fix: support non-indexed deepObject array unmarshaling (#22) (#96)

OpenAPI 3.0's deepObject style has undefined behavior for arrays.
Two conventions exist in the wild:

- Indexed: p[vals][0]=a&p[vals][1]=b (oapi-codegen's current format)
- Non-indexed: p[vals]=a&p[vals]=b (Swagger UI / Rails convention)

Swagger UI generates the non-indexed format, which previously failed
with "[field] has multiple values". This change makes UnmarshalDeepObject
accept both formats by detecting repeated query parameter keys and
expanding them into synthetic indexed entries (e.g. [vals][0], [vals][1])
before feeding them into the existing tree-construction and assignment
logic.

Marshaling (MarshalDeepObject) intentionally remains unchanged and
continues to emit the indexed format. The indexed format is
unambiguous, already consumed correctly by all known implementations,
and is what oapi-codegen's own generated clients expect. Changing it
to the non-indexed format would be a breaking change for consumers
that rely on the current wire format, with no practical benefit since
both formats are now accepted on the unmarshaling side.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mromaszewicz

deepobject.go

@@ -136,11 +136,16 @@ func UnmarshalDeepObject(dst interface{}, paramName string, params url.Values) e
 		if strings.HasPrefix(pName, searchStr) {
 			// trim the parameter name from the full name.
 			pName = pName[len(paramName):]
-			fieldNames = append(fieldNames, pName)
-			if len(pValues) != 1 {
-				return fmt.Errorf("%s has multiple values", pName)
+			if len(pValues) == 1 {
+				fieldNames = append(fieldNames, pName)
+				fieldValues = append(fieldValues, pValues[0])
+			} else {
+				// Non-indexed array format: expand repeated keys into indexed entries
+				for i, value := range pValues {
+					fieldNames = append(fieldNames, pName+"["+strconv.Itoa(i)+"]")
+					fieldValues = append(fieldValues, value)
+				}
 			}
-			fieldValues = append(fieldValues, pValues[0])
 		}
 	}
 

deepobject_test.go

@@ -206,3 +206,38 @@ func TestDeepObject_ArrayOfObjects(t *testing.T) {
 	assert.Equal(t, "second", dstArray[1].Name)
 	assert.Equal(t, "value2", dstArray[1].Value)
 }
+
+func TestDeepObject_NonIndexedArray(t *testing.T) {
+	t.Run("primitive string array", func(t *testing.T) {
+		params := url.Values{}
+		params.Add("p[vals]", "a")
+		params.Add("p[vals]", "b")
+
+		type Obj struct {
+			Vals []string `json:"vals"`
+		}
+
+		var dst Obj
+		err := UnmarshalDeepObject(&dst, "p", params)
+		require.NoError(t, err)
+		assert.Equal(t, []string{"a", "b"}, dst.Vals)
+	})
+
+	t.Run("object with mixed scalar and non-indexed array", func(t *testing.T) {
+		params := url.Values{}
+		params.Set("p[op]", "eq")
+		params.Add("p[vals]", "a")
+		params.Add("p[vals]", "b")
+
+		type Filter struct {
+			Op   string   `json:"op"`
+			Vals []string `json:"vals"`
+		}
+
+		var dst Filter
+		err := UnmarshalDeepObject(&dst, "p", params)
+		require.NoError(t, err)
+		assert.Equal(t, "eq", dst.Op)
+		assert.Equal(t, []string{"a", "b"}, dst.Vals)
+	})
+}