如何使用代码触发或活动记录查询返回单维数组中的表字段

I am fetching user role ids from user_roles table for logged in user. A user can have multiple roles and I am getting the result in multi-dimensional array. I am using foreach loop to return the role_ids in single array to use in other query IN clause. Is there a way in code-ignitor to retrieve the role ids in single dimensional array. Here is what I did so far:I am fetching user role ids from user_roles tab