(No version information available, might only be in Git)
Collapses the result set to a single document per group
public SolrQuery SolrQuery::collapse ( SolrCollapseFunction $collapseFunction )
Collapses the result set to a single document per group before it forwards the result set to the rest of the search components.
So all downstream components (faceting, highlighting, etc...) will work with the collapsed result set.