NAME

       UR::Context::ObjectFabricator - Track closures used to fabricate objects from data sources

DESCRIPTION

       Object Fabricators are closures that accept listrefs of data returned by data source iterators, take
       slices out of them, and construct UR objects out of the results.  They also handle updating the query
       cache and merging changed DB data with previously cached objects.

       UR::Context::ObjectFabricator objects are used internally by UR::Context, and not intended to be used
       directly.

METHODS

       create_for_loading_template
             my $fab = UR::Context::ObjectFabricator->create_for_loading_template(
                           $context, $loading_tmpl_hashref, $template_data,
                           $rule, $rule_template, $values, $dsx);

           Returns an object fabricator instance that is able to construct objects of the rule's target class
           from rows of data returned by data source iterators.  Object fabricators are used a part of the
           object loading process, and are called by UR::Context::get_objects_for_class_and_rule() to transform
           a row of data returned by a data source iterator into a UR object.

           For each class involved in a get request, the system prepares a loading template that describes which
           columns of the data source data are to be used to construct an instance of that class.  For example,
           in the case where a get() is done on a child class, and the parent and child classes store data in
           separate tables linked by a relation-property/foreign-key, then the query against the data source
           will involve and SQL join (for RDBMS data sources).  That join will produce a result set that
           includes data from both tables.

           The $loading_tmpl_hashref will have information about which columns of that result set map to which
           properties of each involved class.  The heart of the fabricator closure is a list slice extracting
           the data for that class and assigning it to a hash slice of properties to fill in the initial object
           data for its class.  The remainder of the closure is bookkeeping to keep the object cache
           ($UR::Context::all_objects_loaded) and query cache ($UR::Context::all_params_loaded) consistent.

           The interaction of the object fabricator, the query cache, object cache pruner and object loading
           iterators that may or may not have loaded all their data requires that the object fabricators keep a
           list of changes they plan to make to the query cache instead of applying them directly.  When the
           Underlying Context Loading iterator has loaded the last row from the Data Source Iterator, it calls
           "finalize()" on the object fabricator to tell it to go ahead and apply its changes; essentially
           treating that data as a transaction.

       all_object_fabricators
             my @fabs = UR::Context::ObjectFabricator->all_object_fabricators();

           Returns a list of all object fabricators that have not yet been finalized

       fabricate
             my $ur_object = $fab->fabricate([columns,from,data,source]);

           Given a listref of data pulled from a data source iterator, it slices out the appropriate columns
           from the list and constructs a single object to return.

       is_loading_in_progress_for_boolexpr
               my $bool = $fab->is_loading_in_progress_for_boolexpr($boolexpr);

           Given a UR::BoolExpr instance, it returns true if the given fabricator is prepared to construct
           objects matching this boolexpr.  This is used by UR::Context to know if other iterators are still
           pulling in objects that could match another iterator's boolexpr, and it should therefore not trust
           that the object cache is conplete.

       finalize
             $fab->finalize();

           Indicates to the iterator that the caller is done using it for constructing objects, probably because
           the data source has no more data or the iterator that was using this fabricator has gone out of
           scope.

       apply_all_params_loaded
             $fab->apply_all_params_loaded();

           As the fabricator constructs objects, it buffers changes to all_params_loaded (the Context's query
           cache) to maintain consistency if multiple iterators are working concurrently.  At the appripriate
           time, call apply_all_params_loaded() to take those changes and apply them to the current Context's
           all_params_loaded.