Go and Gin: Passing around struct for database context?

I've just started trying out Go, and I'm looking to re-implement an API server written in node with it.

I've hit a hurdle with trying to use dependency injection to pass around a database context as a gin middleware. So far I've set it up as this:

main.go:

package main

import (
        "fmt"
        "runtime"
        "log"
        "github.com/gin-gonic/gin"
        "votesforschools.com/api/public"
        "votesforschools.com/api/models"
)

type DB struct {
        models.DataStore
}

func main() {
        ConfigRuntime()
        ConfigServer()
}

func Database(connectionString string) gin.HandlerFunc {
        dbInstance, err := models.NewDB(connectionString)
        if err != nil {
                log.Panic(err)
        }

        db := &DB{dbInstance}

        return func(c *gin.Context) {
                c.Set("DB", db)
                c.Next()
        }
}


func ConfigRuntime() {
        nuCPU := runtime.NumCPU()
        runtime.GOMAXPROCS(nuCPU)
        fmt.Printf("Running with %d CPUs\n", nuCPU)
}

func ConfigServer() {

        gin.SetMode(gin.ReleaseMode)

        router := gin.New()
        router.Use(Database("<connectionstring>"))
        router.GET("/public/current-vote-pack", public.GetCurrentVotePack)
        router.Run(":1000")
}

models/db.go

package models

import (
        "database/sql"
        _ "github.com/go-sql-driver/mysql"
)

type DataStore interface {
        GetVotePack(id string) (*VotePack, error)
}

type DB struct {
        *sql.DB
}

func NewDB(dataSource string) (*DB, error) {
        db, err := sql.Open("mysql", dataSource)
        if err != nil {
                return nil, err
        }
        if err = db.Ping(); err != nil {
                return nil, err
        }
        return &DB{db}, nil
}

models/votepack.go

package models

import (
        "time"
        "database/sql"
)

type VotePack struct {
        id string
        question string
        description string
        startDate time.Time
        endDate time.Time
        thankYou string
        curriculum []string
}

func (db *DB) GetVotePack(id string) (*VotePack, error) {

        var votePack *VotePack

        err := db.QueryRow(
                "SELECT id, question, description, start_date AS startDate, end_date AS endDate, thank_you AS thankYou, curriculum WHERE id = ?", id).Scan(
                &votePack.id, &votePack.question, &votePack.description, &votePack.startDate, &votePack.endDate, &votePack.thankYou, &votePack.curriculum)

        switch {
        case err == sql.ErrNoRows:
                return nil, err
        case err != nil:
                return nil, err
         default:
                return votePack, nil
        }
}

So with all of the above, I want to pass the models.DataSource around as a middleware so it can be accessed like this:

public/public.go

package public

import (
        "github.com/gin-gonic/gin"
)

func GetCurrentVotePack(context *gin.Context) {
        db := context.Keys["DB"]

        votePack, err := db.GetVotePack("c5039ecd-e774-4c19-a2b9-600c2134784d")
        if err != nil{
                context.String(404, "Votepack Not Found")
        }
        context.JSON(200, votePack)
}

However I get public\public.go:10: db.GetVotePack undefined (type interface {} is interface with no methods)

When I inspect in the debugger (using Webstorm with plugin) the db is just an empty object. I'm trying to be good and avoid global variable use

I don't think context should be used as DI container: https://golang.org/pkg/context/
>Package context defines the Context type, which carries deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes.

I would rather use:

package public

type PublicController struct {
        Database *DB
}

func (c *PublicController) GetCurrentVotePack(context *gin.Context) {
        votePack, err := c.Database.GetVotePack("c5039ecd-e774-4c19-a2b9-600c2134784d")
        if err != nil{
                context.String(404, "Votepack Not Found")
        }
        context.JSON(200, votePack)
}

and configure your controller in main:

func main() {
        pCtrl := PublicController { Database: models.NewDB("<connectionstring>") }

        router := gin.New()
        router.GET("/public/current-vote-pack", pCtrl.GetCurrentVotePack)
        router.Run(":1000")
}

The values within context.Keys are all of type interface{}, so db will not be able to call methods from type *DB until it's converted back to that type.

The safe way:

db, ok := context.Keys["DB"].(*DB)
if !ok {
        //Handle case of no *DB instance
}
// db is now a *DB value

The less safe way, which will panic if context.Keys["DB"] is not a value of type *DB:

db := context.Keys["DB"].(*DB)
// db is now a *DB value

Effective Go has a section on this.

You would need type assertion to convert the interface (db := context.Keys["DB"]) into something useful. See for example this post: https://stackoverflow.com/questions/18041334/convert-interface-to-int-in-go-lang

There's another way to do it when DB is set to the context during startup.

 db := ctx.MustGet("DB").(*gorm.DB)

MustGet returns the value for the given key if it exists, otherwise it panics.