解决vue过滤器filters获取不到this对象的问题

<template>
  <div>{{a|sum(that)}}</div>
</template>

<script>
  export default {
    name: "test",
    data() {
      return {
        that: this,
        a: 1,
        b: 2
      }
    },
    filters: {
      sum(a, that) {
        console.log(that);
        return a + that.b;
      }
    },
  }
</script>

data: {
    shopItemType: {}
},
methods: { 
    shopItemType2str(id){
        return this.shopItemType[id];
    }
}

<tr v-for="shopItem in shopItems">
    <td>{{shopItemType2str(shopItem.item_type)}}</td>
</tr>