Filter and FilterHits

Use filters to narrow down search results, or use them for database-like queries.

There are two extension methods for the ITypeSearch<TSource> class filter: Filter and FilterHits. Both add filters to a search request but do so in different ways.

The Filter method applies a filter to the search query, while FilterHits adds a filter to the search request body. Use the Filter method in most cases, as it filters both the search result and the facets. Use FilterHits when you want to apply a filter to search results (the hits), but not to the facets.

You index three blog posts. Two have the same author.

var post1 = new BlogPost
  {
    Author = new Author {Name = "Agatha Christie"}
  };

var post2 = new BlogPost
  {
    Author = new Author {Name = "Agatha Christie"}
  };

var post3 = new BlogPost
  {
    Author = new Author {Name = "Charles Dickens"}
  };

Next, search for blog posts, filter by author to match the one with two posts, and request a terms facet for the author name. First, use the Filter method; then, the FilterHits method.

var result = client.Search<BlogPost>()
  .Filter(x => x.Author.Name.Match("Agatha Christie"))
  .TermsFacetFor(x => x.Author.Name)
  .GetResult();

var facetItems = result.TermsFacetFor(x => x.Author.Name);
Console.WriteLine("Using Filter");
foreach (var facetItem in facetItems)
  {
    Console.WriteLine(facetItem.Term + " (" + facetItem.Count + ")");
  }

result = client.Search<BlogPost>()
  .FilterHits(x => x.Author.Name.Match("Agatha Christie"))
  .TermsFacetFor(x => x.Author.Name)
  .GetResult();

facetItems = result.TermsFacetFor(x => x.Author.Name);
Console.WriteLine("Using FilterHits");
foreach (var facetItem in facetItems)
  {
    Console.WriteLine(facetItem.Term + " (" + facetItem.Count + ")");
  }

This code produces:

Using Filter
Agatha Christie (2)

Using FilterHits
Agatha Christie (2)
Charles Dickens (1)