= Composite Primary Key Design Notes =

Collating design considerations, options and decisions here to help get this feature moving.

== Design Questions ==

=== How do you define multiple primary keys? ===

The "neatest" solution would be to allow primary_key=True on multiple fields but this may be precluded by other design considerations.

=== How do you foreign key onto a table with a composite foreign key? ===

The primary key is a set of the columns.

=== What do you use for .get(pk=foo) with a composite primary key? ===

The primary key is a set of the columns.

There is mention of the comment framework overlapping with this.

=== How does Admin map a url to an object? ===

Possible solutions (for a key with the value {{{ { val_foo, val_bar, val_baz } }}}:

* {{{/appname/modulename/val_foo,val_bar,val_baz/}}}

  How to deal with values containing commas? Should be able to be handled with some nice url escaping.

* {{{/appname/modulename/val_foo/val_bar/val_baz/}}}

=== How to deal with identity between different orders of the same set? ===

The columns of a key are a set, and have no implicit order. This means the key {{{ { val_foo, val_bar, val_baz } }}} is identical to {{{ { val_baz, val_foo, val_bar } }}} and so on for all permutations. How should URLs be parsed to reflect this identity?

== Affected Areas ==

=== Models ===

[wiki:CompositePrimaryKeyModelNotes On going in depth notes]

Need a way of defining the primary key.

Generation of the create table SQL.

A solution for foreign keying onto a table with a composite primary key.

Many to many relationships should use composite primary keys if they are unique . . .

=== Queries ===

Model.objects.get(pk=foo) needs to be supported.

Solution: the primary key is a set of the column values.

=== Forms ===

=== Admin ===

Expect there is a large amount of work in here making sure all the model inspection and form generation works.

URL to object mapping.

=== Serialization ===

Code needs to be looked at further but all the foreign keying is likely to need attention.

== Relevant Discussions ==

* Jul 2008 http://groups.google.com/group/django-developers/browse_thread/thread/4b2370a0652d9135/2344ca721a4213fd (David Cramer initial thoughts)

* Sep 2008 http://groups.google.com/group/django-developers/browse_thread/thread/32f861c8bd5366a5/22732dc88e644f77 (David Cramer and Malcolm Tredinnick pk talk)

* Nov 2008 http://groups.google.com/group/django-developers/browse_thread/thread/eac7c296c3797f5a/d94cd7cdd0d69b18 (David Cramer proposal)

* Nov 2009 http://groups.google.com/group/django-developers/browse_thread/thread/eac31c483e6ef885/6e80505a789fa750