Partial update nullable fields in Go

Using pointers and SELECT before UPDATE, we solved the partial update problem, but leave out a minor detail: how do we set nullable fields to NULL when the pointer will be nil whether we set it to null or undefined (i.e. not send the field at all)?

Answer: Use blank value as null value!

Let’s say we got a struct like this:

type Student struct {
    Name        *string
    OnboardedAt *time.Time
}

1. For null string

To set the Name field to NULL, set it to an empty string and update the field to NULL when that condition is satisfied. Simple enough.

2. For null time

To set OnboardedAt field to NULL, set it to empty string, and your code will… panic! Because an empty string is not a valid time. It’s a little bit tricky here because time doesn’t have a “blank” value, but fortunately, it has a zero one.

Let’s create a custom blank time instead:

type BlankTime struct {
    time.Time
}

func (n *BlankTime) UnmarshalJSON(b []byte) (err error) {
    if string(b) == `""` {
        zero := time.Time{}
        *n = BlankTime{zero}
        return
    }

    tt, err := time.Parse(`"`+time.RFC3339+`"`, string(b))
    *n = BlankTime{tt}
    return
}

And rewrite our struct:

type Student struct {
    Name        *string
    OnboardedAt *BlankTime
}

Now whenever we set the time field to an empty string, it will get a zero value, do a check to set NULL for your time field.