Fix BelongsToMany Relation to Support Array Foreign Keys in Laravel MongoDB

[peymanaslani70]

🚀 What’s in This PR?

This PR fixes the BelongsToMany relationship to support array-based foreign keys, allowing multiple related ObjectIds to be stored in a single field.

🐞 The Issue

• Previously, BelongsToMany expected foreign keys to be a single value, causing issues when an array was used.
• This resulted in errors like Illegal offset type when working with MongoDB.

🔍 Issue Reference
Fixes [#3015]

🔧 Changes Made

✅ Updated attach() in BelongsToMany.php to store ObjectIds as an array instead of a single value.
✅ Modified buildDictionary() to correctly handle arrays when resolving relationships.
✅ Added a new unit test: testBelongsToManyRelationSupportsArrayForeignKeys (in tests/RelationsTest.php).

• Ensures that array-based foreign keys work as expected.
• Confirms that relationships retrieve the correct data.

🧪 How to Test It?

1. Run the tests with:

   vendor/bin/phpunit tests/RelationsTest.php
2. All tests should pass ✅

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📜 Documentation Update Checklist

✔️ Updated Many-to-Many Relationship section in the docs.
✔️ Explained how to use array-based foreign keys with BelongsToMany().
✔️ Added an example showing how to define relationships with an array field.

Checklist

• Add tests and ensure they pass
• Updated Many-to-Many Relationship section in the docs.