Terms facets

Facets are used for grouping documents based on some criteria, for example specific terms or date range. 

Term facets group terms, words, or whole strings, and provide a count for each. To retrieve a term facet using the fluent API, use the TermsFacetFor and TermsFacetForWordsIn methods.

• The TermsFacetFor method works on exact values in strings in a case sensitive manner. So, "Foo", "foo," and "foo bar" are treated as three different terms.
• The TermsFacetForWordsIn method works on analyzed values of strings in a case-insensitive manner. So, it treats each word in a string a separate term. With this method "Foo", "foo," and "foo bar" are treated as two terms, "foo" and "bar".

Both methods work on values of type string and IEnumerable of string. Also, both have an overload that lets you specify the number of terms to return. Unless specified, 10 is the default number of items returned for a facet. To return more items, use x.Size as in the example below.

GetDealersQuery()
  .TermsFacetFor(x => x.CountiesList, x => x.Size = 20)
  .Take(0)
  .GetPagesResult();

Before providing examples, we need to provide a context. Assume there is a class with a string property, a list of strings property, and a property of a complex type that also has a string property.

public class Book
  {
    public string Title { get; set; }
    public string Author { get; set; }
    public List<string> Tags { get; set; }
  }

public class Author
  {
    public string Name { get; set; }
  }

Assume you indexed three instances of this class with the following values:

Retrieve a tag cloud

Use the TermsFacetFor method to retrieve a list of the most common tags in a search request.

var searchResults = client.Search<Book>()
  .TermsFacetFor(x => x.Tags)
  .Take(0)
  .GetResult();

📘

Note

Use the Take method to exclude actual search results, since you are not interested in them in this scenario.

After getting the search results, you can extract the terms facet for the tags.

var tagCounts = searchResults
  .TermsFacetFor(x => x.Tags).Terms;
foreach(var tagCount in tagCounts)
  {
    string tag = tagCount.Term;
    int count = tagCount.Count;
    Console.WriteLine(tag + ": " + count);
  }

The above code prints:
fiction: 2
crime: 1
science: 1
scifi: 1

Group authors

If you modify the code to instead retrieve a terms facet for author name, the code would look like this.

var searchResults = client.Search<Book>()
  .TermsFacetFor(x => x.Author.Name)
  .Take(0)
  .GetResult();

var authorCounts = searchResults
  .TermsFacetFor(x => x.Author.Name).Terms;
foreach(var authorCount in authorCounts)
  {
    string authorName = authorCount.Term;
    int count = authorCount.Count;
    Console.WriteLine(authorName + ": " + count);
  }

The above code prints:
Agatha Christie: 1
Charles Darwin: 1
Charles Dickens: 1

Group by single words

In contrast, if you used the TermsFacetForWordsIn method (instead of the TermsFacetFor method) in the above code, it prints:
charles: 2
agatha: 1
christie: 1
darwin: 1
dickens: 1