feat(preload): ✨ Add support for custom SQL joins

* Introduce SqlJoins and JoinAliases in PreloadOption.
* Preserve SqlJoins and JoinAliases during filter processing.
* Implement logic to apply custom SQL joins in handler.
* Add tests for SqlJoins handling and join alias extraction.

pkg/restheadspec/headers_test.go

@@ -2,6 +2,8 @@ package restheadspec
 
 import (
 	"testing"
+
+	"github.com/bitechdev/ResolveSpec/pkg/common"
 )
 
 func TestDecodeHeaderValue(t *testing.T) {
@@ -37,6 +39,121 @@ func TestDecodeHeaderValue(t *testing.T) {
 	}
 }
 
+func TestAddXFilesPreload_WithSqlJoins(t *testing.T) {
+	handler := &Handler{}
+	options := &ExtendedRequestOptions{
+		RequestOptions: common.RequestOptions{
+			Preload: make([]common.PreloadOption, 0),
+		},
+	}
+
+	// Create an XFiles with SqlJoins
+	xfile := &XFiles{
+		TableName: "users",
+		SqlJoins: []string{
+			"LEFT JOIN departments d ON d.id = users.department_id",
+			"INNER JOIN roles r ON r.id = users.role_id",
+		},
+		FilterFields: []struct {
+			Field    string `json:"field"`
+			Value    string `json:"value"`
+			Operator string `json:"operator"`
+		}{
+			{Field: "d.active", Value: "true", Operator: "eq"},
+			{Field: "r.name", Value: "admin", Operator: "eq"},
+		},
+	}
+
+	// Add the XFiles preload
+	handler.addXFilesPreload(xfile, options, "")
+
+	// Verify that a preload was added
+	if len(options.Preload) != 1 {
+		t.Fatalf("Expected 1 preload, got %d", len(options.Preload))
+	}
+
+	preload := options.Preload[0]
+
+	// Verify relation name
+	if preload.Relation != "users" {
+		t.Errorf("Expected relation 'users', got '%s'", preload.Relation)
+	}
+
+	// Verify SqlJoins were transferred
+	if len(preload.SqlJoins) != 2 {
+		t.Fatalf("Expected 2 SQL joins, got %d", len(preload.SqlJoins))
+	}
+
+	// Verify JoinAliases were extracted
+	if len(preload.JoinAliases) != 2 {
+		t.Fatalf("Expected 2 join aliases, got %d", len(preload.JoinAliases))
+	}
+
+	// Verify the aliases are correct
+	expectedAliases := []string{"d", "r"}
+	for i, expected := range expectedAliases {
+		if preload.JoinAliases[i] != expected {
+			t.Errorf("Expected alias '%s', got '%s'", expected, preload.JoinAliases[i])
+		}
+	}
+
+	// Verify filters were added
+	if len(preload.Filters) != 2 {
+		t.Fatalf("Expected 2 filters, got %d", len(preload.Filters))
+	}
+
+	// Verify filter columns reference joined tables
+	if preload.Filters[0].Column != "d.active" {
+		t.Errorf("Expected filter column 'd.active', got '%s'", preload.Filters[0].Column)
+	}
+	if preload.Filters[1].Column != "r.name" {
+		t.Errorf("Expected filter column 'r.name', got '%s'", preload.Filters[1].Column)
+	}
+}
+
+func TestExtractJoinAlias(t *testing.T) {
+	tests := []struct {
+		name        string
+		joinClause  string
+		expected    string
+	}{
+		{
+			name:       "LEFT JOIN with alias",
+			joinClause: "LEFT JOIN departments d ON d.id = users.department_id",
+			expected:   "d",
+		},
+		{
+			name:       "INNER JOIN with AS keyword",
+			joinClause: "INNER JOIN users AS u ON u.id = orders.user_id",
+			expected:   "u",
+		},
+		{
+			name:       "JOIN without alias",
+			joinClause: "JOIN roles ON roles.id = users.role_id",
+			expected:   "",
+		},
+		{
+			name:       "Complex join with multiple conditions",
+			joinClause: "LEFT OUTER JOIN products p ON p.id = items.product_id AND p.active = true",
+			expected:   "p",
+		},
+		{
+			name:       "Invalid join (no ON clause)",
+			joinClause: "LEFT JOIN departments",
+			expected:   "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := extractJoinAlias(tt.joinClause)
+			if result != tt.expected {
+				t.Errorf("Expected alias '%s', got '%s'", tt.expected, result)
+			}
+		})
+	}
+}
+
 // Note: The following functions are unexported (lowercase) and cannot be tested directly:
 // - parseSelectFields
 // - parseFieldFilter