populate fails with refpath

Question

I have a group document containing the "Users" field. I use refPath for dynamic addition. Fill does not work, an array of identifiers is returned.

Schema:

  {
    name: {
      type: String,
      required: true,
    },
    participants: [
      {
        type: mongoose.Schema.Types.ObjectId,
        refPath: 'onModel',
      }
    ],
    onModel: {
      type: String,
      enum: ['User', 'UsersOffline'],
    },
    description: {
      type: String,
    },
  },
  {
    versionKey: false,
    timestamps: true,
  },
);

Then I have my request :

 async findOne(id: string) {
      return await this.GroupModel.findById(id).populate('participants')
  }

receive:

{
    "participants": [
      "60b5f9ba890d8625c076841b",
      "60ec7117941ced002b950474",
      "61e9498b84646a293c4d3718"
    ],
    "_id": "62022f8af1f8624450066902",
    "name": "Title",
    "description": "<p>Description</p>",
    "createdAt": "2022-02-08T08:53:30.735Z",
    "updatedAt": "2022-02-08T09:00:14.582Z"
  }