Handling non present field in struct

[zekth]

This is an annoying JSON x Golang edge case but found a solution for this

Let's assume we have a struct like so:

type testStruct struct {
	Foo gonull.Nullable[*string] `json:"foo"`
}

we want to be able to handle those cases:

• {"foo":"f"}
• {}
• {"foo": null}

In the current api we cannot check if the field is present. As {} and {"foo":null} will result with Valid: false and nil value. However we can do this:

diff --git a/gonull.go b/gonull.go
index b20a29e..deab37d 100644
--- a/gonull.go
+++ b/gonull.go
@@ -19,8 +19,9 @@ var (
 // It keeps track of the value (Val) and a flag (Valid) indicating whether the value has been set.
 // This allows for better handling of nullable values, ensuring proper value management and serialization.
 type Nullable[T any] struct {
-       Val   T
-       Valid bool
+       Val     T
+       Valid   bool
+       Present bool
 }
 
 // NewNullable creates a new Nullable with the given value and sets Valid to true.
@@ -59,6 +60,7 @@ func (n Nullable[T]) Value() (driver.Value, error) {
 // UnmarshalJSON implements the json.Unmarshaler interface for Nullable, allowing it to be used as a nullable field in JSON operations.
 // This method ensures proper unmarshalling of JSON data into the Nullable value, correctly setting the Valid flag based on the JSON data.
 func (n *Nullable[T]) UnmarshalJSON(data []byte) error {
+       n.Present = true
        if string(data) == "null" {
                n.Valid = false
                return nil

Then we have this successful test:

func TestPresent(t *testing.T) {
	var nullable1 testStruct
	var nullable2 testStruct
	var nullable3 testStruct

	err := json.Unmarshal([]byte(`{"foo":"f"}`), &nullable1)
	assert.NoError(t, err)
	assert.Equal(t, true, nullable1.Foo.Valid)
	assert.Equal(t, true, nullable1.Foo.Present)

	err = json.Unmarshal([]byte(`{}`), &nullable2)
	assert.NoError(t, err)
	assert.Equal(t, false, nullable2.Foo.Valid)
	assert.Equal(t, false, nullable3.Foo.Present)
	assert.Nil(t, nullable2.Foo.Val)

	err = json.Unmarshal([]byte(`{"foo": null}`), &nullable3)
	assert.NoError(t, err)
	assert.Equal(t, false, nullable3.Foo.Valid)
	assert.Equal(t, true, nullable3.Foo.Present)
	assert.Nil(t, nullable3.Foo.Val)
}

Happy to raise a PR if the implementation is ok.

[LukaGiorgadze]

Hey @zekth, thanks for pointing out this issue. It's a good topic to discuss. frankly, I'm not sure when you'd need to use Foo gonull.Nullable[*string] 'json:"foo"', but as you said, it might be a rare but useful case. Freel free to open PR, I'll gladly check and approve it!
Thank you!

[LukaGiorgadze]

https://github.com/LukaGiorgadze/gonull/releases/tag/v1.0.0