Avoid ChoiceField duplicate enum values for allow_null, allow_blank

drf_spectacular/plumbing.py

@@ -410,9 +410,9 @@ def build_choice_field(field):
     else:
         type = None
 
-    if field.allow_blank:
+    if field.allow_blank and '' not in choices:
         choices.append('')
-    if field.allow_null:
+    if field.allow_null and None not in choices:
         choices.append(None)
 
     schema = {

tests/test_plumbing.py

@@ -20,8 +20,9 @@
 
 from drf_spectacular.openapi import AutoSchema
 from drf_spectacular.plumbing import (
-    analyze_named_regex_pattern, build_basic_type, detype_pattern, follow_field_source,
-    force_instance, get_list_serializer, is_field, is_serializer, resolve_type_hint,
+    analyze_named_regex_pattern, build_basic_type, build_choice_field, detype_pattern,
+    follow_field_source, force_instance, get_list_serializer, is_field, is_serializer,
+    resolve_type_hint,
 )
 from drf_spectacular.validation import validate_schema
 from tests import generate_schema
@@ -358,3 +359,21 @@ def test_analyze_named_regex_pattern(no_warnings, pattern, output):
 def test_unknown_basic_type(capsys):
     build_basic_type(object)
     assert 'could not resolve type for "<class \'object\'>' in capsys.readouterr().err
+
+
+def test_choicefield_choices_enum():
+    schema = build_choice_field(serializers.ChoiceField(['bluepill', 'redpill']))
+    assert schema['enum'] == ['bluepill', 'redpill']
+    assert schema['type'] == 'string'
+
+    schema = build_choice_field(serializers.ChoiceField(
+        ['bluepill', 'redpill'], allow_null=True, allow_blank=True
+    ))
+    assert schema['enum'] == ['bluepill', 'redpill', '', None]
+    assert schema['type'] == 'string'
+
+    schema = build_choice_field(serializers.ChoiceField(
+        choices=['bluepill', 'redpill', '', None], allow_null=True, allow_blank=True
+    ))
+    assert schema['enum'] == ['bluepill', 'redpill', '', None]
+    assert 'type' not in schema