Span field masking query

Wrapper to allow span queries to participate in composite single-field span queries by lying about their search field. The span field masking query maps to Lucene’s SpanFieldMaskingQuery

This can be used to support queries like span-near or span-or across different fields, which is not ordinarily permitted.

Span field masking query is invaluable in conjunction with multi-fields when same content is indexed with multiple analyzers. For instance we could index a field with the standard analyzer which breaks text up into words, and again with the english analyzer which stems words into their root form.

Example:

response = client.search(
  body: {
    query: {
      span_near: {
        clauses: [
          {
            span_term: {
              text: 'quick brown'
            }
          },
          {
            span_field_masking: {
              query: {
                span_term: {
                  'text.stems' => 'fox'
                }
              },
              field: 'text'
            }
          }
        ],
        slop: 5,
        in_order: false
      }
    }
  }
)
puts response

GET /_search
{
  "query": {
    "span_near": {
      "clauses": [
        {
          "span_term": {
            "text": "quick brown"
          }
        },
        {
          "span_field_masking": {
            "query": {
              "span_term": {
                "text.stems": "fox"
              }
            },
            "field": "text"
          }
        }
      ],
      "slop": 5,
      "in_order": false
    }
  }
}

Note: as span field masking query returns the masked field, scoring will be done using the norms of the field name supplied. This may lead to unexpected scoring behaviour.