file.go

// I created Delete method on Post struct

func (p *Post) Delete(db *DB) error {
	_, err := db.Exec("DELETE FROM posts WHERE id = ?", p.ID)
	if err != nil {
		return err
	}
	return nil
}

// I added method DeletePost to Datastore interface

type Datastore interface {
  ...
	DeletePost(post *Post) error
}

// Implementation of DeletePost

func (db *DB) DeletePost(post *Post) error {
	return post.Delete(db)
}

// I created Destroy method on Env receiver (I'm using gin-gonic)

func (e *Env) Destroy(c *gin.Context) {
	post, err := ctrl.DB.GetPost(c.Param("id"))
	...
}