Handling PATCH partial updates when a key is not present

I'm trying to figure out how to get past this issue.

I have a User struct and it has a few fields on it. However, when decoding a JSON object for a PATCH users call, a missing key leads to the value set as *nil. The corresponding database attribute is of type TEXT NULL, so the result will always be stored as NULL when the key is missing.

type UpdateUserDTO struct {
  ID         uuid.UUID
  FirstName  string
  LastName   string
  ImageURL  *string
}

ImageURL can be nil, but when this object is sent from clients:

{ firstName: "Jimmy" }

This decodes into ImageURL = nil because the imageUrl was not present in JSON.

How can I handle partial updates without resorting to checking the presence of each field using a map[string]struct{} instead of my DTO?

You can implement a custom json.Unmarshaler to determine whether the field was omitted entirely, was provided but its value was null, or was provided with non-null value.

type OptString struct {
    IsValid bool
    String  *string
}
// If a field with this type has no corresponding field in the
// incoming JSON then this method will not be invoked and the
// IsValid flag's value will remain `false`.
func (s *OptString) UnmarshalJSON(data []byte) error {
    if err := json.Unmarshal(data, &s.String); err != nil {
        return err
    }
    s.IsValid = true
    return nil
}

type UpdateUserDTO struct {
    ID        uuid.UUID
    FirstName string
    LastName  string
    ImageURL  OptString
}

https://go.dev/play/p/hvS5E46sgkD

An alternative approach, one that does not require a custom type, would be to set the Go field's value to the current database column's value just before unmarshaling the JSON. The json.Decoder (used by json.Unmarshal) will not "touch" a target's field if the incoming JSON doesn't contain a matching field.

dto := loadUpdateUserDTOFromDB(conn)
if err := json.Unmarshal(data, dto); err != nil {
    return err
}

https://go.dev/play/p/grJhr4P7sgQ