Preventing Aggregation Pipeline Access to the courses Collection

To maintain service boundaries and ensure proper separation of concerns within our microservice architecture, direct aggregation lookups ($lookup) from the languages context into the courses collection must be avoided.

$lookup: {
  from: 'courses',
  let: { languageId: '$_id' },
  pipeline: [
    { $match: { $expr: { $eq: ['$languageId', '$$languageId'] } } },
    { $match: queryFilter },
  ],
  as: 'courses',
}

…creates a tight coupling between the languages and courses domains by assuming internal schema knowledge and exposing cross-context joins. This breaks the principle of encapsulation and introduces hidden dependencies that make services harder to evolve independently.