[11.x] Allow factory to link to parent model without defining relationship model

[noefleury]

Situation

When doing some testing in a project, I've face a thing which could happened in some situations.

Imagine we have a model Computer and another which is ComputerReport.

Let's say we would like to build a view to show a list of every computers (Computer), and inside of a specific computer view, show all the linked reports (ComputerReport). In this really simple situation, I think we'll have a relationship method like ->reports() on our Computer model, but I think it's useless to have a relation like ->computer() on the ComputerReport model as we'll never use it.

The problem

When writing the testing logic :

$computer = Computer::factory()->create(['name' => 'My Computer']);
ComputerReport::factory()->for($computer)->create();

We'll be facing the following problem :

Call to undefined method App\Models\ComputerReport::computer()

Because the logic just don't find the missing ->computer() method in our ComputerReport model.

Solution

So, this PR make it work in this type of situation and don't force us to fill our model with logic which is only needed during testing. This is done though a belongsTo relation.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.