Introduction

This package provides a reference implementation.

The specific properties of this implementation are:

intended to be used for intrinsic references
provides integrity enforcement
modelled partially after relational foreign keys

Contents

Motivation
Model comparison
Observations
Conclusions & Requirements for the reference implementation
Implementation notes

Motivation

When developing an application we often find the need to reference objects that are stored as application data. Examples of such objects include centrally managed 'master data'.

The reference to those objects is typically intrinsic to the application we develop so they should behave like normal Python object references while being under the control of our application.

Within the world of Zope and ZODB there are different ways to achieve this. The various approaches have different semantics and side effects. Our goal is to unify the way of intrinsically referencing objects and to provide the ability to switch between different semantics as needed without rewriting application code and without the need to migrate persistent data structures (at least from the application's point of view).

Model comparison

Our goal was to determine the advantages and disadvantages of the different existing approaches. We included three general approaches from the world of Python/Zope/ZODB as well as the standard relational approach to normalisation tables.

We used four criteria to describe each solution:

Reference data: What data is stored to describe the reference?
Reference semantics: What meaning does the reference have? How can its meaning change?
Integrity: What might happen to the application if data that is involved in the reference changes or is deleted?
Set/Lookup: What does the application developer have to do to set a reference or look up a referenced object?

Property	Python references	Weak references	Key reference	Relational DBs
Reference data	OID	OID	application-specific key	application-specific (primary key + table name)
Reference semantics	Refers to a specific Python object	Refers to a specific Python object	Refers to an object which is associated with the saved key at the time of the lookup.	Refers to an object (row) that is associated with the primary key at the time of the lookup.
Integrity	The reference stays valid, however, the target object might have lost its meaning for the application.	The reference might have become stale and leave the referencing object in an invalid state.	The reference might have become stale.	Dependening on the use of foreign keys and the databases implementation of constraints. Can usually be forced to stay valid.
Set/Lookup	Normal Python attribute access.	Use WeakRef wrapper to store and __call__ to lookup. Might use properties for convenience.	Depends on the implementation. Might use properties for convenience.	Explicitly store the primary key. Use JOIN to look up.

Observations

Relational: every object (row) has a canonical place that defines a primary key.

The ZODB (like a filesystem) can have multiple hard links to an object. Objects are deleted when the last hard link to an object is removed. This makes it impossible to use hard links for referencing an object because object deletion will not be noticed and the objects will continue to live. The ZODB itself does not have a notion of a canonical place where an object is defined.
Relational: When referencing an object we can enforce integrity by declaring a foreign key. This is orthogonal to the data stored.
Relational: As an application-level key is used for identifying the target of a reference, the application can choose to delete a row and re-add a row with the same primary key later. If the integrity is enforced this requires support on the database level to temporarily ignore broken foreign keys.
Normal Python references embed themselves naturally in the application. Properties allow hiding the implementation of looking up and storing references.

Conclusions & Requirements for the reference implementation

Allow configuration of foreign key constraints (none, always, at the end of the transaction). This configuration must be changable at any time with an automatic migration path provided.
Use application level keys to refer to an object.
Use a canonical location and a primary key to store objects and to determine whether an object was deleted.
Distinguish between two use cases when modifying an object's key:
1. The application references the right object but has the wrong key (as the key itself might have meaning for the application). In this case the object must be updated to receive the new, correct key and the references must be updated to refer to this new key.
2. The application references the wrong object with the right key. In this case the object referenced by the key must be replaced with a different object.

Implementation notes

Canonical location is determined by location/containment. The primary key for a reference is the referenced object's location.
Constraints are enforced by monitoring containment events.
The different ways of updating/changing a key's meaning are supported by an indirection that enumerates all keys and stores a reference id on the referencing object instead of the location. The two use cases for changing the meaning are implemented by:
1. associating a new path with an existing reference id
2. associating a new reference id with an existing path