Create conditional in Model methods

Hi guys i wish to create a custom method on my Category Model but i need to create some conditionals with it.

table post:
- id;
- category_id
-name...


table Category:
- id
- parent_id
- name

Imagin that i have a post, and in this post there is a category called SomeSubcategoryName, i my posts table i have a column called category_id.

When i call the post record i have a relation with the Category Model, but i wish to have in my Category Model a method called masterCategory, where i give the mster category by checking if the parent_id is null or not, in case that the record have a parent_id of null great, i return the record but if is not i need to use the parent_id value and search on the Model category column id for the record and return the result.

Imagine this scenario:

$post = Post::find(2);

//this call should return the main category and not the subcategory info.

$post->masterCategory();

via Chebli Mohamed