[5.6] Added unsetRelation method to Eloquent models

[IsraelOrtuno]

There are many ways to set and load relationships in Eloquent models. However it becomes tricky for unsetting an already loaded relation.

I do work a lot in packages which interact with Eloquent and relationships with paradigms such as EAV and others where you have to manipulate relationships directly from traits. I think it'll be useful to provide as much methods as possible to manage them.

Right now, the only way I found to unset a relationship from outside the model was:

// Lets assume "user" and "image" relations and we try to remove "user"
$relations = $model->getRelations();
unset($relations['user']);
$model->setRelations($relations);

With this PR:

$model->unsetRelation('user');

[Miguel-Serejo]

Is this necessary? As long as $this->relations isn't null, unset($this->relations('foo')) will always work

[IsraelOrtuno]

You are right, thought it could cause an undefined index error, will change

[laurencei]

Out of curisosity - what is the use case for this? Why would you want to unset a relationship?

[IsraelOrtuno]

This came up because I am developing a permalink based routing system. I am setting a Permalink model to the Route instance via a Macro method. When you cache your routes, you will get all these and their relationships encoded to base64. I do not want to cache anything else than the model itself, do not want to get a huge base64 string.

I guess this may be also useful for not serializing relations.