Unit testing contentsearch with facets

Question

I am trying to write a test for this code:

var publicationYears = new List<string>();
using (var context = searchContextBuilder.GetSearchContext(Indexes.NewsIndex))
{
    var query = context.GetQueryable<NewsArticleItem>().Where(predicate).FacetOn(n => n.Year);
    var queryResults = query.GetResults();
    ...
    publicationYears = GetYearFacets(queryResults.Facets).ToList();
}

return publicationYears;
}

The "searchContextBuilder" is injected, the "predicate" a parameter and "GetYearFacets" just a function that get the values from the facet needed.

I tried some solution (from Kevin Brechbühl) that should work, but apparently only do if the tests is in the same assembly as the code - which is not the case. So I started using the this code: https://gist.github.com/vivianroberts/1a632d8bfb8fa62e23c4679b086728ce which gave me working tests on searches without facets. In my test, I inject a "testable" version of the searchContextBuilder (using Moq):

public class TestableSearchBuilder : ISearchContextBuilder
{
    private readonly IList<NewsArticleItem> items;

    public TestableSearchBuilder()
    {
        items = new List<NewsArticleItem>();

        items.Add(...);
        items.Add(...);
        ...
    }

    public IEnumerable<NewsArticleItem> Items => items;

    public IProviderSearchContext GetSearchContext(string index)
    {
        // create the mock context
        var searchContext = new Mock<IProviderSearchContext>();
        var queryable = new SearchProviderQueryableCollection<NewsArticleItem>(items);
        searchContext.Setup(x => x.GetQueryable<NewsArticleItem>()).Returns(queryable);
        searchContext.Setup(x => x.GetQueryable<NewsArticleItem>(It.IsAny<IExecutionContext>())).Returns(queryable);
        return searchContext.Object;
    }
}

where my "SearchProviderQueryableCollection" is the "LuceneProviderQueryableStub" from the Gist.

The problem is that when using FacetOn an error occurs:

There is no method 'FacetOn' on type 'Sitecore.ContentSearch.Linq.QueryableExtensions.

Same error as Søren mentions on the page btw.. (with Filter).

I did try to get past this by putting the FacetOn also on my injected searchContextBuilder - pointing it towards Sitecores extension method in the real code and to a custom linq solution in the test. That got me a step further, but then I bumped into the call to "Facets" on the SearchResults object. In my test it was null as the facets are not passed to the SearchResults constructor. To fix that, I should include them as extra parameter in the constructor in my SearchProviderQueryableCollection:

object results = new SearchResults<TElement>(items.Select(s => new SearchHit<TElement>(0, s)), items.Count, ...);

which might be possible, but then I need to be able to construct the facets myself. And there I got stuck.. and running out of time.. and actually hoping someone already did this..

So, does anyone have an idea on how to continue getting the facets in my test - or has an alternative solution to testing queries with facets?