Handle dynamic parameters in URL in Golang

Question

currently am working on an API Rest in Golang. I have the process to CRUD all the tables. Now someone is asking me to develop an endpoint to search in one of that tables based on the parameters send in the URL. Let's say that this is my struct for that table:

type Media struct {
    ID                         uint       
    Key                   string     
    RecordKey           string     
    RecordID            string     
    SystemMediaKey      string          
    MediaObjectID       string   
    ChangedByID         string   
    ChangedByKey        string   
    MediaCategory              string   
    MimeType                   string   
    ShortDescription           string   
    LongDescription            string   
    EntryTimestamp             time.Time
    ModificationTimestamp      time.Time  
    DeletedAtTimestamp         *time.Time 
    MediaModificationTimestamp time.Time
    MediaURL                   string   
    MediaHTML                  string   
    Order                      int      
    Group                      string   
    Width                      int      
    Height                     int      
    ImageSize                  string   
    ResourceName               string  
    ClassName                  string 
    Permission                 *string
    MediaStatus                string
}

Now, he can send me all or some of that fields in the URL, the I need to assign that values to my struct to be able to search in the database based on the data assigned to the object.

I am using Gorm to handle everything with the database, gorilla/schema to assign the values on the POST requests and the Julien Schmidt Router. Now,my questions are:

1. What should I configure in the route to accept dynamic parameters?
2. How can I assign the values that comes in the URL to the a type Media object? Thank you!

Answer 1

I finally managed to use this library that converts a map to a struct. The only thing, is that I had to pre-process the map that is returned by URL.Query() Values because it returns an array for each value and I needed only the value and not inside an array.

Answer 2

You could use the reflect package to iterate over the fields and set them by name. Be aware that the reflect package is arduous to use and comes with some dangers of panics if not used properly.

Also be aware that url.Values.Get only returns the first value (see https://godoc.org/net/url#Values.Get for details)

EDIT: I added code to account for the pointers in the struct. They are handled differently.

https://play.golang.org/p/AO4lYx7xka

package main

import (
    "fmt"
    "net/url"
    "reflect"
    "time"
)

type Media struct {
    ID                         uint
    Key                        string
    RecordKey                  string
    RecordID                   string
    SystemMediaKey             string
    MediaObjectID              string
    ChangedByID                string
    ChangedByKey               string
    MediaCategory              string
    MimeType                   string
    ShortDescription           string
    LongDescription            string
    EntryTimestamp             time.Time
    ModificationTimestamp      time.Time
    DeletedAtTimestamp         *time.Time
    MediaModificationTimestamp time.Time
    MediaURL                   string
    MediaHTML                  string
    Order                      int
    Group                      string
    Width                      int
    Height                     int
    ImageSize                  string
    ResourceName               string
    ClassName                  string
    Permission                 *string
    MediaStatus                string
}

func main() {

    testUrl := "www.example.com/test?MimeType=themimetype&Key=thekey&Permission=admin"

    u, err := url.Parse(testUrl)
    if err != nil {
        fmt.Println(err)
        return
    }

    params := u.Query()

    media := &Media{}

    val := reflect.ValueOf(media)

    for i := 0; i < val.Elem().NumField(); i++ {
        // get the reflect.StructField so we can get the Name
        f := val.Elem().Type().Field(i)

        // check if URL.Values contains the field
        if v := params.Get(f.Name); v != "" {
            // get the reflect.Value associated with the Field
            field := val.Elem().FieldByName(f.Name)

            kind := field.Kind()

            // you must switch for each reflect.Kind (associated with the type in your struct)
            // so you know which Set... method to call
            switch kind {
            case reflect.String:
                field.SetString(v)
            case reflect.Ptr:
                // pointers are a special case that must be handled manually unfortunately.
                // because they default to nil, calling Elem() won't reveal the underlying type
                // so you must simply string match the struct values that are pointers.
                switch f.Name {
                case "Permission":
                    newVal := reflect.New(reflect.TypeOf(v))
                    newVal.Elem().SetString(v)
                    field.Set(newVal.Elem().Addr())
                case "DeletedAtTimestamp":
                }
            }

        }
    }

    fmt.Printf("%#v\n", media)
    fmt.Println(*media.Permission)
}