Resolve #39 -- Make default search more like Django admin

Different to before we now match search terms only if all
bits match or the entire statement. The last part differs
from the behavior in Django admin, which does not inlcude
exact machtes of the entire search term inlcude spaces.

Partially revert 07054b2

django_select2/forms.py

@@ -46,12 +46,14 @@
     :parts: 1
 
 """
-import re
+import operator
 import uuid
+from functools import reduce
 from itertools import chain
 from pickle import PicklingError  # nosec
 
 from django import forms
+from django.contrib.admin.utils import lookup_needs_distinct
 from django.contrib.admin.widgets import SELECT2_TRANSLATIONS
 from django.core import signing
 from django.db.models import Q
@@ -315,8 +317,6 @@ class HeavySelect2TagWidget(HeavySelect2Mixin, Select2TagWidget):
 class ModelSelect2Mixin:
     """Widget mixin that provides attributes and methods for :class:`.AutoResponseView`."""
 
-    _word_split_pattern = re.compile(r"\t|\n| ")
-
     model = None
     queryset = None
     search_fields = []
@@ -400,19 +400,24 @@ def filter_queryset(self, request, term, queryset=None, **dependent_fields):
         search_fields = self.get_search_fields()
         select = Q()
 
-        if term != "":
-            for field in search_fields:
-                field_select = Q(**{field: term})
-                if "contains" in field:
-                    for word in filter(None, self._word_split_pattern.split(term)):
-                        field_select |= Q(**{field: word})
-
-                select |= field_select
+        use_distinct = False
+        if search_fields and term:
+            for bit in term.split():
+                or_queries = [Q(**{orm_lookup: bit}) for orm_lookup in search_fields]
+                select &= reduce(operator.or_, or_queries)
+            or_queries = [Q(**{orm_lookup: term}) for orm_lookup in search_fields]
+            select |= reduce(operator.or_, or_queries)
+            use_distinct |= any(
+                lookup_needs_distinct(queryset.model._meta, search_spec)
+                for search_spec in search_fields
+            )
 
         if dependent_fields:
             select &= Q(**dependent_fields)
 
-        return queryset.filter(select).distinct()
+        if use_distinct:
+            queryset.filter(select).distinct()
+        return queryset.filter(select)
 
     def get_queryset(self):
         """

tests/test_forms.py

@@ -482,7 +482,7 @@ def test_filter_queryset__contains(self, genres):
         assert qs.exists()
 
         qs = widget.filter_queryset(None, "NOT Gen")
-        assert qs.exists(), "contains works even if only one part matches"
+        assert not qs.exists(), "contains works even if all bits match"
 
     def test_filter_queryset__multiple_fields(self, genres):
         genre = Genre.objects.create(title="Space Genre")