ElasticSearch 5 Sort by Keyword Field Case Insensitive

Yes, there is a way to do this, using multi_fields.

In Elasticsearch 5.0 onwards, string field types were split out into two separate types, text field types that are analyzed and can be used for search, and keyword field types that are not analyzed and are suited to use for sorting, aggregations and exact value matches.

With dynamic mapping in Elasticsearch 5.0 (i.e. let Elasticsearch infer the type that a document property should be mapped to), a json string property is mapped to a text field type, with a sub-field of "keyword" that is mapped as a keyword field type and the setting ignore_above:256.

With NEST 5.x automapping, a string property on your POCO will be automapped in the same way as dynamic mapping in Elasticsearch maps it as per above e.g. given the following document

public class Document{    public string Property { get; set; }}

automapping it

var pool = new SingleNodeConnectionPool(new Uri("http://localhost:9200"));var defaultIndex = "default-index";var connectionSettings = new ConnectionSettings(pool)        .DefaultIndex(defaultIndex);var client = new ElasticClient(connectionSettings);client.CreateIndex(defaultIndex, c => c    .Mappings(m => m        .Map<Document>(mm => mm            .AutoMap()        )    ));

produces

{  "mappings": {    "document": {      "properties": {        "property": {          "fields": {            "keyword": {              "ignore_above": 256,              "type": "keyword"            }          },          "type": "text"        }      }    }  }}

You can now use property for sorting using Field(f => f.Property.Suffix("keyword"). Take a look at Field Inference for more examples.

keyword field types have doc_values enabled by default, which means that a columnar data structure is built at index time and this is what provides efficient sorting and aggregations.

To add a custom analyzer at index creation time, we can automap as before, but then provide overrides for fields that we want to control the mapping for with .Properties()

client.CreateIndex(defaultIndex, c => c    .Settings(s => s        .Analysis(a => a            .Analyzers(aa => aa                .Custom("lowercase_analyzer", ca => ca                    .Tokenizer("keyword")                    .Filters(                        "standard",                        "lowercase",                        "trim"                    )                )            )        )    )    .Mappings(m => m        .Map<Document>(mm => mm            .AutoMap()            .Properties(p => p                .Text(t => t                    .Name(n => n.Property)                    .Analyzer("lowercase_analyzer")                    .Fields(f => f                        .Keyword(k => k                            .Name("keyword")                            .IgnoreAbove(256)                        )                    )                )            )        )    ));

which produces

{  "settings": {    "analysis": {      "analyzer": {        "lowercase_analyzer": {          "type": "custom",          "filter": [            "standard",            "lowercase",            "trim"          ],          "tokenizer": "keyword"        }      }    }  },  "mappings": {    "document": {      "properties": {        "property": {          "type": "text",          "fields": {            "keyword": {              "type": "keyword",              "ignore_above": 256            }          },          "analyzer": "lowercase_analyzer"        }      }    }  }}