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Type:
Bug
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Resolution: Fixed
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Priority:
Major - P3
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Affects Version/s: 7.0.0, 6.0.6
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Component/s: None
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Query Optimization
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Fully Compatible
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ALL
When I try to run the aggregation below, which has a nesting of $facet -> $lookup -> $facet -> $lookup, I get the error message "$lookup is not allowed to be used within a $facet stage". This message is wrong, since $lookup is indeed allowed in a $facet stage and works fine. The problem with the pipeline is that it has a $facet inside a $facet, which is not allowed.
This is the pipeline that results in the wrong error message:
db.data.aggregate([
{ "$facet" : {
"result" : [
{ "$match" : { "ref1" : { "$exists" : true }}},
{ "$lookup" : {
"from" : "data",
"localField" : "ref1",
"foreignField" : "_id",
"pipeline" : [
{ "$facet" : {
"result" : [
{ "$match" : { "ref2" : { "$exists" : true }}},
{ "$lookup" : {
"from" : "data",
"localField" : "ref2",
"foreignField" : "_id",
"as" : "ref2" }}]}}],
"as" : "ref1" }}]}}])
On the other hand, for a pipeline with a $facet immediately nested within another $facet, the message is correct. This pipeline:
db.data.aggregate([{"$facet" : { "x" : [ {"$facet" : { "y" : [] }}]}}])
results in the correct message "$facet is not allowed to be used within a $facet stage"
The problem is the same in both 6.0.6 and 7.0.0.