# SQLAlchemy 0.4 Documentation

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Version: 0.4.6 Last Updated: 05/14/08 14:36:43

### module sqlalchemy.orm.interfaces

Semi-private implementation objects which form the basis of ORM-mapped attributes, query options and mapper extension.

Defines the MapperExtension class, which can be end-user subclassed to add event-based functionality to mappers. The remainder of this module is generally private to the ORM.

### Module Functions

def build_path(mapper, key, prev=None)

### class AttributeExtension(object)

An abstract class which specifies append, delete, and set event handlers to be attached to an object property.

def append(self, obj, child, initiator)
def remove(self, obj, child, initiator)
def set(self, obj, child, oldchild, initiator)
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### class ExtensionOption(MapperOption)

a MapperOption that applies a MapperExtension to a query operation.

def __init__(self, ext)

Construct a new `ExtensionOption`.

def process_query(self, query)
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### class LoaderStrategy(object)

Describe the loading behavior of a StrategizedProperty object.

The `LoaderStrategy` interacts with the querying process in three ways:

- it controls the configuration of the `InstrumentedAttribute` placed on a class to handle the behavior of the attribute. this may involve setting up class-level callable functions to fire off a select operation when the attribute is first accessed (i.e. a lazy load)
- it processes the `QueryContext` at statement construction time, where it can modify the SQL statement that is being produced. simple column attributes may add their represented column to the list of selected columns, eager loading properties may add `LEFT OUTER JOIN` clauses to the statement.
- it processes the `SelectionContext` at row-processing time. This includes straight population of attributes corresponding to rows, setting instance-level lazyloader callables on newly constructed instances, and appending child items to scalar/collection attributes in response to eagerly-loaded relations.

def __init__(self, parent)

Construct a new `LoaderStrategy`.

def create_row_processor(self, selectcontext, mapper, row)

Return row processing functions which fulfill the contract specified by MapperProperty.create_row_processor.

StrategizedProperty delegates its create_row_processor method directly to this method.

def init(self)
def init_class_attribute(self)
def setup_query(self, context, **kwargs)
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### class MapperExtension(object)

Base implementation for customizing Mapper behavior.

For each method in MapperExtension, returning a result of EXT_CONTINUE will allow processing to continue to the next MapperExtension in line or use the default functionality if there are no other extensions.

Returning EXT_STOP will halt processing of further extensions handling that method. Some methods such as `load` have other return requirements, see the individual documentation for details. Other than these exception cases, any return value other than EXT_CONTINUE or EXT_STOP will be interpreted as equivalent to EXT_STOP.

EXT_PASS is a synonym for EXT_CONTINUE and is provided for backward compatibility.

def after_delete(self, mapper, connection, instance)

Receive an object instance after that instance is DELETEed.

def after_insert(self, mapper, connection, instance)

Receive an object instance after that instance is INSERTed.

def after_update(self, mapper, connection, instance)

Receive an object instance after that instance is UPDATEed.

def append_result(self, mapper, selectcontext, row, instance, result, **flags)

Receive an object instance before that instance is appended to a result list.

If this method returns EXT_CONTINUE, result appending will proceed normally. if this method returns any other value or None, result appending will not proceed for this instance, giving this extension an opportunity to do the appending itself, if desired.

mapper
  The mapper doing the operation.
selectcontext
  SelectionContext corresponding to the instances() call.
row
  The result row from the database.
instance
  The object instance to be appended to the result.
result
  List to which results are being appended.
**flags
  extra information about the row, same as criterion in create_row_processor() method of MapperProperty

def before_delete(self, mapper, connection, instance)

Receive an object instance before that instance is DELETEed.

Note that no changes to the overall flush plan can be made here; this means any collection modification, save() or delete() operations which occur within this method will not take effect until the next flush call.

def before_insert(self, mapper, connection, instance)

Receive an object instance before that instance is INSERTed into its table.

This is a good place to set up primary key values and such that aren't handled otherwise.

Column-based attributes can be modified within this method which will result in the new value being inserted. However no changes to the overall flush plan can be made; this means any collection modification or save() operations which occur within this method will not take effect until the next flush call.

def before_update(self, mapper, connection, instance)

Receive an object instance before that instance is UPDATEed.

