feat: fix SavePolicy bug to preserve unique indexes by using DeleteMany instead of Drop

adapter.go

@@ -230,7 +230,7 @@ func (a *adapter) dropTable() error {
 	ctx, cancel := context.WithTimeout(context.TODO(), a.timeout)
 	defer cancel()
 
-	err := a.collection.Drop(ctx)
+	_, err := a.collection.DeleteMany(ctx, bson.D{{}})
 	if err != nil {
 		return err
 	}

adapter_test.go

@@ -15,6 +15,7 @@
 package mongodbadapter
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"strings"
@@ -634,3 +635,64 @@ func TestUpdateFilteredPoliciesTxn(t *testing.T) {
 	e.LoadPolicy()
 	testGetPolicyWithoutOrder(t, e, [][]string{{"alice", "data1", "write"}, {"bob", "data2", "read"}, {"data2_admin", "data2", "read"}, {"data2_admin", "data2", "write"}})
 }
+
+func TestSavePolicyPreservesIndexes(t *testing.T) {
+	// Initialize with some policies
+	initPolicy(t, getDbURL())
+
+	a, err := NewAdapter(getDbURL())
+	if err != nil {
+		panic(err)
+	}
+
+	e, err := casbin.NewEnforcer("examples/rbac_model.conf", a)
+	if err != nil {
+		panic(err)
+	}
+
+	// Save policy which should NOT drop indexes
+	if err := e.SavePolicy(); err != nil {
+		t.Errorf("Expected SavePolicy() to be successful; got %v", err)
+	}
+
+	// Try to add a duplicate policy - should fail due to unique index
+	adapter := a.(*adapter)
+	line := savePolicyLine("p", []string{"alice", "data1", "read"})
+
+	ctx, cancel := context.WithTimeout(context.TODO(), adapter.timeout)
+	defer cancel()
+
+	// Attempting to insert a duplicate should fail
+	_, err = adapter.collection.InsertOne(ctx, line)
+	if err == nil {
+		t.Error("Expected InsertOne of duplicate to fail due to unique index, but it succeeded")
+	}
+}
+
+func TestSavePolicyPreventsDuplicates(t *testing.T) {
+	// Initialize with some policies
+	initPolicy(t, getDbURL())
+
+	a, err := NewAdapter(getDbURL())
+	if err != nil {
+		panic(err)
+	}
+
+	e, err := casbin.NewEnforcer("examples/rbac_model.conf", a)
+	if err != nil {
+		panic(err)
+	}
+
+	// Save policy which should NOT drop indexes
+	if err := e.SavePolicy(); err != nil {
+		t.Errorf("Expected SavePolicy() to be successful; got %v", err)
+	}
+
+	// Try to add a duplicate directly via adapter (bypassing enforcer's in-memory check)
+	// This should be prevented by the unique index
+	adapter := a.(*adapter)
+	err = adapter.AddPolicy("p", "p", []string{"alice", "data1", "read"})
+	if err == nil {
+		t.Error("Expected AddPolicy of duplicate to fail due to unique index, but it succeeded")
+	}
+}