Improve error message for undefined arrays of objects

internal/fields/testdata/fields/fields.yml

@@ -86,3 +86,5 @@
   type: keyword
   normalize:
     - array
+- name: user.group.id
+  type: keyword

internal/fields/testdata/undefined-array-of-objects.json

@@ -0,0 +1,12 @@
+{
+  "user": {
+    "group": [
+      {
+        "id": "42"
+      },
+      {
+        "id": "0"
+      }
+    ]
+  }
+}

internal/fields/validate.go

@@ -584,7 +584,12 @@ func (v *Validator) validateScalarElement(key string, val interface{}, doc commo
 	}
 
 	if definition == nil {
-		return fmt.Errorf(`field "%s" is undefined`, key)
+		switch val.(type) {
+		case []any, []map[string]interface{}:
+			return fmt.Errorf(`field "%s" is used as array of objects, expected explicit definition with type group or nested`, key)
+		default:
+			return fmt.Errorf(`field "%s" is undefined`, key)
+		}
 	}
 
 	// Convert numeric keyword fields to string for validation.

internal/fields/validate_test.go

@@ -148,6 +148,17 @@ func TestValidate_ipAddress(t *testing.T) {
 	require.Empty(t, errs)
 }
 
+func TestValidate_undefinedArrayOfObjects(t *testing.T) {
+	validator, err := CreateValidatorForDirectory("testdata", WithSpecVersion("2.0.0"), WithDisabledDependencyManagement())
+	require.NoError(t, err)
+	require.NotNil(t, validator)
+
+	e := readSampleEvent(t, "testdata/undefined-array-of-objects.json")
+	errs := validator.ValidateDocumentBody(e)
+	require.Len(t, errs, 1)
+	require.Contains(t, errs[0].Error(), `field "user.group" is used as array of objects, expected explicit definition with type group or nested`)
+}
+
 func TestValidate_WithSpecVersion(t *testing.T) {
 	validator, err := CreateValidatorForDirectory("testdata", WithSpecVersion("2.0.0"), WithDisabledDependencyManagement())
 	require.NoError(t, err)