How to update SOLR document? This question is related to https://drupal.stackexchange.com/questions/193603/solr-views-flag-newly-flagged-users-do-not-appear-in-search-results

Because previous question can be difficult to understand I'm trying to simplify things.

If I understand SOLR - Drupal integration well, every entity is represents as Document in SOLR. 

How to invoke update of document for specified entity?

List of modules that I'm using for search page (a View)

dependencies[] = date_facets
dependencies[] = facetapi
dependencies[] = facetapi_bonus
dependencies[] = search_api
dependencies[] = search_api_facetapi
dependencies[] = search_api_location_views
dependencies[] = search_api_solr


I need to update SOLR document when user flag another user (Flag module).
If you're writing code:

search_api_track_item_change('entity_type', array($entity_id));


That will mark an entity as 'dirty', and it'll be (re-)indexed on the next run.

If you're not writing code, install the rules module, add a rule with an event of "A flag type has been flagged...", and an action of type "Index an entity", with the relevant parameters.