json.Unmarshal convert map to slice

Given the following JSON string:

{
 "username":"bob",
 "name":"Robert",
 "locations": [
   {
    "city": "Paris",
    "country": "France"
   },
   {
    "city": "Los Angeles",
    "country": "US"
   }
 ]
}

I need a way to unmarshal this into the a struct like this:

type User struct {
 Username string
 Name string
 Cities []string
}

Where Cities is a slice containing the "city" values, and "country" is discarded.

I think this can be done using a custom JSON.Unmarshal function, but not sure how to do that.

>Solution :

You can define new type for Cities and implement custom Unmarshaler:

type User struct {
    Username string   `json:"username"`
    Name     string   `json:"name"`
    Cities   []Cities `json:"locations"`
}

type Cities string

func (c *Cities) UnmarshalJSON(data []byte) error {
    tmp := struct {
        City string `json:"city"`
    }{}
    err := json.Unmarshal(data, &tmp)
    if err != nil {
        return err
    }
    *c = Cities(tmp.City)
    return nil
}

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