fix(injectModel): fix pre-existing index rollback and required-field duplicate precheck

[coderabbitai]

Follow-up from PR #52

This issue tracks two edge-case bugs identified during review of PR #52 in packages/common/src/utils/injectModel.js that were deferred to keep the merge scope focused.

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Fix 1 — Rollback loop may drop pre-existing indexes (Critical)

File: packages/common/src/utils/injectModel.js

createdIndexes.push(indexName) runs unconditionally after createIndex(). Since MongoDB's createIndex is idempotent and succeeds silently on pre-existing indexes, the rollback loop can call dropIndex(indexName) on indexes that already existed before this request, causing potential data integrity loss on live collections.

Proposed fix: Snapshot existing index names before the loop and only push newly created indexes:

// Before the loop
const existingIndexes = await Model.collection.indexes();
const existingIndexNames = new Set(existingIndexes.map((idx) => idx.name));

// Inside the loop, after createIndex:
const createdName = await Model.collection.createIndex({ [normalizedKey]: 1 }, indexOptions);
if (!existingIndexNames.has(createdName)) {
  createdIndexes.push(createdName);
}

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Fix 2 — getUniqueFieldFilter excludes missing-field docs for required fields (Major)

File: packages/common/src/utils/injectModel.js

getUniqueFieldFilter returns { [fieldKey]: { $exists: true } } for required fields. This excludes documents missing the field from the duplicate precheck. However, MongoDB's unique index still treats multiple missing-field documents as duplicates (they all resolve to null), so createIndex() can still fail with error 11000 even after the precheck passes.

Proposed fix: Use an unrestricted filter for required fields:

function getUniqueFieldFilter(fieldKey, isRequired) {
  if (isRequired) {
    return {}; // scan ALL docs, including those missing the field
  }
  return { [fieldKey]: { $exists: true, $ne: null } };
}

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Requested by: @yash-pouranik
Reference PR: #52
Review thread: #52 (review)