapper’s selectable unit. This is normally simply the primary key of the local_table, but can be overridden here.
  with_polymorphic
    A tuple in the form `(<classes>, <selectable>)` indicating the default style of “polymorphic” loading, that is, which tables are queried at once. <classes> is any single or list of mappers and/or classes indicating the inherited classes that should be loaded at once. The special value `'*'` may be used to indicate all descending classes should be loaded immediately. The second tuple argument <selectable> indicates a selectable that will be used to query for multiple classes. Normally, it is left as None, in which case this mapper will form an outer join from the base mapper’s table to that of all desired sub-mappers. When specified, it provides the selectable to be used for polymorphic loading. When with_polymorphic includes mappers which load from a “concrete” inheriting table, the <selectable> argument is required, since it usually requires more complex UNION queries.
  select_table
    Deprecated. Synonymous with `with_polymorphic=('*', <selectable>)`.
  version_id_col
    A `Column` which must have an integer type that will be used to keep a running version id of mapped entities in the database. this is used during save operations to ensure that no other thread or process has updated the instance during the lifetime of the entity, else a `ConcurrentModificationError` exception is thrown.

## Mapper Properties

A basic mapping of a class will simply make the columns of the database table or selectable available as attributes on the class. Mapper properties allow you to customize and add additional properties to your classes, for example making the results one-to-many join available as a Python list of `related` objects.

Mapper properties are most commonly included in the `mapper()` call:

    mapper(Parent, properties={
       'children': relation(Children)
    }

`sqlalchemy.orm.` `backref`(name, **kwargs)

  Create a BackRef object with explicit arguments, which are the same arguments one can send to `relation()`.

  Used with the backref keyword argument to `relation()` in place of a string argument.

`sqlalchemy.orm.` `column_property`(*args, **kwargs)

  Provide a column-level property for use with a Mapper.

  Column-based properties can normally be applied to the mapper’s `properties` dictionary using the `schema.Column` element directly. Use this function when the given column is not directly present within the mapper’s selectable; examples include SQL expressions, functions, and scalar SELECT queries.

  Columns that aren’t present in the mapper’s selectable won’t be persisted by the mapper and are effectively “read-only” attributes.

  > *cols
  >   list of Column objects to be mapped.
  > comparator_factory
  >   a class which extends `sqlalchemy.orm.properties.ColumnProperty.Comparator` which provides custom SQL clause generation for comparison operations.
  > group
  >   a group name for this property when marked as deferred.
  > deferred
  >   when True, the column property is “deferred”, meaning that it does not load immediately, and is instead loaded when the attribute is first accessed on an instance. See also `deferred()`.
  > extension
  >   an `AttributeExtension` instance, or list of extensions, which will be prepended to the list of attribute listeners for the resulting descriptor placed on the class. These listeners will receive append and set events before the operation proceeds, and may be used to halt (via exception throw) or change the value used in the operation.

`sqlalchemy.orm.` `comparable_property`(comparator_factory, descriptor=None)

  Provide query semantics for an unmanaged attribute.

  Allows a regular Python @property (descriptor) to be used in Queries and SQL constructs like a managed attribute. comparable_property wraps a descriptor with a proxy that directs operator overrides such as == (__eq__) to the supplied comparator but proxies everything else through to the original descriptor:

      class MyClass(object):
          @property
          def myprop(self):
              return 'foo'

      class MyComparator(sqlalchemy.orm.interfaces.PropComparator):
          def __eq__(self, other):
              ....

      mapper(MyClass, mytable, properties=dict(
               'myprop': comparable_property(MyComparator)))

  Used with the `properties` dictionary sent to `mapper()`.

  comparator_factory
    A PropComparator subclass or factory that defines operator behavior for this property.
  descriptor

    Optional when used in a `properties={}` declaration. The Python descriptor or property to layer comparison behavior on top of.

    The like-named descriptor will be automatically retreived from the mapped class if left blank in a `properties` declaration.

`sqlalchemy.orm.` `composite`(class_, *cols, **kwargs)

  Return a composite column-based property for use with a Mapper.

  This is very much like a column-based property except the given class is used to represent “composite” values composed of one or more columns.

