FieldMaskingSpanQuery

Wrapper to allow SpanQuery objects participate in composite single-field SpanQueries by 'lying' about their search field. That is, the masked SpanQuery will function as normal, but SpanQuery.getField simply hands back the value supplied in this class's constructor.

This can be used to support Queries like [SpanNearQuery] or [SpanOrQuery] across different fields, which is not ordinarily permitted.

This can be useful for denormalized relational data: for example, when indexing a document with conceptually many 'children':

teacherid: 1
studentfirstname: james
studentsurname: jones

teacherid: 2
studenfirstname: james
studentsurname: smith
studentfirstname: sally
studentsurname: jones

a SpanNearQuery with a slop of 0 can be applied across two [SpanTermQuery] objects as follows:

    SpanQuery q1  = new SpanTermQuery(new Term("studentfirstname", "james"));
    SpanQuery q2  = new SpanTermQuery(new Term("studentsurname", "jones"));
    SpanQuery q2m = new FieldMaskingSpanQuery(q2, "studentfirstname");
    Query q = new SpanNearQuery(new SpanQuery[]{q1, q2m}, -1, false);

to search for 'studentfirstname:james studentsurname:jones' and find teacherid 1 without matching teacherid 2 (which has a 'james' in position 0 and 'jones' in position 1).

Note: as [getField] returns the masked field, scoring will be done using the Similarity and collection statistics of the field name supplied, but with the term statistics of the real field. This may lead to exceptions, poor performance, and unexpected scoring behaviour.