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For a Gin PATCH endpoint, track whether each field was sent separately from the value it contains. A JSON request such as {"active":false,"quota":0} can then explicitly set a boolean to false and a number to 0, while omitted fields remain unchanged. Avoid binding:"required" on optional scalar fields: Gin’s validator treats a type’s zero value as missing.
Why ordinary scalar fields lose PATCH presence
When Go decodes JSON into a struct with plain int, bool, or string fields, an omitted field has the type’s zero value. An explicitly supplied 0, false, or empty string also produces that value. Once decoded this way, the handler cannot tell omission from an explicit zero by inspecting the field alone.
The JSON struct option json:",omitempty" does not solve this: it controls whether zero-valued fields are omitted when encoding JSON, not whether an incoming member was present. Gin uses go-playground/validator/v10 for binding validation, and its required rule rejects default zero values, including 0, false, and "". See Gin’s model binding and validation documentation, the Go encoding/json documentation, and validator’s tag documentation.
Use pointers for the common two-state case
If your API treats JSON null the same as omission, or does not allow clients to clear a field with null, pointers provide a compact typed representation. A nil pointer means there is no decoded value to apply; a non-nil pointer can hold an explicit zero.
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type PatchUser struct {
Age *int `json:"age"`
Active *bool `json:"active"`
Name *string `json:"name"`
}
func patchUser(c *gin.Context) {
var req PatchUser
if err := c.ShouldBindJSON(&req); err != nil {
c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
return
}
if req.Age != nil {
if *req.Age < 0 {
c.JSON(http.StatusUnprocessableEntity, gin.H{"error": "age must be non-negative"})
return
}
// Apply *req.Age, including 0.
}
if req.Active != nil {
// Apply *req.Active, including false.
}
if req.Name != nil {
// Apply *req.Name if the API permits an empty string.
}
}This is an implementation pattern, not a tested complete handler; add the imports, persistence logic, and field-specific rules your endpoint needs. The key is to check presence first, validate the supplied concrete value, and apply it only when present. Do not treat a supplied zero as absence.
Decide what JSON null means
A pointer field generally does not distinguish an omitted property from a property supplied as null; both result in nil. Choose and document the endpoint’s contract rather than letting decoding behavior decide it accidentally.
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- Null means no change: a pointer DTO is suitable; nil covers both omission and null.
- Null is invalid: use presence-aware decoding or a custom optional type so you can reject null instead of treating it as omission.
- Null clears the value: represent three states—omitted (leave unchanged), null (clear), and concrete value (set). A custom optional type or decoding the object into
map[string]json.RawMessagebefore typed value handling can preserve this distinction.
The Go JSON decoder’s behavior and the fact that marshaling options do not report incoming member presence are described in the Go encoding/json documentation.
Validate supplied fields without rejecting valid zeroes
Keep presence checks separate from value rules. For example, a supplied age can be checked for a non-negative value even when it is zero; a supplied boolean can be false; and a supplied string can be empty if the API permits it. The same pattern applies to nested or more complex fields: first determine whether the client supplied the field, then validate its value according to the endpoint’s contract.
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Validator tag names can be easy to confuse because they operate in different contexts:
json:",omitempty"affects JSON encoding output; it is not a request-presence rule.binding:"required"rejects the type’s zero value, so it is usually wrong for optional PATCH scalars where zero is meaningful.- Validator’s
omitemptyskips subsequent validation when the validator considers a field unset. It is distinct from the JSON option with the same name. omitnilskips checks for nil values.omitzeroskips zero values, including a pointer’s pointed-to zero, so it is unsuitable when an explicit zero must be validated or applied.
Gin supports custom validation functions and struct-level validation for reusable rules; presence semantics still need to be represented correctly in the request type. See Gin’s custom validator documentation and validator’s tag documentation.
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Choose the representation that matches the contract
| Approach | Presence and null states | Zero-value handling | Best fit |
|---|---|---|---|
Plain scalar field (int, bool, string) |
Cannot distinguish omitted from a supplied zero after ordinary decoding. | Zero, false, and empty string are indistinguishable from omission. | Not suitable by itself when PATCH must preserve explicit zero values. |
| Pointer field | Distinguishes a decoded value from nil; does not inherently distinguish omission from JSON null. | Non-nil pointer preserves explicit zero, false, or empty string. | Simple typed DTO when null is equivalent to omission or is not a supported clearing operation. |
| Custom optional type or raw-key parsing | Can represent omitted, null, and concrete value separately if implemented to preserve all three states. | Can preserve and validate explicit zero values. | APIs where null has a distinct meaning, such as clearing a field or returning a validation error. |
Use ShouldBindJSON to control API errors
Gin’s ShouldBindJSON returns a binding or validation error to the handler, allowing the API to choose the response and error body. By contrast, BindJSON aborts the request and writes a 400 response on binding errors. For an endpoint with deliberate 400 versus 422 behavior, use ShouldBindJSON and make that decision explicitly. Gin’s binding documentation also shows how a missing field tagged binding:"required" produces a validation error, which is a useful distinction between a create DTO and a partial-update DTO.
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PATCH validation checklist
- Omitted field: leave the stored value unchanged.
- Explicit
0,false, or"": preserve it as supplied and apply the field’s own validation rules. null: follow the documented contract—ignore, reject, or clear.- Malformed JSON: return a deliberate client error from the binding failure path.
- Invalid supplied value: return the endpoint’s chosen validation response without confusing it with omission.
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