  The class must implement a constructor with positional arguments matching the order of columns supplied here, as well as a __composite_values__() method which returns values in the same order.

  A simple example is representing separate two columns in a table as a single, first-class “Point” object:

      class Point(object):
          def __init__(self, x, y):
              self.x = x
              self.y = y
          def __composite_values__(self):
              return self.x, self.y
          def __eq__(self, other):
              return other is not None and self.x == other.x and self.y == other.y

      # and then in the mapping:
      ... composite(Point, mytable.c.x, mytable.c.y) ...

  The composite object may have its attributes populated based on the names of the mapped columns. To override the way internal state is set, additionally implement `__set_composite_values__`:

      class Point(object):
          def __init__(self, x, y):
              self.some_x = x
              self.some_y = y
          def __composite_values__(self):
              return self.some_x, self.some_y
          def __set_composite_values__(self, x, y):
              self.some_x = x
              self.some_y = y
          def __eq__(self, other):
              return other is not None and self.some_x == other.x and self.some_y == other.y

  Arguments are:

  class_
    The “composite type” class.
  *cols
    List of Column objects to be mapped.
  group
    A group name for this property when marked as deferred.
  deferred
    When True, the column property is “deferred”, meaning that it does not load immediately, and is instead loaded when the attribute is first accessed on an instance. See also `deferred()`.
  comparator_factory
    a class which extends `sqlalchemy.orm.properties.CompositeProperty.Comparator` which provides custom SQL clause generation for comparison operations.
  extension
    an `AttributeExtension` instance, or list of extensions, which will be prepended to the list of attribute listeners for the resulting descriptor placed on the class. These listeners will receive append and set events before the operation proceeds, and may be used to halt (via exception throw) or change the value used in the operation.

`sqlalchemy.orm.` `deferred`(*columns, **kwargs)

  Return a `DeferredColumnProperty`, which indicates this object attributes should only be loaded from its corresponding table column when first accessed.

  Used with the properties dictionary sent to `mapper()`.

`sqlalchemy.orm.` `dynamic_loader`(argument, secondary=None, primaryjoin=None, secondaryjoin=None, foreign_keys=None, backref=None, post_update=False, cascade=False, remote_side=None, enable_typechecks=True, passive_deletes=False, order_by=None, comparator_factory=None, query_class=None)

  Construct a dynamically-loading mapper property.

  This property is similar to `relation()`, except read operations return an active `Query` object which reads from the database when accessed. Items may be appended to the attribute via `append()`, or removed via `remove()`; changes will be persisted to the database during a `Sesion.flush()`. However, no other Python list or collection mutation operations are available.

  A subset of arguments available to `relation()` are available here.

  Parameters: | argument – a class or `Mapper` instance, representing the target of the relation. secondary – for a many-to-many relationship, specifies the intermediary table. The secondary keyword argument should generally only be used for a table that is not otherwise expressed in any class mapping. In particular, using the Association Object Pattern is generally mutually exclusive with the use of the secondary keyword argument. query_class – Optional, a custom Query subclass to be used as the basis for dynamic collection.

`sqlalchemy.orm.` `relation`(argument, secondary=None, **kwargs)

  Provide a relationship of a primary Mapper to a secondary Mapper.

  This corresponds to a parent-child or associative table relationship. The constructed class is an instance of `RelationProperty`.

  A typical `relation()`:

      mapper(Parent, properties={
        'children': relation(Children)
      })

