Update specific field on SOLR index

Solr does not support updating individual fields yet, but there is a JIRA issue about this (almost 3 years old as of this writing).

Until this is implemented, you have to update the whole document.

UPDATE: as of Solr 4+ this is implemented, here's the documentation.

Please refer to this document about the "Partial Documents Update" Feature in Solr 4.0

Solr 4.0 is now final and production-ready.

This feature makes it possible to update fields and even adding values to multiValued fields.

Mauricio was right with his answer back in 2010, but this is the way things are today.