Use collectionFormat=multi for MultiChoiceField in query and formData

Fixes #257.

src/drf_yasg/inspectors/field.py

@@ -597,16 +597,22 @@ class ChoiceFieldInspector(FieldInspector):
                     if model_type:
                         enum_type = model_type.get('type', enum_type)
 
+            enum_values = list(field.choices.keys())
             if isinstance(field, serializers.MultipleChoiceField):
-                return SwaggerType(
+                result = SwaggerType(
                     type=openapi.TYPE_ARRAY,
                     items=ChildSwaggerType(
                         type=enum_type,
-                        enum=list(field.choices.keys())
+                        enum=enum_values
                     )
                 )
+                if swagger_object_type == openapi.Parameter:
+                    if result['in'] in (openapi.IN_FORM, openapi.IN_QUERY):
+                        result.collection_format = 'multi'
+            else:
+                result = SwaggerType(type=enum_type, enum=enum_values)
 
-            return SwaggerType(type=enum_type, enum=list(field.choices.keys()))
+            return result
 
         return NotHandled
 

testproj/users/serializers.py

@@ -81,3 +81,4 @@ class UserSerializerrr(serializers.ModelSerializer):
 class UserListQuerySerializer(serializers.Serializer):
     username = serializers.CharField(help_text="this field is generated from a query_serializer", required=False)
     is_staff = serializers.BooleanField(help_text="this one too!", required=False)
+    styles = serializers.MultipleChoiceField(help_text="and this one is fancy!", choices=('a', 'b', 'c', 'd'))

tests/reference.yaml

@@ -740,6 +740,19 @@ paths:
           description: this one too!
           required: false
           type: boolean
+        - name: styles
+          in: query
+          description: and this one is fancy!
+          required: true
+          type: array
+          items:
+            type: string
+            enum:
+              - a
+              - b
+              - c
+              - d
+          collectionFormat: multi
       responses:
         '200':
           description: ''
@@ -1159,6 +1172,7 @@ definitions:
               - fan
               - fancy
               - felix
+              - fennel
               - fish
               - flatline
               - forth
@@ -1184,6 +1198,7 @@ definitions:
               - haskell
               - haxeml
               - hexdump
+              - hlsl
               - hsail
               - html
               - html+cheetah
@@ -1450,6 +1465,7 @@ definitions:
               - xml+php
               - xml+smarty
               - xml+velocity
+              - xorg.conf
               - xquery
               - xslt
               - xtend