  Parameters: | argument – a class or `Mapper` instance, representing the target of the relation. secondary – for a many-to-many relationship, specifies the intermediary table. The secondary keyword argument should generally only be used for a table that is not otherwise expressed in any class mapping. In particular, using the Association Object Pattern is generally mutually exclusive with the use of the secondary keyword argument. backref – indicates the name of a property to be placed on the related mapper’s class that will handle this relationship in the other direction, including synchronizing the object attributes on both sides of the relation. Can also point to a `backref()` for more configurability. cascade – a comma-separated list of cascade rules which determines how Session operations should be “cascaded” from parent to child. This defaults to `False`, which means the default cascade should be used. The default value is `"save-update, merge"`. Available cascades are: `save-update` - cascade the “add()” operation (formerly known as save() and update()) `merge` - cascade the “merge()” operation `expunge` - cascade the “expunge()” operation `delete` - cascade the “delete()” operation `delete-orphan` - if an item of the child’s type with no parent is detected, mark it for deletion. Note that this option prevents a pending item of the child’s class from being persisted without a parent present. `refresh-expire` - cascade the expire() and refresh() operations `all` - shorthand for “save-update,merge, refresh-expire, expunge, delete” collection_class – a class or callable that returns a new list-holding object. will be used in place of a plain list for storing elements. comparator_factory – a class which extends `RelationProperty.Comparator` which provides custom SQL clause generation for comparison operations. extension – an `AttributeExtension` instance, or list of extensions, which will be prepended to the list of attribute listeners for the resulting descriptor placed on the class. These listeners will receive append and set events before the operation proceeds, and may be used to halt (via exception throw) or change the value used in the operation. foreign_keys – a list of columns which are to be used as “foreign key” columns. this parameter should be used in conjunction with explicit `primaryjoin` and `secondaryjoin` (if needed) arguments, and the columns within the `foreign_keys` list should be present within those join conditions. Normally, `relation()` will inspect the columns within the join conditions to determine which columns are the “foreign key” columns, based on information in the `Table` metadata. Use this argument when no ForeignKey’s are present in the join condition, or to override the table-defined foreign keys. join_depth – when non-`None`, an integer value indicating how many levels deep eagerload joins should be constructed on a self-referring or cyclical relationship. The number counts how many times the same Mapper shall be present in the loading condition along a particular join branch. When left at its default of `None`, eager loads will automatically stop chaining joins when they encounter a mapper which is already higher up in the chain. lazy=(True|False|None|’dynamic’) – specifies how the related items should be loaded. Values include: True - items should be loaded lazily when the property is first accessed. False - items should be loaded “eagerly” in the same query as that of the parent, using a JOIN or LEFT OUTER JOIN. None - no loading should occur at any time. This is to support “write-only” attributes, or attributes which are populated in some manner specific to the application. ‘dynamic’ - a `DynaLoader` will be attached, which returns a `Query` object for all read operations. The dynamic- collection supports only `append()` and `remove()` for write operations; changes to the dynamic property will not be visible until the data is flushed to the database. order_by – indicates the ordering that should be applied when loading these items. passive_deletes=False – Indicates loading behavior during delete operations. A value of True indicates that unloaded child items should not be loaded during a delete operation on the parent. Normally, when a parent item is deleted, all child items are loaded so that they can either be marked as deleted, or have their foreign key to the parent set to NULL. Marking this flag as True usually implies an ON DELETE <CASCADE|SET NULL> rule is in place which will handle updating/deleting child rows on the database side. Additionally, setting the flag to the string value ‘all’ will disable the “nulling out” of the child foreign keys, when there is no delete or delete-orphan cascade enabled. This is typically used when a triggering or error raise scenario is in place on the database side. Note that the foreign key attributes on in-session child objects will not be changed after a flush occurs so this is a very special use-case setting. passive_updates=True – Indicates loading and INSERT/UPDATE/DELETE behavior when the source of a foreign key value changes (i.e. an “on update” cascade), which are typically the primary key columns of the source row. When True, it is assumed that ON UPDATE CASCADE is configured on the foreign key in the database, and that the database will handle propagation of an UPDATE from a source column to dependent rows. Note that with databases which enforce referential integrity (i.e. Postgres, MySQL with InnoDB tables), ON UPDATE CASCADE is required for this operation. The relation() will update the value of the attribute on related items which are locally present in the session during a flush. When False, it is assumed that the database does not enforce referential integrity and will not be issuing its own CASCADE operation for an update. The relation() will issue the appropriate UPDATE statements to the database in response to the change of a referenced key, and items locally present in the session during a flush will also be refreshed. This flag should probably be set to False if primary key changes are expected and the database in use doesn’t support CASCADE (i.e. SQLite, MySQL MyISAM tables). post_update – this indicates that the relationship should be handled by a second UPDATE statement after an INSERT or before a DELETE. Currently, it also will issue an UPDATE after the instance was UPDATEd as well, although this technically should be improved. This flag is used to handle saving bi-directional dependencies between two individual rows (i.e. each row references the other), where it would otherwise be impossible to INSERT or DELETE both rows fully since one row exists before the other. Use this flag when a particular mapping arrangement will incur two rows that are dependent on each other, such as a table that has a one-to-many relationship to a set of child rows, and also has a column that references a single child row within that list (i.e. both tables contain a foreign key to each other). If a `flush()` operation returns an error that a “cyclical dependency” was detected, this is a cue that you might want to use `post_update` to “break” the cycle. primaryjoin – a ClauseElement that will be used as the primary join of this child object against the parent object, or in a many-to-many relationship the join of the primary object to the association table. By default, this value is computed based on the foreign key relationships of the parent and child tables (or association table). remote_side – used for self-referential relationships, indicates the column or list of columns that form the “remote side” of the relationship. secondaryjoin – a ClauseElement that will be used as the join of an association table to the child object. By default, this value is computed based on the foreign key relationships of the association and child tables. uselist=(True|False) – a boolean that indicates if this property should be loaded as a list or a scalar. In most cases, this value is determined automatically by `relation()`, based on the type and direction of the relationship - one to many forms a list, many to one forms a scalar, many to many is a list. If a scalar is desired where normally a list would be present, such as a bi-directional one-to-one relationship, set uselist to False. viewonly=False – when set to True, the relation is used only for loading objects within the relationship, and has no effect on the unit-of-work flush process. Relations with viewonly can specify any kind of join conditions to provide additional views of related objects onto a parent object. Note that the functionality of a viewonly relationship has its limits - complicated join conditions may not compile into eager or lazy loaders properly. If this is the case, use an alternative method.

