Context

Currently, simple conditions (simple lookups of one column=value)  are already pushed into $lookup, improving performance by filtering early. However, more complex conditions such as nested expressions, comparisons, or logical operators — are still applied after $lookup, which can be inefficient when working with large related datasets.
MongoDB supports embedding these conditions directly into the $lookup pipeline using the let and pipeline options, allowing more selective joins.

Definition of done

• Extend $lookup handling to support complex conditions ($and, $or, $expr, etc.) within the inner pipeline.

• Allow filter() conditions to be passed down into the inner $lookup stage when feasible.

• Validate that variable scoping (let) and parameter substitution behave as expected.

• Add tests for nested and mixed simple/complex condition cases.

• Remove duplicate condition between outer and inner stages.

Pitfalls

• Complex expressions may require variable rewriting ($$ prefixes).