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CCK Reference Filter for nodes and users

CCK reference filter enhances the user experience and ensures data integrity in a number of common use-cases involving node references:
o from an originating node to a destination node, or
o from an originating node to a destination user.

What does it do?

When using a CCK reference field to link the originating node to one or more other nodes or users, this package will restrict the nodes or users available for selection to those that have not yet been referenced from any other origination node.

When would I use it?

Here are some typical scenarios of users and nodes that must be referenced from no more than one originating node.

  • a CMS containing a directory of companies and organisations, each referencing a contact person who is a registered/authenticated user on the site; each user may be a contact for one company or organisation only
  • a site featuring nodes of type meeting, each of which reference one agenda node and one meeting-minutes node; both agenda and minutes belong to that one meeting only
  • a repository of financial reports whereby each company's financial annual report (node) references up to 12 monthly report nodes belonging to that company only
  • a site with teams of some kind whereby the members/players have logins on the site; each player can be in one team only

In general: any one-to-one or one-to-many parent-child relationship whereby a parent may have multiple children, but each child belongs to only one parent. In other words: child selection is mutual exclusive.

Put another way, this package makes sure that the UI offers only 'orphan' nodes and users as valid destination references.

The tarball contains three small modules.
Enable Node Reference Filter for use with the CCK submodule Node Reference.
Enable User Reference Filter for use with the CCK submodule User Reference.
In either case you also need to enable the third module in this package, Content Reference Filter. It contains common functionality for the other two filter modules.

These modules work with both the radio button and check box widgets of the Node and User Reference modules, as well as the single drop-down and multi-select combo-boxes.
They may be used in the standard and advanced (i.e. View) modes, as supplied by CCK's Node Reference and User Reference modules.

Installation and configuration

The package is a cinch to install. User Reference Filter has no configuration options at all. Just enable and you're away. Node Reference Filter has a couple of options (see its README), but the defaults are usually what you need. Enable the module and have a peek later at Site configuration >> Node Reference Filter for available options.
As before, you configure node and user reference fields on the Content
management >> Content types >> Manage fields >> Configure
page of the content types in question. Don't forget to tick the box Ensure references are mutually exclusive to activate this feature for the field in question.
Users continue to select references when they create or edit content of that type. The difference is that this time the options on offer will be a subset of the original collection.

Frequently asked questions

Q: What will happen when I run out of nodes (or users) to reference?
A: You'll get a message saying that all available nodes (users) have been taken. You must either relinquish an existing reference on another node or create more destination nodes (users).

Q: Can I potentially run into a deadlock when all references are taken and I cannot relinquish any, because when I created the reference field at Content management >> Content types >> Manage fields I configured it to be "required", so it won't let me deselect it?
A: No problem, the modules understand looming deadlock. Say, you have a system with two "company" nodes and two registered users, each referenced from the company nodes through a company contact person reference field. You've realised that in linking the companies to the contacts you swapped the user names, which happen to be required fields. Because of this you wouldn't normally be able to deselect your option. However the modules are clever in detecting required reference fields that have only one option remaining and will unlock the field for deselection by you. This way you are able to swap the references.

Q: If I have nodes or users that are referenced more than once at the time I enable the filter module(s), will second and subsequent references to these nodes and users be automatically curtailed to enforce mutual exclusion?
A: No, you'll have to do this yourself. The modules initially keep any existing selections. Auto-removal would not be a good idea as the modules can't read your mind as to which references are appropriate to clear.

Q: How do I find users that have been referenced, in particular referenced more than once?
A: No convenient solution to this. You could browse all the My account pages, i.e user/% pages (where % is the user id, or name if aliases have been set up). Where referenced, the My account pages display links to the referring nodes under the heading Related content. For this section to appear you must tick the Reverse links checkbox on the Manage fields >> Configure content admin page for the field in question. As an admin (uid=1) you could also switch on the debug info at Site configuration >> Content reference filter and then visit the Edit tab of a node.

Q: How do I find nodes that have been referenced, in particular referenced more than once?
A: The Node Reference Filter module comes with a View, named referenced_node_counts that displays all nodes (i.e. titles) with their reference counts, in descending order. You must have core's optional Search module enabled, as well as Views, of course. For an up-to-date report make sure cron ran recently. You can check when the last cron run was and optionally execute cron manually at /admin/reports/status.
With the canned view backlinks enabled, referring nodes are also listed under the tab labeled What links here (node/%/backlinks) on each node.

Q: Can I implement the functionality provided by this package via a couple of advanced Views slotted into Node Reference and User Reference respectively?
A: Be my guest! The nature of the exclusion problem is such that it cannot easily be tackled with Views in particular with respect to already selected options on the form. Also, any Views solution is likely to be less performant than the dedicated code contained in this package.

Q: How come that unlike the other widgets, the auto-complete widget hasn't been adapted to operate in exclusion mode?
A: CCK's auto-complete widget does not cater for filtering of values in any way. hook_allowed_values() is not available for auto-complete.

Q: If I want to use Node Reference Filter for its Views only, do I still have to enable Content Reference Filter?
A: No, in this case you do not need Content Reference Filter.

Related modules

The following modules, while different, also operate in the space of CCK reference fields:

Supporting organizations: 

Project information