Add ability to configure NGRAM's

README.rst

@@ -92,6 +92,8 @@ The backend has the following optional settings:
   See `here <http://xapian.org/docs/apidoc/html/classXapian_1_1QueryParser.html#ac7dc3b55b6083bd3ff98fc8b2726c8fd>`__ for
   more information about the different strategies.
 
+- ``XAPIAN_NGRAM_MIN_LENGTH``, ``XAPIAN_NGRAM_MAX_LENGTH``: options for custom configuration of ngrams (phrases) length.
+
 - ``HAYSTACK_XAPIAN_USE_LOCKFILE``: Use a lockfile to prevent database locking errors when running management commands with multiple workers.
   Defaults to `True`.
 

xapian_backend.py

@@ -19,8 +19,8 @@ from haystack.inputs import AutoQuery
 from haystack.models import SearchResult
 from haystack.utils import get_identifier, get_model_ct
 
-NGRAM_MIN_LENGTH = 2
-NGRAM_MAX_LENGTH = 15
+NGRAM_MIN_LENGTH = getattr(settings, 'XAPIAN_NGRAM_MIN_LENGTH', 2)
+NGRAM_MAX_LENGTH = getattr(settings, 'XAPIAN_NGRAM_MAX_LENGTH', 15)
 
 try:
     import xapian