mybatis中时不能自动给id设值

做mybatis项目时遇到这种情况，<insert>时<selectKey>不能自动给id设值，报错：

ORA-01400: 无法将 NULL 插入 ("TEST"."T_STUDENT_INFO"."ID")；

配置文件如下：

<insert id="save" parameterType="entity.Student">
    <selectKey resultClass="java.lang.Integer" keyProperty="id" order="BEFORE">
        select SEQ_STUDENT_ID.nextval as id from dual
    </selectKey>
    insert into t_student_info
    <trim prefix="(" suffix=")" suffixOverrides=",">
        <if test="userName != null and userName != ''">USER_NAME,</if>
        <if test="age != null">AGE,</if>
        <if test="gender != null and gender != ''">GENDER,</if>
    </trim>
    <trim prefix="values(" suffix=")" suffixOverrides=",">
        <if test="userName != null and userName != ''">
            #{userName,jdbcType=VARCHAR},
        </if>
        <if test="age != null">
            #{age,jdbcType=BIGINT},
        </if>
        <if test="gender != null and gender != ''">
            #{gender,jdbcType=VARCHAR},
        </if>
    </trim>
</insert>

<selectKey>写了order="BEFORE"也不能自动给要insert的id字段设值；有的时候这样写是可以的，我想可能是版本原因或者是mybatis的bug。在这里，在<insert>中加上 id 就可以了，如下：

<insert id="save" parameterType="entity.Student">
    <selectKey resultClass="java.lang.Integer" keyProperty="id" order="BEFORE">
        select SEQ_STUDENT_ID.nextval as id from dual
    </selectKey>
    insert into t_student_info
    <trim prefix="(" suffix=")" suffixOverrides=",">
        ID,
        <if test="userName != null and userName != ''">USER_NAME,</if>
        <if test="age != null">AGE,</if>
        <if test="gender != null and gender != ''">GENDER,</if>
    </trim>
    <trim prefix="values(" suffix=")" suffixOverrides=",">
        #{id,jdbcType=BIGINT},
        <if test="userName != null and userName != ''">
            #{userName,jdbcType=VARCHAR},
        </if>
        <if test="age != null">
            #{age,jdbcType=BIGINT},
        </if>
        <if test="gender != null and gender != ''">
            #{gender,jdbcType=VARCHAR},
        </if>
    </trim>
</insert>

记录一下，以后为了保险起见，都在<insert>中加上 ID字段就OK啦。。。