For a Go API to distinguish an omitted field from an explicit null or a supplied zero value, preserve the JSON key’s presence before decoding it into an ordinary struct. A plain int cannot tell whether its value of 0 came from the request or from Go’s default value. The right treatment of null depends on the patch format: JSON Merge Patch treats it as removal, while JSON Patch has a separate remove operation.
First choose the patch format
“PATCH” describes an HTTP method, not one universal JSON format. Decide which document format the endpoint accepts before defining what a null means. The two common formats below use different shapes and semantics.
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| Requirement | 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 the member | Include no operation for its path |
| Remove a field | Set its 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 a null value |
| Arrays | Replaced as values; it cannot patch part of a non-object target | Operations can address array paths and indices |
| Typical fit | Straightforward object updates that do not need stored explicit nulls | Precise operation-level changes or explicit null assignment |
RFC 7396 specifies that “Null values in the merge patch are given special meaning to indicate the removal of existing values in the target.” Read the RFC 7396 specification for its processing rules and examples. RFC 6902 instead defines an array of operations with JSON Pointer paths; its JSON Patch specification describes operations including remove, add, and replace.
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Why an ordinary Go field loses request presence
When JSON is decoded into a regular Go struct, a scalar field that is absent can retain its Go zero value. If a request explicitly supplies that same value, such as 0 for an int, the resulting field value alone does not reveal which case occurred. The same modeling problem applies to meaningful values such as false and an empty string.
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Presence is separate from value. If the endpoint must distinguish “leave unchanged” from “set this to its zero value,” retain whether the key appeared as part of decoding or patch application. The Go project’s encoding/json documentation describes struct decoding and encoding behavior.
Decode Merge Patch fields without losing presence
For an object-shaped Merge Patch, decode into map[string]json.RawMessage. Map membership records whether a key was sent, while each raw message lets the handler distinguish a JSON null token from a concrete JSON value.
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- Parse the request object into a map of raw JSON values.
- For each field the endpoint permits, check whether its key exists in the map. If it does not, make no change to that field.
- If the key exists, identify whether its raw JSON token is
null. Apply the API’s documented clear/remove behavior, or reject null if the field does not allow it. - For any other token, decode it into the field’s concrete Go type. This preserves supplied values such as
0,false, and"". - Validate the assembled changes, authorize each permitted update, then apply them to the current resource.
This pattern leaves policy decisions explicit: decoding tells the handler what the client sent, while the endpoint contract determines which updates are valid and what removal means for the resource.
When a pointer or wrapper is not enough
A pointer can represent a concrete value versus nil, but on a fresh ordinary struct an omitted pointer field and an explicitly null pointer field can both end up nil. A pointer alone therefore does not reliably preserve all three states: absent, null, and concrete value.
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A wrapper can hold a value plus a Set flag, but the request decoder must set that flag only when the member is present. Do not assume the wrapper’s field value by itself records whether the key appeared. For larger APIs, centralize presence-aware decoding or patch application instead of reimplementing subtly different rules in each handler.
Keep marshaling options separate from PATCH input semantics
omitempty controls JSON marshaling; it does not track whether a request key was present 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. That can affect an encoded response, but it cannot recover request presence that was not retained while decoding.
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omitzero is also a marshaling option: it omits Go zero values or values whose IsZero method reports true. The documented JSON v2 behavior uses a different test for omitempty: whether the encoded JSON value is empty. Check the project’s Go version and imported package before relying on version-sensitive tag behavior; the Go project documents these distinctions in encoding/json and its JSON v2 documentation.
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Test the states independently
Tests should verify the request’s meaning, not just the decoded Go value. For each patchable field, exercise omission, explicit null, and a concrete value. Include zero, false, and an empty string where those values are valid for the field. Assert that omission leaves the existing resource unchanged, and that null and concrete values follow the endpoint’s documented contract.
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