$lookup with pipeline match and projection does not work for guid - mongodb

Question

I have two collections that I want to join with $lookup based on two id fields. Both fields are from type guid and looke like this in mongodb compass: 'Binary('cavTZa/U2kqfHtf08sI+Fg==', 3)'
This syntax in the compass aggregation pipeline builder gives the expected result:
{
from: 'clients',
localField: 'ClientId',
foreignField: '_id',
as: 'ClientData'
}
But i want to add some projection and tried to change it like this:
{
from: 'clients',
'let': {
id: '$_id.clients'
},
pipeline: [
{
$match: {
$expr: {
$eq: [
'$ClientId',
'$$id'
]
}
}
},
{
$project: {
Name: 1,
_id: 0
}
}
],
as: 'ClientData'
}
But the result here is that every client from collection 'clients' is added to every document in the starting table. I have to use MongoDB 3.6 so the new lookup syntax from >=5.0 is not available.
Any ideas for me? Does $eq work for binary stored guid data?

Answer 1

In the first example, you say that the local field is ClientId and the foreign field is _id. But that's not what you used in your second example.
This should work better:
{
from: 'clients',
'let': {
ClientId: '$ClientId'
},
pipeline: [
{
$match: {
$expr: {
$eq: [
'$$ClientId',
'$_id'
]
}
}
},
{
$project: {
Name: 1,
_id: 0
}
}
],
as: 'ClientData'
}