Cannot Load BelongsToMany Relation

It looks like you're encountering an issue with loading the belongsToMany relationship using the load method, but it works when you directly call get on the relationship. This discrepancy can sometimes be due to how the relationships are defined or how the data is structured in the database.

Here are a few steps to troubleshoot and potentially resolve the issue:

1. Check the Pivot Model: Ensure that your ApplicationSpace model extends Pivot and not Model. This is crucial for belongsToMany relationships.

   namespace App\Models;
   
   use Illuminate\Database\Eloquent\Relations\Pivot;
   
   class ApplicationSpace extends Pivot
   {
       // Any custom logic for the pivot model
   }
   

2. Ensure Correct Foreign Key Constraints: Verify that the foreign key constraints in your migration match the actual column names in your tables. Since you are using ulid, make sure the foreignUlid method is correctly applied.

3. Check the Relationship Definitions: Ensure that the relationship definitions in both models are correct. It looks like you have them set up correctly, but double-check for any typos or inconsistencies.

   namespace App\Models;
   
   use Illuminate\Database\Eloquent\Model;
   use Illuminate\Database\Eloquent\Relations\BelongsToMany;
   
   class Space extends Model
   {
       public function applications(): BelongsToMany
       {
           return $this->belongsToMany(Application::class, 'application_space', 'space_id', 'application_id')
               ->using(ApplicationSpace::class)
               ->withPivot('is_sync')
               ->withTimestamps();
       }
   }
   
   class Application extends Model
   {
       public function spaces(): BelongsToMany
       {
           return $this->belongsToMany(Space::class, 'application_space', 'application_id', 'space_id')
               ->using(ApplicationSpace::class)
               ->withPivot('is_sync')
               ->withTimestamps();
       }
   }
   

4. Loading the Relationship: When you use $space->load('applications');, ensure that $space is an instance of the Space model and that it has been retrieved from the database.

   $space = Space::find($spaceId); // Ensure $spaceId is a valid ID
   $space->load('applications');
   

5. Debugging the Issue: Add some debugging statements to check the state of the $space object before and after loading the relationship.

   $space = Space::find($spaceId);
   if ($space) {
       dd($space->applications); // Check if applications are loaded
       $space->load('applications');
       dd($space->applications); // Check if applications are loaded after load
   } else {
       dd('Space not found');
   }
   

6. Check for Null Values: The error message "Attempt to read property space_id on null" suggests that there might be a null value where the code expects an object. Ensure that all related records exist and that there are no null values in the application_space table.

By following these steps, you should be able to identify and resolve the issue with loading the belongsToMany relationship. If the problem persists, consider providing more context or specific code snippets that might be causing the issue.