Mongo $lookup where id fields are of different types

I have 2 collections : Transactions & Accounts.

I need to join Accounts to Transactions so I can group on the Accounts.acctType field. The problem is that Transactions.accountId is of type 'string' and Accounts._id is of type 'Int32'.

is there any way to get around this problem without having to change the Transactions.accountId type?

Current lookup code:

$lookup: {
  from: 'accounts', 
  localField: accountId, 
  foreignField: '_id', 
  as: 'accountData'
}

What I need:

$lookup: {
  from: 'accounts', 
  localField: Number(accountId), //something like this
  foreignField: '_id', 
  as: 'accountData'
}

Or:

$lookup: {
  from: 'accounts', 
  localField: accountId, 
  foreignField: '_id.toString()', //or something like this
  as: 'accountData'
}