`sqlalchemy.orm.` `synonym`(name, map_column=False, descriptor=None, comparator_factory=None, proxy=False)

  Set up name as a synonym to another mapped property.

  Used with the `properties` dictionary sent to `mapper()`.

  Any existing attributes on the class which map the key name sent to the `properties` dictionary will be used by the synonym to provide instance-attribute behavior (that is, any Python property object, provided by the `property` builtin or providing a `__get__()`, `__set__()` and `__del__()` method). If no name exists for the key, the `synonym()` creates a default getter/setter object automatically and applies it to the class.

  name refers to the name of the existing mapped property, which can be any other `MapperProperty` including column-based properties and relations.

  If map_column is `True`, an additional `ColumnProperty` is created on the mapper automatically, using the synonym’s name as the keyname of the property, and the keyname of this `synonym()` as the name of the column to map. For example, if a table has a column named `status`:

      class MyClass(object):
          def _get_status(self):
              return self._status
          def _set_status(self, value):
              self._status = value
          status = property(_get_status, _set_status)

      mapper(MyClass, sometable, properties={
          "status":synonym("_status", map_column=True)
      })

  The column named `status` will be mapped to the attribute named `_status`, and the `status` attribute on `MyClass` will be used to proxy access to the column-based attribute.

  The proxy keyword argument is deprecated and currently does nothing; synonyms now always establish an attribute getter/setter function if one is not already available.

## Decorators

`sqlalchemy.orm.` `reconstructor`(fn)

  Decorate a method as the ‘reconstructor’ hook.

  Designates a method as the “reconstructor”, an `__init__`-like method that will be called by the ORM after the instance has been loaded from the database or otherwise reconstituted.

  The reconstructor will be invoked with no arguments. Scalar (non-collection) database-mapped attributes of the instance will be available for use within the function. Eagerly-loaded collections are generally not yet available and will usually only contain the first element. ORM state changes made to objects at this stage will not be recorded for the next flush() operation, so the activity within a reconstructor should be conservative.

`sqlalchemy.orm.` `validates`(*names)

  Decorate a method as a ‘validator’ for one or more named properties.

  Designates a method as a validator, a method which receives the name of the attribute as well as a value to be assigned, or in the case of a collection to be added to the collection. The function can then raise validation exceptions to halt the process from continuing, or can modify or replace the value before proceeding. The function should otherwise return the given value.

## Utilities

`sqlalchemy.orm.` `object_mapper`(instance)

  Given an object, return the primary Mapper associated with the object instance.

  Raises UnmappedInstanceError if no mapping is configured.

