codegen: handle nullable foreign keys using 'ifnull' and add a test case for them

pkg/codegen/modelgenerate/generate_model.go

@@ -351,7 +351,11 @@ func GenerateDeleteItemFunc(tbl schema.Table) *ast.FuncDecl {
 func GenerateSQLFieldsConst(tbl schema.Table) *ast.GenDecl {
 	columns := make([]string, 0, len(tbl.Columns))
 	for _, col := range tbl.Columns {
-		columns = append(columns, col.Name)
+		if col.IsNullableForeignKey() {
+			columns = append(columns, fmt.Sprintf("ifnull(%s, 0) %s", col.Name, col.Name))
+		} else {
+			columns = append(columns, col.Name)
+		}
 	}
 	// Join with comma and space
 	value := "`" + strings.Join(columns, ", ") + "`"