[5.8] Fix column overriding in HasManyThrough relationships

[staudenmeir]

When the related table of a HasManyThrough relationship contains a column with the same name as the $firstKey in the intermediate table (selected for eager loading), the intermediate value overrides the related value:

Schema::create('users', function ($table) {
    $table->increments('id');
});

Schema::create('teams', function ($table) {
    $table->increments('id');
    $table->integer('owner_id');
});

Schema::create('projects', function ($table) {
    $table->increments('id');
    $table->integer('owner_id');
});

class User extends Model
{
    public function projects()
    {
        return $this->hasManyThrough(Project::class, Team::class, 'owner_id', 'owner_id');
    }
}

User::with('projects')->get();

// Contains "teams.owner_id" instead of "projects.owner_id".
dump($users[0]->projects[0]->owner_id);

We can fix this by adding an alias to the $firstKey column (that hopefully nobody uses as a column name):

select "projects".*, "teams"."owner_id" as "_first_key"
from "projects" inner join "teams" on "teams"."id" = "projects"."owner_id"
where "teams"."owner_id" in (?)

There already are eager loading tests.

Fixes #17918.