lean() memory savings lost after adding populate() in Mongoose 6.x queries

Direct Answers

1. populate() on a lean() query returns plain objects for populated paths. In Mongoose 5.x, 6.x, and 7.x, chaining .lean() before .populate() yields plain JavaScript objects for both the root documents and every populated reference. The memory benefit of lean() is not lost.
2. No version boundary changed this behavior. The lean-plus-populate semantics have been stable since at least Mongoose 4.x and remain unchanged through the current 7.x line.
3. Yes, query.cursor() with .lean() is the documented streaming approach. For large result sets, Model.find().lean().cursor() (optionally with .populate()) returns a readable stream that yields one plain object at a time, avoiding full result-set buffering.

Why the Memory Savings Hold

When lean() is applied, Mongoose skips document hydration entirely—no getters, virtuals, change-tracking, or prototype methods are attached. populate() respects that flag: it fetches the referenced documents and merges them as plain objects into the same lean result. The populated fields are not re-wrapped as Mongoose documents.

Practical Pattern for Large Reads

const cursor = MyModel.find({ status: 'active' })
  .lean()
  .populate('owner', 'name email')
  .cursor();

for await (const doc of cursor) {
  // doc and doc.owner are plain objects
  process(doc);
  // do NOT accumulate in an array
}

Key points:

• Call .lean() before .populate() (order does not matter in current versions, but this is the conventional order).
• Use .cursor() instead of .exec() to get an async iterable.
• Process each item immediately; pushing into an array defeats the streaming benefit.

Caveats to Verify in Your Codebase

• lean() strips getters, virtuals, and instance methods unless you pass { getters: true, virtuals: true }. If your populated paths rely on virtuals, they will be undefined.
• If you need toObject() or toJSON() transforms on populated data, they will not run—plain objects are returned as-is.

One Diagnostic Detail

Are any of your populated paths using virtual populate or custom populate options (e.g., match, select, perDocumentLimit)? Those features work with lean, but if you depend on virtual getters on the populated documents, they will be absent unless you enable { virtuals: true } on the root lean() call.