For a Go JSON PATCH endpoint, keep three inputs distinct when the API needs them: an omitted member means “leave unchanged,” JSON null follows the patch format’s removal or null rule, and a concrete value such as 0 is an update. A plain Go scalar does not record whether its JSON key appeared. Decode presence explicitly before applying changes, and decide the meaning of null from the endpoint’s patch format and contract.
First identify which PATCH format the endpoint accepts
PATCH is an HTTP method, not a single JSON format. The two common formats below give null different meanings, so document the media type and behavior rather than treating them as interchangeable.
| Behavior | JSON Merge Patch (RFC 7396) | JSON Patch (RFC 6902) |
|---|---|---|
| Document shape | An object resembling the target document | An array of operation objects |
| Leave a field unchanged | Omit its member | Include no operation for its path |
| Remove a field | Set the member to null |
Use a remove operation |
| Assign explicit JSON null | Not representable as an ordinary member value: null means removal | Use add or replace with value: null |
| Arrays | Replaced as values; Merge Patch cannot edit part of a non-object target | Operations can address array paths and indices |
| Typical fit | Straightforward object updates without a need to store explicit nulls | Precise operation-level changes or explicit null assignment |
RFC 7396 defines null in a merge patch as removal of an existing value. If a resource must retain an explicit JSON null as distinct from a missing member, JSON Merge Patch cannot express that ordinary member update; JSON Patch can express removal and assignment separately. See the RFC 7396 specification and RFC 6902 specification.
Why a plain Go field loses the distinction
Suppose a request struct has an int field. After decoding, its value is zero both when the request omitted that member and when it supplied "count": 0. The same ambiguity applies to scalar zero values such as false and "". If the endpoint treats supplied values as updates, it must preserve key presence before decoding into a model that only holds values.
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A pointer alone is not a universal fix. On a fresh ordinary struct, an absent pointer field and a field explicitly set to JSON null can both decode as nil. A pointer can distinguish a non-null value from nil, but it does not by itself preserve all three states of absent, null, and concrete value.
Decode Merge Patch fields while preserving presence
For an object-shaped Merge Patch request, decode into map[string]json.RawMessage. Map membership tells you whether a key was sent; the raw JSON lets you distinguish null from a concrete value before decoding that value to its Go type.
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var patch map[string]json.RawMessage
if err := json.NewDecoder(r.Body).Decode(&patch); err != nil {
// Return a client error for invalid JSON.
}
raw, present := patch["count"]
if !present {
// No requested change to count.
} else if bytes.Equal(bytes.TrimSpace(raw), []byte("null")) {
// Apply the API's documented null behavior (or reject null).
} else {
var count int
if err := json.Unmarshal(raw, &count); err != nil {
// Return a client error for a value that is not an integer.
}
// A supplied 0 reaches this branch as a real value.
}This example shows the decision points, not a complete HTTP handler: production code should handle malformed input, unknown keys, type errors, validation, and authorization explicitly. For each supported field, test map presence first, then handle null according to the contract, then decode and validate a concrete value. Apply only authorized changes to the current resource.
Represent the update decision explicitly
Keep the parsed request separate from the stored resource until you know which changes were requested. For example, a field-level update can be modeled conceptually as a presence flag plus a value, with an additional null state if the API needs one. A wrapper type can carry a Set flag, but the containing request decoder must set it only when the member is present; a value field alone cannot infer that fact.
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For a larger API, centralize presence and null handling in reusable decoding or patch-application code. Test each important case independently: absent, null, zero, false, and empty string. Also test invalid types and unknown members according to the endpoint’s policy.
Do not use omitempty to detect request presence
omitempty is a marshaling option, not a record of which keys arrived during unmarshaling. In the documented legacy encoding/json behavior, it omits false, numeric zero, nil pointers and interfaces, and empty arrays, slices, maps, and strings when encoding. That can be undesirable when a response must include a meaningful zero or false value, but it does not solve PATCH input tracking. See the Go encoding/json documentation.
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The same documentation describes omitzero as omitting Go zero values, or values whose IsZero method reports true. JSON v2 documents a different meaning for omitempty: it tests whether the encoded JSON value is empty. These are version- and package-sensitive details; check whether the project uses the legacy package or JSON v2, and which Go version applies, before copying a tag pattern. The JSON v2 documentation describes that package’s behavior.
Build the handler around the contract
- Confirm the wire format. Decide whether the endpoint accepts Merge Patch objects or JSON Patch operation arrays, and document the media type and semantics.
- Preserve input presence. For a Merge Patch object, retain key membership and raw values, such as with
map[string]json.RawMessage. For JSON Patch, parse and validate each operation and its path. - Apply null semantics deliberately. Under Merge Patch, null removes the corresponding object member. Under JSON Patch,
removeremoves a path, whileaddorreplacecan carry a null value. - Decode concrete values by field type. Keep supplied values such as zero, false, and an empty string as actual updates rather than confusing them with omission.
- Validate and authorize changes. Check values and permitted fields before applying updates to the current resource; reject malformed, disallowed, or invalid changes under the API’s defined error policy.
- Test the state distinctions. Cover omitted, null, zero, false, empty string, and invalid-type inputs, plus the removal and explicit-null behavior relevant to the chosen format.
Go’s JSON tutorial documents struct-field encoding and nil-pointer behavior. Those encoding rules do not replace an explicit presence strategy for PATCH requests.
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