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When data has not yet been loaded for an entity, some additional logic is used to allow
asynchronous requests via chained method accessors.

Given a chain $book->author->address->city, where the first three entries are regular entities
and the last item in the chain is an accessor for a scalar method:

First, we check $book to see whether it has an author yet. If the author information is loaded
(to the extent that ->author contains an entity instance), then we can use this existing instance.
If not, we instantiate a new entity of this type, marked as incomplete and as a pending request, and
continue.

This means that a chain where some of the elements can be null is still valid. As data is populated,
entries in this chain will be filled out, and cases where the foreign key value was null will end up
marked as invalid entities.

In general, unless you know beforehand that all entities in the chain have been populated, all access
to chained entities should go through the L<EntityModel::Gather> interface to ensure that values are
consistent.

This also allows the backend storage to apply optimisations if available - if several pending requests
address related storage areas, it may be possible for the storage engine to combine queries and return
data faster.

=head1 EVENTS

Two types of events can be defined:

=over 4

=item * task - this is a one-shot event, typically used to call a piece of code after data has been written
to or read from storage

=item * event - this is an event handler that will be called every time an event occurs.

=back

These are handled by the L</_queue_task> and L</_attach_event> methods respectively.

=head2 new

Instantiate from an ID or a pre-fabricated object (hashref).

=over 4

=item * Create a new, empty object:

 EntityModel::Support::Perl::Base->new(1)

=item * Instantiate from ID:

 EntityModel::Support::Perl::Base->new(1)
 EntityModel::Support::Perl::Base->new('123-456')
 EntityModel::Support::Perl::Base->new([123,456])

=item * Create an object and assign initial values:

 EntityModel::Support::Perl::Base->new({ x => 1, y => 2 })

=back

Any remaining options indicate callbacks:

=over 4

=item * before_commit - just before commit

=item * after_commit - after this has been committed to the database

=item * on_load - when the data has been read from storage

=item * on_not_found - when storage reports that this item is not found

=back

The before_XXX callbacks are also aliased to on_XXX for convenience.

=head2 _queue_task

Queues a new one-shot task for the given event type.

Supports the following event types:

=over 4

=item * on_load - data has been read from storage

=item * on_create - initial data has been written to storage

=item * on_update - values have been updated, but not necessarily written to storage

=item * on_remove - this entry has been removed from storage

=item * on_not_found - could not find this entry in backend storage

=back

=head2 _event

Pass the given event through to any defined callbacks.

=head2 _spec_from_hashref

Private method to generate hashref containing spec information suitable for bless to requested class,
given a hashref which represents the keys/values for the object.

This will flatten any Entity objects down to the required ID key+values.

=head2 create

Create a new object.

Takes a hashref, and sets the flag so that ->commit does the insert.

=head2 _update

Write current values back to storage.

=head2 _select

Populate this instance with values from the database.

=head2 _pending_insert

Returns true if this instance is due to be committed to the database.

=head2 _pending_update

Returns true if this instance is due to be committed to the database.

=head2 _insert

Insert this instance into the db.

=head2 commit

Commit any pending changes to the database.

=head2 revert

Revert any pending changes to the database.

=head2 id

Primary key (id) for this instance.

=head2 fromID

Instantiate from an ID.

=head2 remove

Remove this instance from the database.

=head2 _view

Returns the view corresponding to this object.

=head1 INHERITED METHODS