fix: Enable interface field access in rule expressions (#484)

- Enhanced IsObject() method to recognize interfaces containing structs
- Updated GetObjectValueByField() to handle interface types
- Updated SetObjectValueByField() to support interface field modification
- Added comprehensive tests for interface field access
- Enables direct field access like Data.Payload.Status in rules
- Maintains backward compatibility with existing functionality

Fixes issue where interface{} fields could not be accessed directly
in rule expressions, requiring workaround accessor methods.

Author: WilliamDeBruin

examples/interface_field_access_test.go

@@ -0,0 +1,122 @@
+package examples
+
+import (
+	"testing"
+
+	"github.com/hyperjumptech/grule-rule-engine/ast"
+	"github.com/hyperjumptech/grule-rule-engine/builder"
+	"github.com/hyperjumptech/grule-rule-engine/engine"
+	"github.com/hyperjumptech/grule-rule-engine/pkg"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// PayloadContainer represents a struct with an interface{} field
+type PayloadContainer struct {
+	Type    string
+	Payload interface{}
+}
+
+// NestedData represents the concrete type stored in the interface
+type NestedData struct {
+	Status string
+	Value  int
+}
+
+func TestInterfaceFieldAccess(t *testing.T) {
+	t.Parallel()
+	t.Run("should not fail when accessing interface fields directly", func(t *testing.T) {
+		// Rule that tries to access interface field directly
+		ruleText := `
+rule InterfaceFieldRule "test interface field access" {
+	when
+		Data.Type == "test" &&
+		Data.Payload.Status == "active"
+	then
+		Data.Type = "processed";
+		Log("Success");
+}`
+
+		// Test data with interface{} field
+		testData := &PayloadContainer{
+			Type: "test",
+			Payload: NestedData{
+				Status: "active",
+				Value:  42,
+			},
+		}
+
+		dataCtx := ast.NewDataContext()
+		require.NoError(t, dataCtx.Add("Data", testData))
+
+		knowledgeLibrary := ast.NewKnowledgeLibrary()
+		ruleBuilder := builder.NewRuleBuilder(knowledgeLibrary)
+
+		byteArr := pkg.NewBytesResource([]byte(ruleText))
+		err := ruleBuilder.BuildRuleFromResource("Test", "1.0.0", byteArr)
+		require.NoError(t, err)
+
+		knowledgeBase, err := knowledgeLibrary.NewKnowledgeBaseInstance("Test", "1.0.0")
+		require.NoError(t, err)
+
+		gruleEngine := engine.NewGruleEngine()
+		gruleEngine.ReturnErrOnFailedRuleEvaluation = true
+		err = gruleEngine.Execute(dataCtx, knowledgeBase)
+
+		assert.NoError(t, err, "should not fail to access interface field")
+		assert.Equal(t, "processed", testData.Type, "rule should have executed and modified the data")
+
+		// Verify the interface field value remains unchanged since we're only reading it
+		payload, ok := testData.Payload.(NestedData)
+		require.True(t, ok, "type assertion should succeed")
+		assert.Equal(t, "active", payload.Status, "interface field should remain unchanged")
+	})
+
+	t.Run("should allow modifying interface fields when using pointer to struct", func(t *testing.T) {
+		// Rule that modifies interface field
+		ruleText := `
+rule InterfaceFieldModifyRule "test interface field modification" {
+	when
+		Data.Type == "test" &&
+		Data.Payload.Status == "active"
+	then
+		Data.Type = "processed";
+		Data.Payload.Status = "handled";
+		Log("Modified interface field");
+}`
+
+		// Test data with interface{} field containing a pointer to struct (addressable)
+		testData := &PayloadContainer{
+			Type: "test",
+			Payload: &NestedData{
+				Status: "active",
+				Value:  42,
+			},
+		}
+
+		dataCtx := ast.NewDataContext()
+		require.NoError(t, dataCtx.Add("Data", testData))
+
+		knowledgeLibrary := ast.NewKnowledgeLibrary()
+		ruleBuilder := builder.NewRuleBuilder(knowledgeLibrary)
+
+		byteArr := pkg.NewBytesResource([]byte(ruleText))
+		err := ruleBuilder.BuildRuleFromResource("Test", "1.0.0", byteArr)
+		require.NoError(t, err)
+
+		knowledgeBase, err := knowledgeLibrary.NewKnowledgeBaseInstance("Test", "1.0.0")
+		require.NoError(t, err)
+
+		gruleEngine := engine.NewGruleEngine()
+		gruleEngine.ReturnErrOnFailedRuleEvaluation = true
+		err = gruleEngine.Execute(dataCtx, knowledgeBase)
+
+		assert.NoError(t, err, "should not fail to modify interface field when using pointer")
+		assert.Equal(t, "processed", testData.Type, "rule should have executed and modified the data")
+
+		// Verify the interface field was modified
+		payload, ok := testData.Payload.(*NestedData)
+		require.True(t, ok, "type assertion should succeed")
+		assert.Equal(t, "handled", payload.Status, "interface field should have been modified")
+	})
+}

