http://stackoverflow.com/questions/20317626/restrict-multi-field-facet-calculation-to-subset-of-possible-values

gistfile1.txt

### SCHEMA

<schema name="elogging" version="1.5">
  <fields>
    <field name="id" type="string" indexed="true" stored="true" required="true" multiValued="false"/>
    <field name="title" type="string" indexed="true" stored="true" required="true" />
    <field name="category" type="int" indexed="true" stored="true" required="true" multiValued="true"/>
    <field name="_version_" type="long" indexed="true" stored="true"/>
  </fields>
  <uniqueKey>id</uniqueKey>
  <types>
    <fieldType name="date" class="solr.TrieDateField" precisionStep="0" positionIncrementGap="0"/>
    <fieldType name="string" class="solr.StrField" sortMissingLast="true"/>
    <fieldType name="long" class="solr.TrieLongField" precisionStep="0" positionIncrementGap="0"/>
    <fieldType name="int" class="solr.TrieIntField" precisionStep="0" positionIncrementGap="0"/>
  </types>
</schema>


### DATA

{
  "responseHeader": {
    "status": 0,
    "QTime": 1,
    "params": {
      "q": "*:*",
      "indent": "true",
      "wt": "json",
      "_": "1386505576524"
    }
  },
  "response": {
    "numFound": 4,
    "start": 0,
    "docs": [
      {
        "category": [
          1,
          2
        ],
        "title": "test",
        "id": "1",
        "_version_": 1453856367234252800
      },
      {
        "category": [
          2,
          3
        ],
        "title": "Doc from cat 2,3",
        "id": "2",
        "_version_": 1453856424077557800
      },
      {
        "category": [
          7
        ],
        "title": "Doc from cat 7",
        "id": "3",
        "_version_": 1453856462346387500
      },
      {
        "category": [
          8
        ],
        "title": "Doc from cat 8",
        "id": "4",
        "_version_": 1453856469762965500
      }
    ]
  }
}