Using object path in $lookup mongo aggregation pipeline - mongodb

Question

For today's task I am trying aggregating documents in a collection (let's call it collection1 and in one of the pipeline's stages I am trying to use $lookup to retrieve documents from another collection (let's call it collection2).
collection1 object model:
{
"field1": "value1",
"field2": "value2"
"field3": "value3"
}
collection2 object model:
{
"field1: "value1",
"field2"; "value2",
"field3: {
"field31": "value31",
"field32": "value32"
}
}
What I am exactly trying to do is to retrieve the documents from collection2 where field3.field31 equals value of the collection1s field1.
My $lookup stage looks like approx like this but currently it doesn't seem to work. I did not find any clue if this should work but looking forward to your replies.
{
$lookup: {
from: "collection2",
let: {
"c": "$field1",
"l": "$field2",
"t": "$field3",
},
pipeline: [
{
$match: {
$expr: {
$and: [
{ $eq: ["$field1", "$$c"] },
{ $eq: ["$field2", "$$l"] },
{ $eq: ["$field3.field31", "$$t"] },
]
}
},
},
],
as: "awesomejoin"
}
}
I want to avoid having a project or a group and then unwinding and filtering again. My wish is to get the records directly from the match stage thinking this is better in terms of performance...
Let me know your thoughts on this.
Thank you

Answer 1

Please try this :
db.Collection1.aggregate([
{
$lookup:
{
from: "Collection2",
localField: "field1",
foreignField: "field3.field31",
as: "docs"
}
}
])
It should be simple with plain $lookup, not exactly sure why you're creating local variables and looking for equals on same variables, Also $unwind will be used on arrays, over objects you could access inner elements using . notation same as like in programming languages.
Ref : $lookup