IndexSampleOrders.cs

public class IndexSampleOrders : ContactIndexableLoadFieldsProcessor
{
    protected override IEnumerable<IIndexableDataField> GetFields(ContactIndexableLoadFieldsPipelineArgs args)
    {
        IContact contact = args.Contact;
           
        var keyInteractionsFacet = contact.GetFacet<IKeyInteractionsFacet>(KeyInteractionsFacet.FacetName);

        var fields = new List<IIndexableDataField>();
        if (keyInteractionsFacet != null)
        {
            IndexableDataField<string> favouriteSampleTypeField = CreateFavouriteSampleTypeField(keyInteractionsFacet.SamplesOrdered);
            IndexableDataField<string[]> samplesOrderedField = CreateSamplesOrderedField(keyInteractionsFacet.SamplesOrdered);

            if (favouriteSampleTypeField != null)
            {
                fields.Add(favouriteSampleTypeField);
            }
            if (samplesOrderedField != null)
            {
                fields.Add(samplesOrderedField);
            }
        }

        return fields;
    }

    private IndexableDataField<string> CreateFavouriteSampleTypeField(IElementCollection<ISampleOrder> samplesOrdered)
    {
        string favouriteSampleType = samplesOrdered.GroupBy(s => s.Type)
            .OrderByDescending(gp => gp.Count())
            .Select(g => g.Key)
            .FirstOrDefault();

        IndexableDataField<string> favouriteSampleTypeField = null;
        if (favouriteSampleType != null)
        {
            favouriteSampleTypeField = new IndexableDataField<string>("contact.SampleOrder.FavouriteType", favouriteSampleType);
        }

        return favouriteSampleTypeField;
    }

    private IndexableDataField<string[]> CreateSamplesOrderedField(IElementCollection<ISampleOrder> samplesOrdered)
    {
        List<string> skus = (from s in samplesOrdered
            select s.Sku).Distinct().ToList();

        IndexableDataField<string[]> samplesOrderedField = null;
        if (skus.Any())
        {
            samplesOrderedField = new IndexableDataField<string[]>("contact.SampleOrder.Skus", skus.ToArray());
        }

        return samplesOrderedField;
    }
}