"Has A" or "Belongs To" relationship or something else? - php

Question

I'm not sure why I'm struggling with this it seems like a very simple concept. So my struggling makes me think that perhaps my data modeling needs another component...
I'm using Laravel 5 and am trying to define some model relationships. BelongsTo,HasA, etc. Before I can write the code, I need to at least conceptually understand what type of relationship I'm creating.
I have an application to where users can send people referral links, if a person clicks on the link and signs up, their user record makes note of the code that referred them. This way I can trace back and see who referred a particular user. But a referral is NOT necessary to sign up
Tables:
USERS
+----+-------------+
| id | referral_id |
+----+-------------+
| 1 | 1 |
| 2 | null |
| 3 | 2 |
+----+-------------+
REFERRALS
+----+---------------+---------+
| id | referral_code | user_id |
+----+---------------+---------+
| 1 | 12345 | 2 |
| 2 | 54321 | 2 |
| 3 | 99999 | 2 |
+----+---------------+---------+
USERS.REFERRAL_ID references REFERRALS.ID
and
REFERRALS.USER_ID references USERS.ID
But what kind of relationships are these?
The only one that seems obvious to me is that REFERRALS.USER_ID belongs to USERS.
But what about USERS.REFERRAL_ID, saying it belongsTo Referrals doesn't feel right, as that record isn't required and I don't feel like it 'owns' the user by any means. Saying it hasA referral doesn't feel correct either, as again the user doesn't own or even require the referral.
I guess what is confusing me is that REFERRALS is an optional entity.
How should I conceptualize the relationship between USERS.REFERRAL_ID and REFERRALS.ID?
Is it bad to have this sort of "circular reference"? Would I be better off creating a pivot table?

Answer 1

No need to add any reference to the Referrals table in the User table, you already have that relation defined in the referral table ( user_id column )
Further reading: https://en.wikipedia.org/wiki/Database_normalization
The Relationship is
USER has many REFERRALS
REFERRAL belongs to USER ( inviter )
REFERRAL belongs to USER ( invitee )
Modify your REFERRALS table
+----+---------------+---------+------------+
| id | referral_code | user_id | invitee_id |
+----+---------------+---------+------------+
| 1 | 12345 | 2 | 1 |
| 2 | 54321 | 1 | null |
| 3 | 99999 | 3 | 1 |
+----+---------------+---------+------------+
user_id is the id of the user that sends the invitation
invitee_id is the id of the user that accepts and registers
invitee_id column is nullable() and will contain the id of the invitee from users table when they join.
Think of it as a JOIN table between inviter and invitee.