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RevisionBehavior is a solution for adding undo and other versioning functionality
to your database models. It is set up for easy application to your project,
ease of use and to not get in the way of your other model activity.
It is also intended to work well with it's sibling, LogableBehavior.

Feature list

1. Easy to install
2. Automagically save revision on model save
3. Able to ignore model saves which only contain certain fields
4. Limit number of revisions to keep, will delete oldest
5. Undo functionality (or update to any revision directly)
6. Revert to a datetime (and even do so cascading)
7. Get a diff model array to compare two or more revisions
8. Inspect any or all revisions of a model

Install instructions

1. Place the newest version of RevisionBehavior in your app/models/behaviors folder
2. Add the behavior to AppModel (or single models if you prefer)
3. For each model that you want revision for, create a shadow table
4. Behavior will gracefully do nothing for models that has behavior, but not shadow table
5. If adding Revision to an existing project, run the initializeRevisions() method once for each model.

About shadow tables

You should make these AFTER you have baked your ordinary tables as they may interfer. By default
the tables should be named rev_[normal table name]. If you wish to change the prefix you may
do so in the property called $revision_prefix found in the behavior. Also by default the behavior expects
the shadow tables to be in the same dbconfig as the model, but you may change this on a per
model basis with the useDbConfig config option.

Add the same fields as in the live table, with 3 important differences.

1. The 'id' field should NOT be the primary key, nor auto increment.
2. Add the fields 'version_id' (int, primary key, autoincrement) and 'version_created' (datetime).
3. Skipp fields that should not be saved in shadowtable (lft,right,weight for instance).

Configuration

When adding 'Revision' the a model's actsAs array, you may configure the behavior with these options:

1. limit : int number of revisions to keep, must be at least 2 (as current is 1).
2. ignore : array containing the name of fields to ignore.
3. auto : boolean when false the behavior will NOT generate revisions in afterSave.
4. useDbConfig : string/null Name of dbConfig to use. Null to use Model's.

Limit functionality

The shadow table will save a revision copy when it saves live data, so the newest
row in the shadow table will (in most cases) be the same as the current live data.
The exception is when the ignore field functionality is used and the live data is
updated only in those fields.

Ignore field(s) functionality

If you wish to be able to update certain fields without generating new revisions,
you can add those fields to the configuration ignore array. Any time the behavior's
afterSave is called with just primary key and these fields, it will NOT generate
a new revision. It WILL however save these fields together with other fields when it
does save a revision. You will probably want to set up cron or otherwise call
createRevision() to update these fields at some points.

Auto functionality

By default the behavior will insert itself into the Model's save process by implementing
beforeSave and afterSave. In afterSave, the behavior will save a new revision of the dataset
that is now the live data. If you do NOT want this automatic behavior, you may set the config
option 'auto' to false. Then the shadow table will remain empty unless you call createRevisions
manually.