[kerem-acer]

Closes #83.

SearchField.String(analyzer, searchAnalyzer) couldn't declare multi: the alternate analyzers MongoDB Search also indexes a string with, and which a query picks by name with path: { value, multi }. A field that needed one had to be written whole with SearchField.Raw, or the whole index with Definition(BsonDocument). That's what keeps core's 12 search indexes on raw BSON: 134 of their 135 string fields carry a keyword alternate for wildcard.

What it does

• SearchField.Multi(name, alternate) returns the text mapping with one more alternate. Each call adds one, so a field can have several, and the mapping it's called on is left as it was:

  products.SearchIndex("default", index => index
      .Analyzer("lucene.turkish")
      .Map(x => x.Name,
          SearchField.String("lucene.turkish").Multi("keyword", SearchField.String("lucene.keyword")),
          SearchField.Autocomplete(2, 20),
          SearchField.Token()));

  It renders multi: { keyword: { type: "string", analyzer: "lucene.keyword" } } on the string mapping, exactly as core writes it by hand. A declaration moved from Definition to Map then matches the index already on the server, and nothing is rebuilt. Comparing listed definitions already handled multi, defaults filled into alternates included, so that needed no change.

• What MongoDB Search refuses, Multi refuses up front with ArgumentException. I checked each case against Atlas Local 8.3.4:

  Case	Server's error
  multi on a non-text mapping	unrecognized field "multi"
  A non-text alternate	multi definition must be string field
  An alternate with alternates of its own	multi definition cannot contain nested multi definition

  Multi also throws for a name already taken or a blank one.

docs/search.md shows Multi under the SearchField table, with a wildcard that picks the alternate through SearchPath.Analyzer.

Tests

SearchTests (integration, on Atlas Local and community-search):

• MigrateAllAsync_TextWithAnAlternateAsDeclared_LeavesItForSearchesToPick:
  • the second migration logs both indexes as defined as declared;
  • a wildcard of Ada L* on the keyword alternate finds Ada Lovelace;
  • each server's listing is verified: Atlas Local fills the alternate's defaults in, community-search lists it as created.

SearchFieldTests (unit):

• Multi_TextAlternates_AreWrittenUnderTheTextsMulti: two alternates, and one added to a Raw mapping that has one already.
• Multi_OfAMapping_LeavesItAsItWas
• Multi_AlternateMongoDbCantTake_ThrowsArgumentException, which covers:
  • a token mapping;
  • a token alternate;
  • an alternate with alternates;
  • a name taken;
  • a blank name.
• Multi_NullAlternate_ThrowsArgumentNullException

SearchIndexBuilderTests (unit):

• SearchIndex_TextWithAnAlternate_IsMappedWithItBesideTheFieldsOtherMappings: the issue's example renders with the field's autocomplete and token mappings, and the features' fields.

Ran locally on net10.0; CI runs net11.0 as well:

• unit lane: all pass except the two QueryAsync_JoinOn*_IsLeftToTheDriver tests, which fail only locally, against the InsurUp driver fork;
• SearchTests: all 48 pass on both servers.

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