How to check if a document is mongoose sub schema in middleware?

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Summary

The issue at hand is how to prevent a mongoose plugin from adding an organizationId field to sub-documents in a multi-tenant application. The current implementation uses a global plugin to handle adding organizationId during filters, saving, and aggregation, but it incorrectly adds this field to sub-documents.

Root Cause

The root cause of this issue is the inability to distinguish between top-level documents and sub-documents within the mongoose middleware. The this object in the pre middleware hook refers to the document being saved, but it does not provide a straightforward way to determine if the document is a sub-document or a top-level document.

Why This Happens in Real Systems

This issue occurs in real systems because:

  • Mongoose plugins are designed to be reusable and flexible, but they can sometimes lack the context needed to make decisions about specific documents.
  • Sub-documents can be nested arbitrarily deep, making it difficult to determine their position within the document hierarchy.
  • Middleware hooks have limited access to the surrounding context, making it challenging to implement conditional logic based on the document’s position.

Real-World Impact

The real-world impact of this issue includes:

  • Incorrect data: Sub-documents may be assigned an organizationId field, leading to inconsistent and potentially incorrect data.
  • Performance issues: Unnecessary updates to sub-documents can result in slower performance and increased database load.
  • Security concerns: In a multi-tenant application, incorrect assignment of organizationId can lead to unauthorized access to sensitive data.

Example or Code

To illustrate the issue, consider the following example:

const parentObj = {
  subObject: [
    { name: 'subObject1' },
    { name: 'subObject2' },
    { name: 'subObject3' }
  ]
};

In this example, the subObject array contains sub-documents that should not have an organizationId field added to them.

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Checking the document’s constructor: Use the instanceof operator to check if the document is an instance of the mongoose.Document constructor.
  • Inspecting the document’s schema: Use the schema object to determine if the document is a sub-document or a top-level document.
  • Using a custom flag: Add a custom flag to the document or schema to indicate whether the organizationId field should be added.

Why Juniors Miss It

Junior engineers may miss this issue because:

  • Lack of experience: They may not have worked with complex document hierarchies or mongoose plugins before.
  • Insufficient understanding: They may not fully understand how mongoose middleware hooks work or how to access the surrounding context.
  • Overlooking edge cases: They may not consider the potential consequences of adding an organizationId field to sub-documents.