Laravel eloquent - making query with optional value key parameters with relationship

I need to make a query with optional parameters array, that could look something like this:

array:3 [
  "parent" => "prosjektsamarbeidets"
  "category" => "article"
  "slug" => "prosjektsamarbeidets-verdi"
]

It will always have a slug parameter but other parameters are optional in the array. With this arguments I need to make a query in the Model Content, where the relationship for the category which refers in this case to the model Taxonomy is set up like this:

public function taxonomies()
{
    return $this->morphToMany('App\ContentTaxonomy', 'taggable');
}

And the parent key value if it exists is the name of the record from the contents table, which has this columns:

id | cms_id | title | slug | excerpt | body | parent_id  

How can I do such a query, after I get the content by slug?

$post = Content::where('slug',$arguments['slug'])->get();

if (array_key_exists('parent', $arguments)) {
      //do further query by parent name
}

if (array_key_exists('category', $arguments)) {
      //do further query by category
}