I have a document like:

```
{
"Id":"1",
"Name":"product1",      
"Categories":["Cat1",
  "Cat2",
  "Cat3"]
},
{
"Id":"2",
"Name":"product2",      
"Categories":["Cat3",
  "Cat2",
  "Cat6"]
}

```

Now I want return a distinct List of all categories.
I tried CollapseParameter, but it doesn't work.

```
var foo = _solr.Query(new SolrQuery("*"), new QueryOptions
{
    Collapse = new CollapseParameters("Categories")
});

var bar= results.CollapseExpand; //NULL

```

How can I get a list of all categories without iterating through all the documents? Do I have to create new Documents for the categories? Or should I work with Faceting?