Lucene.net contains only last added document

It's a weird problem but every time I add a new document to Lucene.net it overrides the last one and thus it always holds the last inserted document. I've confirmed this behavior using LUKE which lets me open the index files. I'd appreciate if someone could shed light on the problem. Here is my code:

public class SearchService : ISearchService
{
    Directory indexFileLocation;
    Analyzer analyzer;

    public SearchService(String indexLocation)
    {
        indexFileLocation = FSDirectory.GetDirectory(indexLocation, true);
        analyzer = new StandardAnalyzer();
    }

    public void AddToSearchIndex(ISearchableData data)
    {
        IndexWriter indexWriter = new IndexWriter(indexFileLocation, analyzer, true);
        Document    doc         = new Document();

        foreach (var entry in data)
        {
            Field field = new Field(
                entry.Key, 
                entry.Value, 
                Lucene.Net.Documents.Field.Store.NO, 
                Lucene.Net.Documents.Field.Index.TOKENIZED, 
                Lucene.Net.Documents.Field.TermVector.WITH_POSITIONS_OFFSETS);

            doc.Add(field);
        }

        Field keyField = new Field(
            SearchField.Key.ToString(), 
            data.Key, 
            Lucene.Net.Documents.Field.Store.YES, 
            Lucene.Net.Documents.Field.Index.UN_TOKENIZED);

        doc        .Add(keyField);
        indexWriter.AddDocument(doc);
        indexWriter.Optimize();
        indexWriter.Close();
    }

    public IDictionary<Int32, float> SearchContent(String term)
    {
        IndexSearcher searcher = new IndexSearcher(indexFileLocation);
        TermQuery     query = new TermQuery(new Term(SearchField.Content.ToString(), term));
        Hits          hits = searcher.Search(query);
        searcher.Close();

        return OrganizeSearchResults(hits);
    }

    public IDictionary<Int32, float> OrganizeSearchResults(Hits hits)
    {
        IDictionary<Int32, float> result = new Dictionary<Int32, float>();
        String keyField = SearchField.Key.ToString();

        for (int i = 0; i < hits.Length(); i++)
        {
            Document doc = hits.Doc(i);
            Field field = doc.GetField(keyField);
            result.Add(Int32.Parse(
                field.StringValue()),
                hits.Score(i));
        }

        return result;
    }
}

I add documents like this:

new SearchService(searchInde开发者_如何转开发xFolderPath).AddToSearchIndex(entry.ToSearchableData());

and search for it like this:

ISearchService search = new SearchService(MvcApplication.SearchIndexPath);
IList<Int32> submissionIds = search.SearchContent(SearchTerm).Select(hit => hit.Key).ToList<Int32>();

The true here:

new IndexWriter(indexFileLocation, analyzer, true);

tells Lucene to create a new index, deleting the old one.