Resolved: Replace field value with first element of an array in the same document

// Product model
const ProductSchema = {
  vendors: [{ type: Schema.Types.ObjectId, ref: 'Vendor' }],
  mainVendor: { type: Schema.Types.ObjectId, ref: 'Vendor' },
}
// Vendor model
const VendorSchema = { ... }

const product = {
  mainVendor: 'mainVendorObjectId'
  vendors: ['secondVendorObjectid', 'thirdVendorObjectId']
}

const product = {
  mainVendor: 'secondVendorObjectId',
  vendors: ['thirdVendorObjectId']
}

const product = {
  mainVendor: 'mainVendorObjectId',
  vendors: []
}

const product = {
  mainVendor: null,
  vendors: []
}

VendorSchema.post('findOneAndDelete', async function (res: TVendorModel) {
    try {
        const products = await ProductModel.updateMany(
            { mainVendor: res._id },
            { $expr: { $set: { mainVendor: '$vendors.0' } } }
        );
    } catch (error) {
        console.error(error);
    }
});

db.collection.update({
  mainVendor: "mainVendorObjectId"
},
[
  {
    $set: {
      mainVendor: {
        $cond: {
          if: {
            $eq: [
              "$vendors",
              []
            ]
          },
          then: null,
          else: {
            $first: "$vendors"
          }
        }
      },
      vendors: {
        $cond: {
          if: {
            $eq: [
              "$vendors",
              []
            ]
          },
          then: [],
          else: {
            $slice: [
              "$vendors",
              1,
              {
                $size: "$vendors"
              }
            ]
          }
        }
      }
    }
  }
],
{
  multi: true
})