fix: Month JSON unmarshaling now enforcing year and month only (#906)

Author: morremeyer

internal/types/month.go

@@ -29,8 +29,17 @@ func (m Month) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
 // The month is expected to be a string in a format accepted by ParseDate.
+// From the parsed string, everything is then ignored except the year and month
 func (m *Month) UnmarshalJSON(data []byte) error {
-	return (*time.Time)(m).UnmarshalJSON(data)
+	var date time.Time
+	err := date.UnmarshalJSON(data)
+	if err != nil {
+		return err
+	}
+
+	month := NewMonth(date.Year(), date.Month())
+	*m = month
+	return nil
 }
 
 // MonthOf returns the Month in which a time occurs in that time's location.

internal/types/month_test.go

@@ -0,0 +1,21 @@
+package types_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/envelope-zero/backend/v3/internal/types"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMonthUnmarshalJSON(t *testing.T) {
+	var target struct {
+		Month types.Month
+	}
+	jsonString := []byte(`{ "month": "2024-05-12T17:59:23+02:00" }`)
+
+	err := json.Unmarshal(jsonString, &target)
+
+	assert.Nil(t, err)
+	assert.Equal(t, types.NewMonth(2024, 5), target.Month)
+}