[pmachapman]

Fixes #1063.

This PR fixes an OOM error with the refs to targetRefs migration that occurred due to each pretranslation being loaded into memory via ToListAsync().

Execution times on a local copy of production were comparable to running the script below (~7 minutes), with QA around 5 minutes. This will likely be slower on MongoDB Atlas.

If you want to manually migrate the collection, run:

db.translation.pretranslations.aggregate([
  {
    $match: {
      refs: { $exists: true }
    }
  },
  {
    $set: {
      targetRefs: {
        $ifNull: ["$targetRefs", "$refs"]
      }
    }
  },
  {
    $unset: "refs"
  },
  {
    $merge: {
      into: "translation.pretranslations",
      on: "_id",
      whenMatched: "replace"
    }
  }
])

I reverted QA by modifying the schema_version, then running this script:

db.translation.pretranslations.aggregate([
  {
    $match: {
      sourceRefs: { $exists: false },
      targetRefs: { $exists: true }
    }
  },
  {
    $set: {
      refs: "$targetRefs"
    }
  },
  {
    $unset: "targetRefs"
  },
  {
    $merge: {
      into: "translation.pretranslations",
      on: "_id",
      whenMatched: "replace"
    }
  }
])

---

This change is 

[ddaspit]

@ddaspit reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Enkidu93).

[Enkidu93]

@Enkidu93 reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on pmachapman).