Note that this method is called for all instances that are marked as "dirty", even those which have no net changes to their column-based attributes. An object is marked as dirty when any of its column-based attributes have a "set attribute" operation called or when any of its collections are modified. If, at update time, no column-based attributes have any net changes, no UPDATE statement will be issued. This means that an instance being sent to before_update is not a guarantee that an UPDATE statement will be issued (although you can affect the outcome here).

To detect if the column-based attributes on the object have net changes, and will therefore generate an UPDATE statement, use `object_session(instance).is_modified(instance, include_collections=False)`.

Column-based attributes can be modified within this method which will result in their being updated. However no changes to the overall flush plan can be made; this means any collection modification or save() operations which occur within this method will not take effect until the next flush call.

def create_instance(self, mapper, selectcontext, row, class_)

Receive a row when a new object instance is about to be created from that row.

The method can choose to create the instance itself, or it can return EXT_CONTINUE to indicate normal object creation should take place.

mapper
  The mapper doing the operation
selectcontext
  SelectionContext corresponding to the instances() call
row
  The result row from the database
class_
  The class we are mapping.
return value
  A new object instance, or EXT_CONTINUE

def get(self, query, *args, **kwargs)

Override the get method of the Query object.

The return value of this method is used as the result of `query.get()` if the value is anything other than EXT_CONTINUE.

def get_by(self, query, *args, **kwargs)

Override the get_by method of the Query object.

The return value of this method is used as the result of `query.get_by()` if the value is anything other than EXT_CONTINUE.

DEPRECATED.

def get_session(self)

Retrieve a contextual Session instance with which to register a new object.

Note: this is not called if a session is provided with the __init__ params (i.e. _sa_session).

def init_failed(self, mapper, class_, oldinit, instance, args, kwargs)
def init_instance(self, mapper, class_, oldinit, instance, args, kwargs)
def instrument_class(self, mapper, class_)
def load(self, query, *args, **kwargs)

Override the load method of the Query object.

The return value of this method is used as the result of `query.load()` if the value is anything other than EXT_CONTINUE.

def populate_instance(self, mapper, selectcontext, row, instance, **flags)

Receive a newly-created instance before that instance has its attributes populated.

The normal population of attributes is according to each attribute's corresponding MapperProperty (which includes column-based attributes as well as relationships to other classes). If this method returns EXT_CONTINUE, instance population will proceed normally. If any other value or None is returned, instance population will not proceed, giving this extension an opportunity to populate the instance itself, if desired.

def select(self, query, *args, **kwargs)

Override the select method of the Query object.

The return value of this method is used as the result of `query.select()` if the value is anything other than EXT_CONTINUE.

DEPRECATED.

def select_by(self, query, *args, **kwargs)

Override the select_by method of the Query object.

The return value of this method is used as the result of `query.select_by()` if the value is anything other than EXT_CONTINUE.

DEPRECATED.

def translate_row(self, mapper, context, row)

Perform pre-processing on the given result row and return a new row instance.

This is called as the very first step in the `_instance()` method.

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### class MapperOption(object)

Describe a modification to a Query.

def process_query(self, query)
def process_query_conditionally(self, query)

same as process_query(), except that this option may not apply to the given query.

Used when secondary loaders resend existing options to a new Query.

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### class MapperProperty(object)

Manage the relationship of a `Mapper` to a single class attribute, as well as that attribute as it appears on individual instances of the class, including attribute instrumentation, attribute access, loading behavior, and dependency calculations.

def cascade_iterator(self, type_, state, visited_instances=None, halt_on=None)

Iterate through instances related to the given instance for a particular 'cascade', starting with this MapperProperty.

See PropertyLoader for the related instance implementation.

def compare(self, operator, value)

Return a compare operation for the columns represented by this `MapperProperty` to the given value, which may be a column value or an instance. 'operator' is an operator from the operators module, or from sql.Comparator.

By default uses the PropComparator attached to this MapperProperty under the attribute name "comparator".

def create_row_processor(self, selectcontext, mapper, row)

Return a 3-tuple consiting of two row processing functions and an instance post-processing function.

Input arguments are the query.SelectionContext and the first applicable row of a result set obtained within query.Query.instances(), called only the first time a particular mapper's populate_instance() method is invoked for the overall result.

The settings contained within the SelectionContext as well as the columns present in the row (which will be the same columns present in all rows) are used to determine the presence and behavior of the returned callables. The callables will then be used to process all rows and to post-process all instances, respectively.