model/GoDataAccessLayer.go

@@ -256,6 +256,19 @@ func (node *GoValueNode) IsObject() bool {
 
 			return typ.Elem().Kind() == reflect.Struct
 		}
+		if typ.Kind() == reflect.Interface {
+			// Check if the interface contains a struct
+			if !node.thisValue.IsNil() {
+				elem := node.thisValue.Elem()
+				if elem.IsValid() {
+					elemType := elem.Type()
+					if elemType.Kind() == reflect.Ptr {
+						return elemType.Elem().Kind() == reflect.Struct
+					}
+					return elemType.Kind() == reflect.Struct
+				}
+			}
+		}
 
 		return typ.Kind() == reflect.Struct
 	}
@@ -267,14 +280,24 @@ func (node *GoValueNode) IsObject() bool {
 func (node *GoValueNode) GetObjectValueByField(field string) (reflect.Value, error) {
 	if node.IsObject() {
 		var val reflect.Value
-		if node.thisValue.Kind() == reflect.Ptr {
+
+		// Handle interface types by extracting the concrete value
+		if node.thisValue.Kind() == reflect.Interface {
+			if !node.thisValue.IsNil() {
+				elem := node.thisValue.Elem()
+				if elem.Kind() == reflect.Ptr {
+					val = elem.Elem().FieldByName(field)
+				} else if elem.Kind() == reflect.Struct {
+					val = elem.FieldByName(field)
+				}
+			}
+		} else if node.thisValue.Kind() == reflect.Ptr {
 			val = node.thisValue.Elem().FieldByName(field)
-		}
-		if node.thisValue.Kind() == reflect.Struct {
+		} else if node.thisValue.Kind() == reflect.Struct {
 			val = node.thisValue.FieldByName(field)
 		}
-		if val.IsValid() {
 
+		if val.IsValid() {
 			return val, nil
 		}
 
@@ -293,12 +316,20 @@ func (node *GoValueNode) GetObjectTypeByField(field string) (typ reflect.Type, e
 				typ = nil
 			}
 		}()
-		if node.thisValue.Kind() == reflect.Ptr {
 
+		// Handle interface types by extracting the concrete type
+		if node.thisValue.Kind() == reflect.Interface {
+			if !node.thisValue.IsNil() {
+				elem := node.thisValue.Elem()
+				if elem.Kind() == reflect.Ptr {
+					return elem.Elem().FieldByName(field).Type(), nil
+				} else if elem.Kind() == reflect.Struct {
+					return elem.FieldByName(field).Type(), nil
+				}
+			}
+		} else if node.thisValue.Kind() == reflect.Ptr {
 			return node.thisValue.Elem().FieldByName(field).Type(), nil
-		}
-		if node.thisValue.Kind() == reflect.Struct {
-
+		} else if node.thisValue.Kind() == reflect.Struct {
 			return node.thisValue.FieldByName(field).Type(), nil
 		}
 	}
@@ -353,7 +384,19 @@ func SetNumberValue(target, newvalue reflect.Value) error {
 
