for(i in req.body.category_id){
db.query('INSERT INTO guide_categories(category_id) values($1)', [req.body.category_id[i]],function(err,category) {
  if(err) return next(err);                         
  return console.log("success");           
  });                             
}

回答如下：

改成

for(i in req.body.category_id){
db.query('INSERT INTO guide_categories(category_id) values($1) returning *', [req.body.category_id[i]], (err,category) => {
  if(err) {
    console.log(err);     
    } else {                    
    console.log(category);   
  }        
  };                             
}

并填充console.log的输出