You can do like following
[
{
"$match": {
"_id": "A"
}
},
{
$lookup: {
from: "Episodes",
let: {
id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$project",
"$$id"
]
}
}
},
{
$lookup: {
from: "Sequences",
let: {
epi: "$name"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$episode",
"$$epi"
]
}
}
}
],
as: "sequences"
}
}
],
as: "episodes"
}
}
]
**Working [Mongo playground][1]**
[1]: https://mongoplayground.net/p/N9xvadenEku
Just out of curiosity I've taken a look at what happens under the hood, and I've used [dtruss/strace][1] on each test.
C++
./a.out < in
Saw 6512403 lines in 8 seconds. Crunch speed: 814050
syscalls `sudo dtruss -c ./a.out < in`
CALL COUNT
__mac_syscall 1
<snip>
open 6
pread 8
mprotect 17
mmap 22
stat64 30
read_nocancel 25958
Python
./a.py < in
Read 6512402 lines in 1 seconds. LPS: 6512402
syscalls `sudo dtruss -c ./a.py < in`
CALL COUNT
__mac_syscall 1
<snip>
open 5
pread 8
mprotect 17
mmap 21
stat64 29
[1]: http://en.wikipedia.org/wiki/Strace
