How can I tell if something was submitted as zero value or omitted from the request body?

[chrislentz]

I am currently writing a PATCH endpoint with Go Echo using Go Playground for validation. I use the method below to bind my JSON request body to a struct. The issue is that this is a patch endpoint, so properties are optional in the request body JSON. How can I determine if the request body submitted a property as its zero value or was omitted? Because when I look at the struct, I get the zero value even if the field was omitted from the JSON request body.

if err := (&echo.DefaultBinder{}).BindBody(c, &requestData); err != nil {
    return errors.New("invalid-data")
}

For example, let's say I have a property called "count." This is a PATCH endpoint, so the count could be omitted completely or submitted with an integer. If I use Go Echo's bind capabilities, the bound struct will show the "count = 0" (the zero value equivalent) if it is omitted. But it wasn't submitted, so I should leave the value untouched and not update it to "0". If the request contains "count: 0", I should update it.

How would I know if "count" was submitted? I hope I don't have to compare every property to the existing database values to see if it changed before saving.