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Start with the endpoint’s patch contract
HTTP PATCH does not define what a particular JSON body means. It applies changes described by a patch document, whose format is identified by the request’s media type. Document the accepted media type or types and the endpoint’s rules for omitted fields, nulls, unknown fields, and invalid values. RFC 5789 also defines Accept-Patch for advertising supported patch media types: RFC 5789.
The status code and error body for a rejected field are part of your API contract; there is no universal status code for every invalid PATCH input. Tests should encode that contract rather than assume one.
Why a Go pointer may not distinguish missing from null
With encoding/json, an omitted object member leaves the destination field unchanged. JSON null sets pointer, map, slice, and interface values to nil. For most other Go types, null has no effect and does not itself cause a decoding error. These behaviors mean a pointer field alone is not a reliable presence signal when decoding a fresh request value: both an omitted member and an explicit null can leave a pointer nil. See the official encoding/json documentation.
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Seed nonzero destination values in decoder-level tests when you need to confirm how omitted members preserve existing values. For PATCH requests, though, it is usually clearer to decode into a request representation that records whether each member appeared, then apply changes to the stored resource only after decoding and validation succeed.
Represent field presence explicitly
A small wrapper can record both whether a member was present and whether its value was null. The example below uses a pointer for the decoded string value; Set distinguishes omission, while a present null leaves Value nil.
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type PatchField[T any] struct {
Set bool
Value *T
}
func (f *PatchField[T]) UnmarshalJSON(data []byte) error {
f.Set = true
if bytes.Equal(bytes.TrimSpace(data), []byte("null")) {
f.Value = nil
return nil
}
var value T
if err := json.Unmarshal(data, &value); err != nil {
return err
}
f.Value = &value
return nil
}
type PatchRequest struct {
Name PatchField[string] `json:"name"`
}This representation supports three distinct states: Set == false means absent; Set == true and Value == nil means present as null; and Set == true with a non-nil value means present with a value. Define what each state means for the endpoint. For example, null might clear a nullable name, be rejected for a required name, or have a format-specific meaning.
Another option is to decode the top-level object into map[string]json.RawMessage, check whether the key exists, and decode its raw value only when present. That approach makes presence inspection explicit and is useful when field policies vary. A custom wrapper is convenient for a stable request shape. Test whichever approach you use against the Go version and JSON decoder actually deployed; other JSON packages and newer or experimental APIs may have different behavior.
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Test the three states directly
Decoder-level tests should prove the representation preserves presence before business logic applies the update.
func TestPatchFieldPresence(t *testing.T) {
tests := []struct {
name string
input string
wantSet bool
wantValue *string
}{
{name: "absent", input: `{}`, wantSet: false},
{name: "null", input: `{"name":null}`, wantSet: true, wantValue: nil},
{name: "value", input: `{"name":"Ada"}`, wantSet: true, wantValue: ptr("Ada")},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
var got PatchRequest
if err := json.Unmarshal([]byte(tt.input), &got); err != nil {
t.Fatal(err)
}
if got.Name.Set != tt.wantSet {
t.Fatalf("Set = %v, want %v", got.Name.Set, tt.wantSet)
}
if !reflect.DeepEqual(got.Name.Value, tt.wantValue) {
t.Fatalf("Value = %#v, want %#v", got.Name.Value, tt.wantValue)
}
})
}
}The example assumes helpers such as ptr and imports for bytes, encoding/json, reflect, and testing. Extend these cases to cover each field type and null policy your API supports. If null into a non-string type must be rejected, encode that rule in the wrapper or validation layer instead of relying on the default decoder’s handling.
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Exercise the real handler with httptest
Decoder tests isolate representation behavior. Handler tests should exercise the production route, decoding, validation, and update path. Go’s net/http/httptest documentation describes httptest.NewRequest for creating a request for a server handler and httptest.NewRecorder for recording its response.
- Seed a known resource. Use a nonzero starting state so preservation, clearing, and accidental mutation are observable.
- Build a PATCH request. Use
httptest.NewRequest(http.MethodPatch, "/resource/1", strings.NewReader(body)), then setContent-Typeto the media type the endpoint accepts. - Call the real handler. Pass the request through the same router and handler path used in production where practical.
- Assert the response contract. Check the expected status and relevant response body or error structure.
- Inspect stored state. Verify the intended update occurred on success and that rejected input left the resource unchanged.
Use table-driven cases
Keep the cases together so each input’s expected response and state effect are visible. The status labels below are intentionally contract-dependent: substitute the exact result your endpoint specifies.
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| Case | Example body | What to assert |
|---|---|---|
| Field omitted | {} |
Existing value is preserved or handled according to the endpoint contract; assert its specified response. |
| Explicit null | {"name":null} |
Null clears, removes, is rejected, or follows the documented rule; assert the resulting state. |
| Valid replacement | {"name":"Ada"} |
Success response and updated resource value. |
| Wrong JSON type | {"name":42} |
Expected rejection or documented coercion, plus unchanged state if rejected. |
| Malformed JSON | {"name": |
Client-error response under the API contract and unchanged state. |
| Domain-invalid value | {"age":-1} |
Validation response and unchanged state if invalid. |
| Unknown member | {"typo":true} |
Whether the endpoint rejects or ignores unknown members, as documented. |
Do not check only for an error. A handler can return a failure after partially changing state. RFC 5789 requires PATCH application to be atomic: clients must not be exposed to a partially applied patch if the complete patch cannot be applied. Stage updates in a copy or transaction, validate the complete change, and commit only when it can succeed; handler tests should verify that a rejected request leaves the resource unchanged.
Choose tests that match the JSON patch format
PATCH is the HTTP method, not one particular JSON format. Merge Patch and JSON Patch assign different meanings to null and use different document shapes, so the content type and test inputs must match the endpoint.
| Format | Media type | Document shape and null behavior | Useful test focus |
|---|---|---|---|
| JSON Merge Patch | application/merge-patch+json |
An object-shaped patch adds or replaces present members; a member set to null requests removal. A non-object patch replaces the whole target. This format is not suited to storing explicit JSON null as a meaningful member value. | Test omitted members, null removal, replacement, and non-object replacement if the endpoint permits it. See RFC 7396. |
| JSON Patch | application/json-patch+json |
An ordered array of operations, including add, remove, replace, move, copy, and test. Null inside an operation’s value is data, not the Merge Patch removal convention. |
Test valid operation sequences, invalid paths or operations, null values where allowed, and failure without partial application. See RFC 6902. |
If clients need null as a stored value, array-element edits, or explicit ordered operations, evaluate whether JSON Patch fits better than Merge Patch. If updates are mostly object member replacement and removal, Merge Patch may be simpler. Your choice also affects client support, validation, and the failure cases your handler must test.
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