Understanding why relationship works - laravel

Question

I am trying to understand why something works. I have a user model and within it I state that a User can have one Project e.g.
public function project()
{
return $this->hasOne('App\Project', 'userId');
}
Then within my Project model I define a user belongs to a Project
public function user()
{
return $this->belongsTo('App\User', 'userId');
}
So the way I understand this, a User should be allowed only one project? So within my Projects controller, I have my store function. I wont go over all of it but I essentially do the following
$newProject = new Project();
$newProject->projectName = Input::get('projectName');
$newProject->projectValue = Input::get('projectValue');
$newProject->userId = Input::get('user');
$newProject->save();
Now where I get the input user, this is always the logged in users id. So say I log into the system and I create a new project. This project then has my userID. This works fine.
My question is why does it allow me to create a second project with my same ID? So if I am logged into the system, I can essentially create as many projects as I want under my name. Does this not go against what my relatiohsips are defined as? According to the relationship, I should only be allowed to create one Project.
I am really just looking for information as to why this is allowed?
Thanks

Answer 1

In One to One,
You're linking a record to another record in another table.
That means, You're checking for single record rather than entire table.
So in your example, As you defined it's a correct One - to - One relationship.
Why it's adding new more than one project?
As i said before, We are checking for a single record.
How do i restrict that?
use UNIQUE constraint for Foreign key
User table
----------
`id` int,
`project_id` UNIQUE,
Project table
----------
`id` int,
`user_id` UNIQUE,
Example without Unique Constraint http://sqlfiddle.com/#!9/f2b07/1/0
(Try to put same values it will fail to insert)
Example with Unique Constraint http://sqlfiddle.com/#!9/dda24/1