mongodb lookup match-expr as second pipeline step - mongodb

Question

I'm relatively new to mongodb aggregations and I have a tiny problem:
I want to make a join between two collections. The problem is, that the foreign field is placed in an inner array. That means that I have to unwind the array in order to make a proper $match. In my $match, I use an $epxr and an $eq in order to make the join on the unwinded documents (which is intended and therefor not a problem). The $expr is needed to access variables from the orginal collection:
[
...
{
$lookup: {
from: 'foreignCollection',
as: 'field',
let: {
localField: '$someComparisonField'
},
pipeline: [
{
$unwind: '$arr'
},
{
$match: {
$expr: {
$eq: [ '$arr.foreignField', '$$localField' ]
}
}
}
]
}
}
]
However field is always an empty array in my result set. I seriously don't know what I'm doing wrong :D
Can someone help me?
Edit:
As requested, some sample data for the two involved collections:
orginalCollection:
{
...
someComparisonField: 1
},
{
...
someComparisonField: 2
}
foreignCollection:
{
...
arr: [
{
...
foreignField: 1
},
{
...
foreignField: 1
},
{
...
foreignField: 2
},
]
},
{
...
arr: [
{
...
foreignField: 1
},
{
...
foreignField: 2
},
{
...
foreignField: 2
},
]
},
{
...
arr: [
{
...
foreignField: 2
},
{
...
foreignField: 1
},
{
...
foreignField: 2
},
]
},
Edit 2:
I forgot to add a small detail: with $eq in the inner pipeline, I am accessing a fixed index which means that the foreignField actually looks like this: foreignField: [ <value> ].

Answer 1

With some investigation, I came to the following conclusion:
Before reading further, I recommend reading both of my edits to the questions. Especially my 2nd edit.
It seems like $eq within an $expr simply can't compare a fixed index.
This expression simply doesn't work somehow:
{
$expr: {
$eq: [ '$arr.foreignField.0': '$$localField' ]
}
}
To work arround this, I created an projection before this stage which extracts the value from the desired fixed index and stores it in a new field. So now the particular stages from the inner pipeline look like this:
...
{
$project: {
value: { $arrayElemAt: [ '$arr.foreignField', 0 ] }
}
},
{
$match: {
$expr: {
$eq: [ '$value', '$$localField' ]
}
}
}
...