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Type:
Question
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Resolution: Unresolved
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: None
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QE 2021-08-23
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3
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I have some sample data as follows:
> db.first.find()
{ "_id" : 1, "flavor" : "chocolate" }
> db.another.find()
{ "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 }
{ "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 }
{ "_id" : 3 }
{ "_id" : 4, "item" : "ginger" }
When running a pipeline with $merge:
db.first.aggregate([
{
$merge: {
"into": "another",
"whenMatched": [
{
"$project": {
"_id": 0
}
}
]
}
}
])
This results in no error, and no updates to the "another" collection. Is this expected? I assumed this would not be allowed as there is a restriction on the "whenMatched" pipeline not updating the "on" field.
Another example, using a different "on" field:
> db.second.find()
{ "_id" : ObjectId("60dcae7e4a59a35f0ed6b34b"), "item" : "almonds" }
> db.another.find()
{ "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 }
{ "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 }
{ "_id" : 3 }
{ "_id" : 4, "item" : "ginger" }
Pipeline:
db.second.aggregate([
{
$merge: {
"into": "another",
"whenMatched": [
{
"$project": {
"item": 0
}
}
]
}
}
])
uncaught exception: Error: command failed: {
"ok" : 0,
"errmsg" : "E11000 duplicate key error collection: playground.another index: item_1 dup key: { item: \"almonds\" }",
"code" : 11000,
"codeName" : "DuplicateKey",
"keyPattern" : {
"item" : 1
},
"keyValue" : {
"item" : "almonds"
}
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:618:17
assert.commandWorked@src/mongo/shell/assert.js:708:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1046:12
@(shell):1:1
It seems that in this case I run into a duplicate key error as we attempt to insert a results document. What is the correct behavior in this case?