(SP: 5) Migration Task: Remove documents that refer to non-existent users

[PavloButynets]

1. Analyze the attachments collection:

• Identify documents where the author field references a user that no longer exists in the users collection.
• Delete these documents.

2. Analyze the lessons collection:

• Check for documents where the author field refers to a user that does not exist in the users collection.
• Delete those documents.

3. Analyze the quizzes collection:

• Identify documents where the author field references a non-existent user in the users collection.
• Remove those documents.

4. Analyze the questions collection:

• Identify documents where the author field references a user that no longer exists in the users collection.
• Delete these documents.

5. Analyze the resourcescategories collection:

• Locate documents in which the author field refers to a user that does not exist in the users collection.
• Delete these documents.

6. Analyze the courses collection:

• Locate documents where the author field references a non-existent user in the users collection.
• Remove these documents from the collection.

7. Analyze the notes collection:

• Search for documents with an author field that references a non-existent user in the users collection.
• Remove the invalid documents.