This seems like a bug - I would expect it to work based on your attempted troubleshooting steps and the code.

If I remember correctly, you should be able to work around this by using a FieldQuery with lucene syntax: "field: value" in the query expression. If you take a look at the index in Luke, you can grab the field name for the category.

Edit: I just remembered that the query expression syntax doesn't work as a pass through, that is in Commerce. Ted Nyberg has an example of a CustomFieldQuery to be able to pass in the field you wish to scope your FieldQuery to - the example is in this article as "CustomFieldQuery": http://tedgustaf.com/blog/2013/4/add-custom-fields-to-the-episerver-search-index-with-episerver-7/

Pasted below for redundancy - all credit to Ted.

public class CustomFieldQuery : IQueryExpression
{
    public CustomFieldQuery(string queryExpression, string fieldName)
    {
        Expression = queryExpression;
        Field = fieldName;
        Boost = null;
    }
 
    public CustomFieldQuery(string queryExpression, string fieldName, float boost)
    {
        Expression = queryExpression;
        Field = fieldName;
        Boost = boost;
    }
 
    public string GetQueryExpression()
    {
        return string.Format("{0}:({1}{2})", 
            Field, 
            LuceneHelpers.EscapeParenthesis(Expression),
            Boost.HasValue ? string.Concat("^", Boost.Value.ToString(CultureInfo.InvariantCulture).Replace(",", ".")) : string.Empty );
    }
 
    public string Field { get; set; }
 
    public string Expression { get; set; }
 
    public float? Boost { get; set; }
}

Hope this helps,

/Matt

Thanks you. 

I am aware of the Ted's blog post regarding custom fields in Lucene, and I might use that approch. But I would prefer to use the built in category field query.

I'll reach out to Epi-support and see what they have to say.

When looking at the index in Luke, I noticed that it is the CategoryId that is saved, not the Category value.

I was able to get it working by passing the category ID instead, by using CategoryRepository like so:

var categoryRepository = ServiceLocator.Current.GetInstance<CategoryRepository>();

var myCategory = categoryRepository.Get("Something");

categoryQuery.Items.Add(myCategory.ID.ToString());