Solr multi-select facets occurs bad request error I faced with a weird problem using apache solr faceting search. While selecting two facet constraints, solr respond with 400 error:


  "400" Status: null: null HTTP ERROR 400
  
  Problem accessing /solr/project/select. Reason:
  
  null


Basically I have one index configured and number of fields indexed inside. I'm using facet for "content type" field. There is no problem at all when only one facet constraint is selected. 


  http://localhost:8983/solr/project/select?fl=item_id,score&amp;fq={!tag=facet:type}ss_type:"system"&amp;start=0&amp;rows=10&amp;sort=ds_created
  desc&amp;facet=true&amp;facet.sort=count&amp;facet.limit=10&amp;facet.mincount=1&amp;facet.missing=false&amp;facet.field={!ex=facet:type}ss_type&amp;f.ss_type.facet.limit=50&amp;wt=json&amp;json.nl=map&amp;q.alt=(ss_search_api_language:"sv")
  (index_id:"project_nodes_index") (hash:gf1tmd)


Problem starts where two constraints are selected:


  http://localhost:8983/solr/project/select?fl=item_id,score&amp;start=0&amp;rows=10&amp;sort=ds_created
  desc&amp;facet=true&amp;facet.sort=count&amp;facet.limit=10&amp;facet.mincount=1&amp;facet.missing=false&amp;facet.field={!ex=facet:type}ss_type&amp;f.ss_type.facet.limit=50&amp;wt=json&amp;json.nl=map&amp;q.alt=(ss_search_api_language:"sv")
  ((({!tag=facet:type}ss_type:"system") OR
  ({!tag=facet:type}ss_type:"product")))
  (index_id:"project_nodes_index") (hash:gf1tmd)


Standart modules are used:
* search_api_solr
* search_api
* facetapi

Solr version 3.6.2
I think you can update to the latest dev version.

For more info you could check this issue:

https://www.drupal.org/node/2290601