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Steps To Reproduce:
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Create a schema as shown in the "Description" section.

Run the query given in the "Description" section on one of the versions listed in "Affects Version/s" section.

PS: My assumption is that the given aggregation doesn't give the expected output past 3.4.5, but I have only listed the version that I have tested the aggregation on.
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Create a schema as shown in the "Description" section. Run the query given in the "Description" section on one of the versions listed in "Affects Version/s" section. PS: My assumption is that the given aggregation doesn't give the expected output past 3.4.5, but I have only listed the version that I have tested the aggregation on.

Schema:
{
x:

{type: String, required: true}

,
a: [{
_b:

{type: Schema.Types.ObjectId, ref: "B", required: true}

,
c:

{type: Number, required: true}

,
_d:

{type: Schema.Types.ObjectId, ref: "D", required: true}

,
e: {
f:

{type: Number, required: true}

}
}]
}

lookup stage:
{
"$lookup":

{ from: "B", localField: "a._b", foreignField: "_id", as: "a._b" }

},
{
"$unwind":

{ path: "$a._b" }

}

expected result:
{
x:/.../,
a:{
_b:[Object],
c:/.../,
_d:/.../,
e:

{ f:/.../ }

}
}

result:
{
x:/.../,
a:

{ _b: [Object] }

,
}

As you can see above, fields inside the "a" gets overwritten by the $lookup operation.

duplicates

SERVER-42306 $lookup with an array of sub objects should allow for an option to allow each sub object to find join partners