Re: #10983

Also related to #6870, that issue addresses a slightly different case where the refPath is within the same doc array as the id.

Summary

There was a bug in our fix for #10983 where we mishandle refPath in an array of ObjectIds where the refPath refers to a parent of the array.

      const Parent = db.model(
        'Parent',
        new mongoose.Schema({
          docArray: [
            {
              type: {
                type: String,
                enum: ['Child', 'OtherModel']
              },
              ids: [
                {
                  type: mongoose.Schema.ObjectId,
                  refPath: 'docArray.type' // <-- note that `docArray.type` is not within the `ids` array
                }
              ]
            }
          ]
        })
      );

In getModelsMapForPopulate(), we end up with a case where we have a flat array of all ids and a flat array of all model names, but no way to match the two. In flat.filter((val, i) => modelNamesForRefPath[i] === modelName); we then end up accidentally filtering out incorrect ids.

Without flattening, we end up with the following list of model names:

[
  undefined, 'Child',
  'Child',   undefined,
  'Child',   undefined,
  'Child'
]

and the following ids:

[
  [],
  [
    new ObjectId('6671a008596112f0729c2045'),
    new ObjectId('667195f3596112f0728abe24'),
    new ObjectId('6671bd39596112f072cda69c'),
    new ObjectId('6672c351596112f072868565')
  ],
  [ new ObjectId('66734edd596112f0727304a2') ],
  [],
  [ new ObjectId('66726eff596112f072f8e834') ],
  [],
  [ new ObjectId('667267ff596112f072ed56b1') ]
]

We just need to correctly filter which ids belong to the child model by recursively going through the model names and ids.

Examples