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Structをomitemptyする方法

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↓こんなStructがあったとして、JSONに変換した時にOrderItemプロパティが空だったら、JSONから省略したい。

HogeSales struct {
    IsDirty     bool        `json:"-"`
    Id          string      `json:"Id,omitempty"`
    Amount      int         `json:"Amount,omitempty"`
    OrderItem   OrderItem   `json:"OrderItem,omitempty"`
}

json.Marshal(HogeSales) でJSONに変換した時に "OrderItem": {} となることを防ぐ方法。
このOrderItem項目すら消したい場合。

ちなみに omitempty される条件は下記

false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero

そう、Struct型である以上、Omitされない。。。

対処方法は簡単 Pointer にして nil を入れておけばOK

HogeSales struct {
    IsDirty     bool        `json:"-"`
    Id          string      `json:"Id,omitempty"`
    Amount      int         `json:"Amount,omitempty"`
    OrderItem   *OrderItem  `json:"OrderItem,omitempty"`
}

func (h *HogeSales) Clear() {
    h.OrderItem = nil
}
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