REST API Update by partial json

If the Update request uses the same Customer struct then the struct fields could be pointers to differentiate between zero value and value not being set in the JSON.
Now all you need to do is merge existing struct into updated Consumer struct.
For this you can use https://github.com/imdario/mergo library in Go.

package main import ( "fmt" "github.com/imdario/mergo" "encoding/json" "os" ) type Address struct { City string `json:"city"` } type Customer struct { Id int64 `json:"id"` Name string `json:"name"` Age int `json:"age"` Address *Address `json:"address"` } func main() { old1 := &Customer{Id:1, Name:"alpha", Age:5, Address:&Address{City:"Delhi"}} b := []byte(`{"name": "beta"}`) //no address, age specified picks from old up1 := new(Customer) json.Unmarshal(b, up1) if err := mergo.Merge(up1, old1); err != nil { fmt.Printf("err in 1st merge: %v\n", err) os.Exit(1) } m1, _ := json.Marshal(up1) fmt.Printf("merged to: %v\n", string(m1)) old2 := &Customer{Id:1, Name:"alpha", Age:5, Address:&Address{City:"Delhi"}} b2 := []byte(`{ "address": {"city": "mumbai"}}`) //address specified up2 := new(Customer) json.Unmarshal(b2, up2) if err := mergo.Merge(up2, old2); err != nil { fmt.Printf("err in 1st merge: %v\n", err) os.Exit(1) } m2, _ := json.Marshal(up2) fmt.Printf("merged to: %v\n", string(m2)) } 

from your comments it seems you are hitting the zero-value issue a lot of users with go encounter i.e. how does one tell if the input data passed a legitimate value - or was that value zeroed by default omission.

The only way to solve this is to use pointers. So in your example, changing your data struct to this:

type Customer struct { Id *int64 `json:"id"` Name *string `json:"name"` Age *int `json:"age"` } 

Then after unmarshaling, any uninitialized fields will have nil values e.g.

var c Customer err := json.Unmarshal(jsonData, &c) if err != nil { panic(err) } if c.Id != nil { log.Println("TODO: added SQL update parms for c.Id:", *c.Id) } if c.Name != nil { log.Println("TODO: added SQL update params for c.Name:", *c.Name) } if c.Age != nil { log.Println("TODO: added SQL update parms for c.Age:", *c.Age) } 

Note: care must be taken to ensure one does not accidentally reference any nil pointers which would trigger an instant panic.

Working playground example.

Another way to partial unmarshal JSON like Protocol Buffers FieldMask

• First deserialize restful api request JSON to map[string]interface{}
• Then convert map[string]interface{} to a FieldMask through fieldmask
• Finally parse map[string]interface{} into a struct through mapstructure

Here is one code sample

 type Customer struct { Name string `json:"name"` Age int `json:"age"` Address string `json:"address"` } func (p *Customer) Update(other Customer, fm fieldmask.FieldMask) { if len(fm) == 0 { *p = other return } if fm.Has("name") { p.Name = other.Name } if fm.Has("age") { p.Age = other.Age } if fm.Has("address") { p.Address = other.Address } } type UpdateCustomerRequest struct { Customer FieldMask fieldmask.FieldMask `json:"-"` } func (req *UpdateCustomerRequest) UnmarshalJSON(b []byte) error { if err := json.Unmarshal(b, &req.FieldMask); err != nil { return err } return mapstructure.Decode(req.FieldMask, &req.Customer) } func PartialUpdate() { customer := Customer{ Name: "foo", Age: 20, Address: "address 1", } fmt.Printf("init: %#v\n", customer) blob := []byte(`{"age": 10, "address": "address 2"}`) req := UpdateCustomerRequest{} if err := json.Unmarshal(blob, &req); err != nil { fmt.Printf("err: %#v\n", err) } customer.Update(req.Customer, req.FieldMask) fmt.Printf("updated: %#v\n", customer) }