 // SetObjectValueByField will set the underlying value's field with new value.
 func (node *GoValueNode) SetObjectValueByField(field string, newValue reflect.Value) (err error) {
-	fieldVal := node.thisValue.Elem().FieldByName(field)
+	var objValue reflect.Value = node.thisValue
+
+	// If it's an interface, extract the concrete value
+	if node.thisValue.Kind() == reflect.Interface && !node.thisValue.IsNil() {
+		objValue = node.thisValue.Elem()
+	}
+
+	// Handle pointer to struct
+	if objValue.Kind() == reflect.Ptr {
+		objValue = objValue.Elem()
+	}
+
+	fieldVal := objValue.FieldByName(field)
 	if fieldVal.IsValid() && fieldVal.CanAddr() && fieldVal.CanSet() {
 		defer func() {
 			if r := recover(); r != nil {

model/GoDataAccessLayer_test.go

@@ -15,10 +15,11 @@
 package model
 
 import (
-	"github.com/stretchr/testify/assert"
 	"reflect"
 	"testing"
 	"time"
+
+	"github.com/stretchr/testify/assert"
 )
 
 type Person struct {
@@ -358,3 +359,82 @@ func TestConstantFunctionCalls(t *testing.T) {
 	assert.Equal(t, "string", retVal.Type().String())
 	assert.Equal(t, "SomeWithSpace", retVal.String())
 }
+
+// TestStructWithInterface represents a struct with an interface{} field for testing
+type TestStructWithInterface struct {
+	Name    string
+	Payload interface{}
+}
+
+// TestPayload represents the concrete type stored in the interface
+type TestPayload struct {
+	Status string
+	Count  int
+}
+
+func TestGoValueNode_Interface(t *testing.T) {
+	// Test with interface containing struct value
+	testData := &TestStructWithInterface{
+		Name: "test",
+		Payload: TestPayload{
+			Status: "active",
+			Count:  42,
+		},
+	}
+
+	rootNode := NewGoValueNode(reflect.ValueOf(testData), "testData")
+	payloadNode, err := rootNode.GetChildNodeByField("Payload")
+	assert.NoError(t, err)
+	assert.True(t, payloadNode.IsInterface())
+	assert.True(t, payloadNode.IsObject()) // Should return true for interface containing struct
+
+	// Test accessing fields within interface
+	statusNode, err := payloadNode.GetChildNodeByField("Status")
+	assert.NoError(t, err)
+	assert.True(t, statusNode.IsString())
+	assert.Equal(t, "testData.Payload.Status", statusNode.IdentifiedAs())
+
+	statusValue, err := statusNode.GetValue()
+	assert.NoError(t, err)
+	assert.Equal(t, "active", statusValue.String())
+
+	countNode, err := payloadNode.GetChildNodeByField("Count")
+	assert.NoError(t, err)
+	assert.True(t, countNode.IsInteger())
+
+	countValue, err := countNode.GetValue()
+	assert.NoError(t, err)
+	assert.Equal(t, 42, int(countValue.Int()))
+}
+
+func TestGoValueNode_InterfaceWithPointer(t *testing.T) {
+	// Test with interface containing pointer to struct (addressable)
+	testData := &TestStructWithInterface{
+		Name: "test",
+		Payload: &TestPayload{
+			Status: "active",
+			Count:  42,
+		},
+	}
+
+	rootNode := NewGoValueNode(reflect.ValueOf(testData), "testData")
+	payloadNode, err := rootNode.GetChildNodeByField("Payload")
+	assert.NoError(t, err)
+	assert.True(t, payloadNode.IsInterface())
+	assert.True(t, payloadNode.IsObject()) // Should return true for interface containing pointer to struct
+
+	// Test setting fields within interface (should work with pointer)
+	err = payloadNode.SetObjectValueByField("Status", reflect.ValueOf("modified"))
+	assert.NoError(t, err)
+
+	// Verify the change
+	statusNode, err := payloadNode.GetChildNodeByField("Status")
+	assert.NoError(t, err)
+	statusValue, err := statusNode.GetValue()
+	assert.NoError(t, err)
+	assert.Equal(t, "modified", statusValue.String())
+
+	// Also verify through direct access to the struct
+	payload := testData.Payload.(*TestPayload)
+	assert.Equal(t, "modified", payload.Status)
+}