Workout app

Hello, im working on a workout app and I want to set up my relationships.

For example:

• An User can have many routines
• A Routine has a name and must have 7 workout days (i can set up the 7 days and edit those later)
• A WorkoutDay has x number of sets
• A Set has x number of excercises
• An Excercise has a name, #of reps and #of times

I was setting up the very first things and got into this:

I am using laravel 5 default user migration only changed PK to user_id

User.php

<?php namespace App;

use Illuminate\Auth\Authenticatable;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Auth\Passwords\CanResetPassword;
use Illuminate\Contracts\Auth\Authenticatable as AuthenticatableContract;
use Illuminate\Contracts\Auth\CanResetPassword as CanResetPasswordContract;

class User extends Model implements AuthenticatableContract, CanResetPasswordContract {

	use Authenticatable, CanResetPassword;

	/**
	 * The database table used by the model.
	 *
	 * @var string
	 */
	protected $table = 'users';

	/**
	 * The database table primary key.
	 *
	 * @var string
	 */
	protected $primaryKey = 'user_id';

	/**
	 * The attributes that are mass assignable.
	 *
	 * @var array
	 */
	protected $fillable = ['name', 'email', 'password'];

	/**
	 * The attributes excluded from the model's JSON form.
	 *
	 * @var array
	 */
	protected $hidden = ['password', 'remember_token'];

	//relationships

	/**
	 * An User can have many Routines
	 * @return mixed
	 */
	public function routines()
	{
		return $this->hasMany('App\Routine');
	}

}

Routine.php

<?php namespace App;

use Illuminate\Database\Eloquent\Model;

class Routine extends Model {

	/**
	 * The database table used by the model.
	 *
	 * @var string
	 */
	protected $table = 'routines';

	/**
	 * The database table primary key.
	 *
	 * @var string
	 */
	protected $primaryKey = 'routine_id';

	/**
	 * The attributes that are mass assignable.
	 *
	 * @var array
	 */
	protected $fillable = [
		'name',
		'user_id',
	];

	//relationships

	/**
	 * A Routine is owned by an User.
	 *
	 * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
	 */
	public function user()
	{
		return $this->belongsTo('App\User');
	}
}

Until here it worked as expected.

I tried to set up the second relationship by doing this:

1.- Created the table

Schema::create('workout_days', function(Blueprint $table)
		{
			$table->increments('day_id');
			$table->integer('routine_id')->unsigned();
			$table->enum('type', ['WORK', 'OFF']);
			$table->tinyInteger('day_number');
			$table->timestamps();

			$table->foreign('routine_id')->references('routine_id')->on('routines')->onDelete('cascade');
		});

The WorkoutDay Model:

<?php namespace App;

use Illuminate\Database\Eloquent\Model;

class WorkoutDay extends Model {

	/**
	 * The database table used by the model.
	 *
	 * @var string
	 */
	protected $table = 'workout_days';

	/**
	 * The database table primary key.
	 *
	 * @var string
	 */
	protected $primaryKey = 'day_id';

	/**
	 * The attributes that are mass assignable.
	 *
	 * @var array
	 */
	protected $fillable = [
		'routine_id',
		'type',
		'day_number',
	];

	//relationships

	/**
	 * A WorkoutDay is owned by a Routine.
	 *
	 * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
	 */
	public function routine()
	{
		return $this->belongsTo('App\Routine');
	}
}

Added this to Routine.php

        /**
	 * A Routine can have many WorkoutDays
	 *
	 * @return mixed
	 */
	public function workoutdays()
	{
		return $this->hasMany('App\WorkoutDay');
	}

Then tested using 'php artisan tinker' and got this:

$user = App\User::first(); //I only have one user so this wont change from this User::find(1);
$user->routines; //it showed the routines I setted up to the user
//then trying to do this:
$user->routines->workoutdays; //Undefined property: Illuminate\Database\Eloquent\Collection::$workoutdays on line 1 -- When calling routines, do i get a collection?
//also tried 
$user->routines()->workoutdays; //didnt worked
//also tried 
$user->routines()->workoutdays(); //errors poppin everywhere.

Thanks in advance