用 sql.NullFloat64 包装 json.Number

Question

I'm working with an API that has json values that can be a number or string and can sometimes be an empty string. For example:

[
    {
        "Description": "Doorknob",
        "Amount": 3.25
    },
    {
        "Description": "Light bulb",
        "Amount": "4.70"
    },
    {
        "Description": "Screwdriver",
        "Amount": ""
    }
]

I've learned I can use json.Number to deal with cases where the value can be a number or a string. But how do I deal with the cases where it's an empty string?

I need to insert these values into a postgres database so I'd like cases where it's an empty string to show as null and I'm thinking sql.NullFloat64 would be best suited to handle it.

Here's a function I'm using that works on the first 2 cases (Doorknob and Light bulb), but fails to unmarshal the last ("Screwdriver").

Here is an unmarshaling function I'm trying to use, but I can't figure out why it's not working:

type NullNumber struct{ sql.NullFloat64 }

func (nn *NullNumber) UnmarshalJSON(data []byte) error {
    var x *json.Number
    if err := json.Unmarshal(data, &x); err != nil {
        return err
    }
    if len(*x) == 0 {
        nn.Valid = false // This doesn't seem to be working. Why?
    }
    this, err := x.Float64()
    if err != nil {
        return err
    }
    nn.Valid = true
    nn.Float64 = this
    return nil
}

Is this a case where I should be using an open interface for the amount?

Any help is very appreciated.

playground: https://play.golang.org/p/QYQRq94OtV3

Answer 1

You're almost there! json.Unmarshal call errors when a json.Number is given an empty string.

Add this check beforehand:

// edge case: json.Number is given an empty string
if bytes.Equal(data, []byte(`""`)) {
    nn.Valid = false
    return nil
}

https://play.golang.org/p/ILxC8tjYI_G