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50_One_language_per_field.asciidoc

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One Language per Field

For documents that represent entities like products, movies, or legal notices, it is common for the same text to be translated into several languages. Although each translation could be represented in a single document in an index per language, another reasonable approach is to keep all translations in the same document:

{
   "title":     "Fight club",
   "title_br":  "Clube de Luta",
   "title_cz":  "Klub rváčů",
   "title_en":  "Fight club",
   "title_es":  "El club de la lucha",
   ...
}

Each translation is stored in a separate field, which is analyzed according to the language it contains:

PUT /movies
{
  "mappings": {
    "movie": {
      "properties": {
        "title": { (1)
          "type":       "string"
        },
        "title_br": { (2)
            "type":     "string",
            "analyzer": "brazilian"
        },
        "title_cz": { (2)
            "type":     "string",
            "analyzer": "czech"
        },
        "title_en": { (2)
            "type":     "string",
            "analyzer": "english"
        },
        "title_es": { (2)
            "type":     "string",
            "analyzer": "spanish"
        }
      }
    }
  }
}

  1. The title field contains the original title and uses the standard analyzer.

  2. Each of the other fields uses the appropriate analyzer for that language.

Like the index-per-language approach, the field-per-language approach maintains clean term frequencies. It is not quite as flexible as having separate indices. Although it is easy to add a new field by using the update-mapping API, those new fields may require new custom analyzers, which can only be set up at index creation time. As a workaround, you can {ref}/indices-open-close.html[close] the index, add the new analyzers with the {ref}/indices-update-settings.html[update-settings API], then reopen the index, but closing the index means that it will require some downtime.

The documents of a single language can be queried independently, or queries can target multiple languages by querying multiple fields. We can even specify a preference for particular languages by boosting that field:

GET /movies/movie/_search
{
    "query": {
        "multi_match": {
            "query":    "club de la lucha",
            "fields": [ "title*", "title_es^2" ], (1)
            "type":     "most_fields"
        }
    }
}

  1. This search queries any field beginning with title but boosts the title_es field by 2. All other fields have a neutral boost of 1.