Fix JSON unmarshal error handling and tests (#22)

* Enhance JSON unmarshal error handling to provide clearer validation messages

* Refactor JSON type mismatch error handling tests for clarity and reliability

Author: moutonjeremy

common.go

@@ -1,8 +1,6 @@
 package fiberoapi
 
 import (
-	"encoding/json"
-	"errors"
 	"fmt"
 	"reflect"
 	"strconv"
@@ -52,12 +50,33 @@ func parseInput[TInput any](app *OApiApp, c *fiber.Ctx, path string, options *Op
 					// It's OK, the POST has no body - ignore the error
 				} else {
 					// Transform JSON unmarshal type errors into readable validation errors
-					// Using errors.As for more robust error handling (handles wrapped errors)
-					var unmarshalErr *json.UnmarshalTypeError
-					if errors.As(err, &unmarshalErr) {
-						return input, fmt.Errorf("invalid type for field '%s': expected %s but got %s",
-							unmarshalErr.Field, unmarshalErr.Type.String(), unmarshalErr.Value)
+					// Check if error message contains unmarshal type error pattern
+					errMsg := err.Error()
+					if strings.Contains(errMsg, "json: cannot unmarshal") && strings.Contains(errMsg, "into Go struct field") {
+						// Parse the error message to extract field name and type info
+						// Format: "json: cannot unmarshal <type> into Go struct field <StructName>.<Field> of type <GoType>"
+						parts := strings.Split(errMsg, "into Go struct field ")
+						if len(parts) == 2 {
+							afterField := parts[1]
+							fieldParts := strings.Split(afterField, " of type ")
+							if len(fieldParts) == 2 {
+								// Extract field name (after the last dot)
+								fullFieldName := fieldParts[0]
+								fieldNameParts := strings.Split(fullFieldName, ".")
+								fieldName := fieldNameParts[len(fieldNameParts)-1]
+
+								// Extract expected type
+								expectedType := fieldParts[1]
+
+								// Extract actual type from the first part
+								typePart := strings.TrimPrefix(parts[0], "json: cannot unmarshal ")
+
+								return input, fmt.Errorf("invalid type for field '%s': expected %s but got %s",
+									fieldName, expectedType, typePart)
+							}
+						}
 					}
+
 					return input, err
 				}
 			}

json_type_error_test.go

@@ -18,11 +18,14 @@ import (
 // the raw Go error message is not user-friendly:
 //   "json: cannot unmarshal number into Go struct field Request.Description of type string"
 //
-// Solution: Detect json.UnmarshalTypeError and transform it into a readable message:
+// Solution: Parse the error message to extract field name and type information,
+// then transform it into a readable message:
 //   "invalid type for field 'description': expected string but got number"
 //
-// Implementation: Uses errors.As (not type assertion) to handle wrapped errors correctly.
-// This ensures the error detection works even if the error is wrapped by Fiber or middleware.
+// Implementation: The error type from c.BodyParser() is *errors.UnmarshalTypeError
+// (not *json.UnmarshalTypeError), so we parse the error message string to extract
+// the field name, expected type, and actual type. This approach works reliably
+// across different Fiber versions and handles all JSON unmarshal type errors.
 
 // Test for JSON type mismatch errors
 func TestJSONTypeMismatchErrors(t *testing.T) {
@@ -115,48 +118,6 @@ func TestJSONTypeMismatchErrors(t *testing.T) {
 	}
 }
 
-// Test that errors.As correctly handles wrapped errors
-func TestJSONTypeMismatchWithWrappedError(t *testing.T) {
-	app := fiber.New()
-	oapi := New(app)
-
-	type TestRequest struct {
-		Value string `json:"value"`
-	}
-
-	type TestResponse struct {
-		Result string `json:"result"`
-	}
-
-	Post(oapi, "/test", func(c *fiber.Ctx, input TestRequest) (TestResponse, TestError) {
-		return TestResponse{Result: "OK"}, TestError{}
-	}, OpenAPIOptions{})
-
-	// Test with wrong type - even if the error is wrapped, errors.As should detect it
-	req := httptest.NewRequest("POST", "/test", strings.NewReader(`{"value": 123}`))
-	req.Header.Set("Content-Type", "application/json")
-	resp, err := app.Test(req)
-	if err != nil {
-		t.Fatalf("Expected no error, got %v", err)
-	}
-
-	if resp.StatusCode != 400 {
-		t.Errorf("Expected status 400, got %d", resp.StatusCode)
-	}
-
-	body, _ := io.ReadAll(resp.Body)
-	bodyStr := string(body)
-
-	// Should contain our custom error message
-	if !strings.Contains(bodyStr, "invalid type for field 'value'") {
-		t.Errorf("Expected 'invalid type for field' in error message, got %s", bodyStr)
-	}
-
-	if !strings.Contains(bodyStr, "expected string but got number") {
-		t.Errorf("Expected 'expected string but got number' in error message, got %s", bodyStr)
-	}
-}
-
 // Test with custom validation error handler
 func TestJSONTypeMismatchWithCustomHandler(t *testing.T) {
 	app := fiber.New()