HasOne relationship returns null on save

Question

I have two models A and B with a foreign key on A of b_id on the id of B. the model code is as follows

A has a trait BB with the following methods

public function b() // shared by multiple models
    {
        return $this->hasOne(B::class, 'id', 'b_id'); // table has foreign keys setup
    }


public function saveB($params){
$b = $this->b;
        if(is_null($b)) $b = new B;
        else $b->a()->dissociate($b->id); // if there is a b break the link between them

        if($b->setParams($params) === true)
        {
          try {
            $this->b()->save($b);
            $this->save();
          } catch (Exceptio ....... throw an exception if anything
}

B is a model with the method

public method setParams($params) {
        $this->trigger = $params[0];
       $this->colour = $params[1];
  .... arbitrary stuff not related;
}

The problem arises out of the following TestCase logic

public function setUp()
  {
    parent::setUp();

    $this->a = A::find(237); // actual record
    $this->a->saveB(['trigger'=>1,'colour'=>'orange']);
  }
public function testSaveB(){ 
$this->assertGreaterThan(0, $this->a->b_id); 
}

The assertGreaterThan test fails and the b_id is null even though the following is true

• The table foreign keys exists
• Appropriate fields are fillable
• The data for B is inserted( no previous record existed)
• The record exists and b_id was NULL from inception
• No exceptions are thrown

Note the model names have been changed to make things easier to understand but the functions are more or less exactly as I have it

Does this logic model->hasOne()->save(model) inserts the newly created model and updates the foreign key for it? or do i have to still update the key/ value and $a->update() rather than $a->save()?