Details
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Bug
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Status: Backlog
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Critical - P2
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Resolution: Unresolved
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3.4.9, 3.6.0-rc0
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None
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Query Optimization
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ALL
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Description
If an array element is used as the localField for the $lookup aggregation pipe, a $project anywhere else in the pipeline will fail to return results and also take a while to run. There is a workaround by adding a field containing the local element and using that field as the localField:
db.venues.aggregate(
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// Pipeline
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[
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// Stage 1
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{
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$addFields: {
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"firstChainId": {
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"$arrayElemAt": [ "$chainIds", 0 ]
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}
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}
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},
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// Stage 2
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{
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$lookup: {
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"from" : "chains",
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"localField" : "firstChainId",
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"foreignField" : "_id",
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"as" : "venueChain"
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}
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},
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// Stage 3
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{
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$addFields: {
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"chainName": "$venueChain.name"
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}
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},
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// Stage 4
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{
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$project: {
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n: 1,
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chainName: 1
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}
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},
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],
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// Options
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{
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cursor: {
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batchSize: 50
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}
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}
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);
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Attachments
Issue Links
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