Thank you John,
You are a lifesaver. The manual did help. Again thank you.
Below is the code sample for those with a similar problem to mine.
const orgUnitId = req.body.ouId;
const deptId = req.body.deptId;
const name = req.body.name;
const url = req.body.url;
const username = req.body.username;
const password = req.body.password;
const result = await OrganisationalUnit.updateOne(
{
id: orgUnitId,
},
{
$push: {
"departments.$[department].repo": {
name: name,
url: url,
username: username,
password: password,
},
},
},
{ arrayFilters: [{ "department.id": { $eq: deptId } }] }
);