Ruby中获取位置不固定的SubTotalLineDetail类型JSON元素值的最优方法

给定如下API返回的JSON数据：

[
    {
        "id": "1",
        "line_num": 1,
        "description": "Custom Design",
        "amount": "300.0",
        "detail_type": "SalesItemLineDetail",
        "sales_line_item_detail": {
            "item_ref": {
                "value": "4",
                "name": "Design",
                "type": null
            },
            "class_ref": null,
            "unit_price": "75.0",
            "rate_percent": null,
            "price_level_ref": null,
            "quantity": "4.0",
            "tax_code_ref": {
                "value": "NON",
                "name": null,
                "type": null
            },
            "service_date": null,
            "tax_inclusive_amount": null
        },
        "sub_total_line_detail": null,
        "payment_line_detail": null,
        "discount_line_detail": null,
        "group_line_detail": null,
        "description_line_detail": null
    },
    {
        "id": null,
        "line_num": null,
        "description": null,
        "amount": "1005.0",
        "detail_type": "SubTotalLineDetail",
        "sales_line_item_detail": null,
        "sub_total_line_detail": {
            "item_ref": null,
            "class_ref": null,
            "unit_price": null,
            "quantity": null,
            "tax_code_ref": null
        },
        "payment_line_detail": null,
        "discount_line_detail": null,
        "group_line_detail": null,
        "description_line_detail": null
    },
    {
        "id": null,
        "line_num": null,
        "description": null,
        "amount": "50.25",
        "detail_type": "DiscountLineDetail",
        "sales_line_item_detail": null,
        "sub_total_line_detail": null,
        "payment_line_detail": null,
        "discount_line_detail": {
            "discount_ref": null,
            "percent_based": true,
            "discount_percent": "5.0",
            "discount_account_ref": {
                "value": "86",
                "name": "Discounts given",
                "type": null
            },
            "class_ref": null,
            "tax_code_ref": null
        },
        "group_line_detail": null,
        "description_line_detail": null
    }
]

需求是获取detail_type为"SubTotalLineDetail"的行对应的amount值（本例为1005.0），由于该行在数组中的位置不固定，不能依赖last方法取最后一个元素。

解决方案 #

利用Ruby数组的find方法，通过detail_type字段精准匹配目标行，示例代码如下：

# 假设line_items是已经解析完成的Ruby数组（从API返回的JSON转换而来）
sub_total_line = line_items.find { |line| line["detail_type"] == "SubTotalLineDetail" }
# 获取amount值，同时处理找不到目标行的边界情况
sub_total_amount = sub_total_line ? sub_total_line["amount"] : nil
# 若需要将字符串类型的金额转为数值类型
sub_total_amount = sub_total_amount.to_f if sub_total_amount

说明 #

• find方法会遍历数组，返回第一个满足条件的元素，完全不受元素位置影响；
• 如果存在多个detail_type为SubTotalLineDetail的行，可改用select方法获取所有匹配行后再按需处理；
• 代码中加入了空值判断，避免找不到目标行时触发异常。

内容的提问来源于stack exchange，提问作者Will DeBernardi