vue2 “component lists rendered with v-for should have explicit keys”警告

代码：

<gallery-item v-for="item in galleries" :gallery="item" >
</gallery-item>

调试过程报警告：
<gallery-item v-for="item in galleries">: component lists rendered with v-for should have explicit keys. See https://vuejs.org/guide/list.html#key for more info.
不过不影响渲染结果。查了文档原来是没有指定key。

修改后问题解决

<gallery-item v-for="item in galleries" :gallery="item" :key="item.index">
</gallery-item>