Callables are of the following form:

    def new_execute(instance, row, **flags):
        # process incoming instance and given row.  the instance is
        # "new" and was just created upon receipt of this row.
        # flags is a dictionary containing at least the following
        # attributes:
        #   isnew - indicates if the instance was newly created as a
        #           result of reading this row
        #   instancekey - identity key of the instance
        # optional attribute:
        #   ispostselect - indicates if this row resulted from a
        #                  'post' select of additional tables/columns

    def existing_execute(instance, row, **flags):
        # process incoming instance and given row.  the instance is
        # "existing" and was created based on a previous row.

    def post_execute(instance, **flags):
        # process instance after all result rows have been processed.
        # this function should be used to issue additional selections
        # in order to eagerly load additional properties.

    return (new_execute, existing_execute, post_execute)

Either of the three tuples can be `None` in which case no function is called.

def do_init(self)

Perform subclass-specific initialization steps.

This is a template method called by the `MapperProperty` object's init() method.

def get_criterion(self, query, key, value)

Return a `WHERE` clause suitable for this `MapperProperty` corresponding to the given key/value pair, where the key is a column or object property name, and value is a value to be matched. This is only picked up by `PropertyLoaders`.

This is called by a `Query`'s `join_by` method to formulate a set of key/value pairs into a `WHERE` criterion that spans multiple tables if needed.

def init(self, key, parent)

Called after all mappers are compiled to assemble relationships between mappers, establish instrumented class attributes.

def is_primary(self)

Return True if this `MapperProperty`'s mapper is the primary mapper for its class.

This flag is used to indicate that the `MapperProperty` can define attribute instrumentation for the class at the class level (as opposed to the individual instance level).

def merge(self, session, source, dest)

Merge the attribute represented by this `MapperProperty` from source to destination object

def register_dependencies(self, *args, **kwargs)

Called by the `Mapper` in response to the UnitOfWork calling the `Mapper`'s register_dependencies operation. Should register with the UnitOfWork all inter-mapper dependencies as well as dependency processors (see UOW docs for more details).

def set_parent(self, parent)
def setup(self, querycontext, **kwargs)

Called by Query for the purposes of constructing a SQL statement.

Each MapperProperty associated with the target mapper processes the statement referenced by the query context, adding columns and/or criterion as appropriate.

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### class PropComparator(ColumnOperators)

defines comparison operations for MapperProperty objects.

PropComparator instances should also define an accessor 'property' which returns the MapperProperty associated with this PropComparator.

def __init__(self, prop)

Construct a new `PropComparator`.

def any(self, criterion=None, **kwargs)

Return true if this collection contains any member that meets the given criterion.

criterion
  an optional ClauseElement formulated against the member class' table or attributes.
**kwargs
  key/value pairs corresponding to member class attribute names which will be compared via equality to the corresponding values.

def contains(self, other)

Return true if this collection contains other

def expression_element(self)
def has(self, criterion=None, **kwargs)

Return true if this element references a member which meets the given criterion.

criterion
  an optional ClauseElement formulated against the member class' table or attributes.
**kwargs
  key/value pairs corresponding to member class attribute names which will be compared via equality to the corresponding values.

def of_type(self, class_)

Redefine this object in terms of a polymorphic subclass.

Returns a new PropComparator from which further criterion can be evaluated.

e.g.:

    query.join(Company.employees.of_type(Engineer)).\
       filter(Engineer.name=='foo')

class_
  a class or mapper indicating that criterion will be against this specific subclass.

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### class PropertyOption(MapperOption)

A MapperOption that is applied to a property off the mapper or one of its child mappers, identified by a dot-separated key.

def __init__(self, key, mapper=None)

Construct a new `PropertyOption`.

def process_query(self, query)
def process_query_conditionally(self, query)
def process_query_property(self, query, paths)
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### class StrategizedOption(PropertyOption)

A MapperOption that affects which LoaderStrategy will be used for an operation by a StrategizedProperty.

def get_strategy_class(self)
def is_chained(self)
def process_query_property(self, query, paths)
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### class StrategizedProperty(MapperProperty)

A MapperProperty which uses selectable strategies to affect loading behavior.

There is a single default strategy selected by default. Alternate strategies can be selected at Query time through the usage of `StrategizedOption` objects via the Query.options() method.

def create_row_processor(self, selectcontext, mapper, row)
def do_init(self)
def setup(self, querycontext, **kwargs)
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