Table: Users
id       - int (auto increment)
username - varchar

Table: Challenges
id    - int (auto increment)
name  - varchar

Table User_challenge_links
id                          - int (auto increment)
user_id                     - int
challenge_sub_categories_id - int

Table Challenge_sub_categories
id                - int (auto increment)
category_id       -
sub_category_id   -
challenge_id      -

class User extends Eloquent
{
    protected $table = "users";

    public function challenges() {
        // get user-> challenges
    }
}

Users
id    name
1     "Sjaak"
2     "Henk"

id   category_id    sub_category_id    challenge_id
1    1              1                  1
2    2              1                  1
3    1              2                  2
4    2              1                  3
5    2              2                  2

id     user_id      Challenge_sub_categories_id
1      1            1
2      1            3
3      2            2
4      2            3
5      2            4

category - House
    sub_category - Cleaning
         Challenge - getting special soap

category - Car
    sub_category - Cleaning
        Challenge - getting special soap

category - Showering
    Challenge - getting special soap

users
    id
    username

challenge_user
    user_id
    challenge_id

challenges
    id
    name
    topic_id
    category_id

topics
    id
    name

categories
    id
    name

class User extends Eloquent {
    public function challenges() {
        return $this->belongsToMany('Challenge');
    }
}

class Challenge extends Eloquent {
    public function users() {
        return $this->belongsToMany('User');
    }
    public function topic() {
        return $this->belongsTo('Topic');
    }
    public function category() {
        return $this->belongsTo('Category');
    }
}

class Topic extends Eloquent {
    public function challenges() {
        return $this->hasMany('Challenge');
    }
}

class Category extends Eloquent {
    public function challenges() {
        return $this->hasMany('Challenge');
    }
}

// Collection of all Challenges by Topic name
Topic::with('challenges')->whereName($topic_name)->first()->challenges;

// Collection of all Challenges by Category name
Category::with('challenges')->whereName($category_name)->first()->challenges;

// Collection of all Users by Challenge id
Challenge::with('users')->find($challenge_id)->users;

// Collection of Users with atleast 2 Challenges
User::has('challenges', '>', 1)->get();

// Attach Challenge to User
$user = User::find($id);
$user->challenges()->attach($challenge_id);

// Assign a Topic to a Challenge
$challenge = Challenge::find($challenge_id);
$topic     = Topic::find($topic_id);

$challenge->topic()->associate($topic);
$challenge->save();