Querying: MoreLikeThis

To be able to work, MoreLikeThis requires access to the index text.
The queried index needs, therefore, to store the fields or the term vectors for these fields.

public class Article
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string ArticleBody { get; set; }
}

public class Articles_ByArticleBody : AbstractIndexCreationTask<Article>
{
    public Articles_ByArticleBody()
    {
        Map = docs => from doc in docs
                      select new
                      {
                          doc.ArticleBody
                      };

        Stores.Add(x => x.ArticleBody, FieldStorage.Yes);
        Analyzers.Add(x => x.ArticleBody, "StandardAnalyzer");
    }
}

Many MoreLikeThis options are set by default.
The simplest mode will satisfy most usage scenarios.

List<Article> articles = session
    .Query<Article, Articles_ByArticleBody>()
    .MoreLikeThis(builder => builder
        .UsingDocument(x => x.Id == "articles/1"))
    .ToList();

List<Article> articles = session.Advanced
    .DocumentQuery<Article, Articles_ByArticleBody>()
    .MoreLikeThis(builder => builder
        .UsingDocument(x => x.WhereEquals(y => y.Id, "articles/1")))
    .ToList();

from index 'Articles/ByArticleBody' 
where morelikethis(id() = 'articles/1')

MoreLikeThis will use all the fields defined in an index.
To use only specific fields, pass these fields in the MoreLikeThisOptions.Fields property.

List<Article> articles = session
    .Query<Article, Articles_ByArticleBody>()
    .MoreLikeThis(builder => builder
        .UsingDocument(x => x.Id == "articles/1")
        .WithOptions(new MoreLikeThisOptions
        {
            Fields = new[] { nameof(Article.ArticleBody) }
        }))
    .ToList();

List<Article> articles = session.Advanced
    .DocumentQuery<Article, Articles_ByArticleBody>()
    .MoreLikeThis(builder => builder
        .UsingDocument(x => x.WhereEquals(y => y.Id, "articles/1"))
        .WithOptions(new MoreLikeThisOptions
        {
            Fields = new[] { nameof(Article.ArticleBody) }
        }))
    .ToList();

from index 'Articles/ByArticleBody' 
where morelikethis(id() = 'articles/1', '{ "Fields" : [ "ArticleBody" ] }')

Default parameters can be changed by manipulating MoreLikeThisOptions properties and passing them to MoreLikeThis.

Some Lucene analyzers have a built-in list of common English words that are usually not useful for searching, like "a", "as", "the", etc.
These words, called stop words, are considered uninteresting and are ignored.
If a used analyzer does not support stop words, or you need to overload these terms, you can specify your own set of stop words.
A document with a list of stop words can be stored in RavenDB by storing the MoreLikeThisStopWords document:

session.Store(new MoreLikeThisStopWords
{
    Id = "Config/Stopwords",
    StopWords = new List<string> { "I", "A", "Be" }
});

The document ID will then be set in the MoreLikeThisOptions.