Combined Termref

This module allows you to associate upto 3 vocabularies to a term-reference field. Standard drupal term reference fields allows you to reference terms in a single vocabulary.

Use case

Lets say you have a free tagging field on a node which your editors would like to use to tag content with. They would like to tag content with terms from different vocabularies. For e.g. content could be an article of type Insight, and be also tagged with category philosophy. It could futher be tagged with a personality Freud. In this case, these three are terms from different vocabularies.

Insight - is an Article type term.
Philosophy - is a Category term.
Freud - is an unassociated random Tag.

Article type, Category and Tag are vocabularies.

It makes sense to have a term reference field referencing these vocabularies instead of just one to avoid creation of duplicate terms. In the present case (without multiple vocab term ref), all terms would get stored into the Tag vocabulary. There would also be duplicate terms in the Article type & Category vocabs. This would also lead to term urls like /insight & /insight-0 if you are using pathauto to alias your term pages with the term name.

Usage

1. Create a typical term reference field.
2. In the Field settings, choose the extra vocabularies you would like to reference. You can choose upto 3 vocabularies.
3. Continue with rest of the setup and usage as usual for a term reference field. In most cases you may want to make this field multi-valued
4. Now your field can reference terms from all the chosen vocabularies.

Please note that any new terms added to the term reference field will get added to the first vocabulary specified in the field settings.

Drupal 8

There is a feature request for this functionality put up for Drupal 8 - http://drupal.org/node/1623054. This module is intended to be an alternative until that gets backported (if).