`sqlalchemy.orm.` `class_mapper`(class_, compile=True)

  Given a class (or an object), return the primary Mapper associated with the key.

  Raises UnmappedClassError if no mapping is configured.

`sqlalchemy.orm.` `compile_mappers`()

  Compile all mappers that have been defined.

  This is equivalent to calling `compile()` on any individual mapper.

`sqlalchemy.orm.` `clear_mappers`()

  Remove all mappers that have been created thus far.

  The mapped classes will return to their initial “unmapped” state and can be re-mapped with new mappers.

## Internals

class `sqlalchemy.orm.mapper.` `Mapper`(class_, local_table, properties=None, primary_key=None, non_primary=False, inherits=None, inherit_condition=None, inherit_foreign_keys=None, extension=None, order_by=False, always_refresh=False, version_id_col=None, polymorphic_on=None, _polymorphic_map=None, polymorphic_identity=None, polymorphic_fetch=None, concrete=False, select_table=None, with_polymorphic=None, allow_null_pks=False, batch=True, column_prefix=None, include_properties=None, exclude_properties=None, eager_defaults=False)

  Define the correlation of class attributes to database table columns.

  Instances of this class should be constructed via the `mapper()` function.

  `__init__`(class_, local_table, properties=None, primary_key=None, non_primary=False, inherits=None, inherit_condition=None, inherit_foreign_keys=None, extension=None, order_by=False, always_refresh=False, version_id_col=None, polymorphic_on=None, _polymorphic_map=None, polymorphic_identity=None, polymorphic_fetch=None, concrete=False, select_table=None, with_polymorphic=None, allow_null_pks=False, batch=True, column_prefix=None, include_properties=None, exclude_properties=None, eager_defaults=False)

    Construct a new mapper.

    Mappers are normally constructed via the `mapper()` function. See for details.

  `add_properties`(dict_of_properties)
    Add the given dictionary of properties to this mapper, using add_property.
  `add_property`(key, prop)

    Add an individual MapperProperty to this mapper.

    If the mapper has not been compiled yet, just adds the property to the initial properties dictionary sent to the constructor. If this Mapper has already been compiled, then the given MapperProperty is compiled immediately.

  `cascade_iterator`(type_, state, halt_on=None)

    Iterate each element and its mapper in an object graph, for all relations that meet the given cascade rule.

    `type\_`:
      The name of the cascade rule (i.e. save-update, delete, etc.)
    `state`:
      The lead InstanceState. child items will be processed per the relations defined for this object’s mapper.

    the return value are object instances; this provides a strong reference so that they don’t fall out of scope immediately.

  `common_parent`(other)
    Return true if the given mapper shares a common inherited parent as this mapper.
  `compile`()

    Compile this mapper and all other non-compiled mappers.

    This method checks the local compiled status as well as for any new mappers that have been defined, and is safe to call repeatedly.

  `get_property`(key, resolve_synonyms=False, raiseerr=True)
    return a MapperProperty associated with the given key.
  `identity_key_from_instance`(instance)

    Return the identity key for the given instance, based on its primary key attributes.

    This value is typically also found on the instance state under the attribute name key.

  `identity_key_from_primary_key`(primary_key)

    Return an identity-map key for use in storing/retrieving an item from an identity map.

    primary_key
      A list of values indicating the identifier.
  `identity_key_from_row`(row, adapter=None)

    Return an identity-map key for use in storing/retrieving an item from the identity map.

    row
      A `sqlalchemy.engine.base.RowProxy` instance or a dictionary corresponding result-set `ColumnElement` instances to their values within a row.
  `isa`(other)
    Return True if the this mapper inherits from the given mapper.
  `iterate_properties`
    return an iterator of all MapperProperty objects.
  `polymorphic_iterator`()

    Iterate through the collection including this mapper and all descendant mappers.

    This includes not just the immediately inheriting mappers but all their inheriting mappers as well.

    To iterate through an entire hierarchy, use `mapper.base_mapper.polymorphic_iterator()`.

  `primary_key_from_instance`(instance)
    Return the list of primary key values for the given instance.
  `primary_mapper`()
    Return the primary mapper corresponding to this mapper’s class key (class).