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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: 4.0.10
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Component/s: Aggregation Framework
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Fully Compatible
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Linux
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v4.2
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Query 2019-07-01
db.isMaster() result:
{ "ismaster" : true, "maxBsonObjectSize" : 16777216, "maxMessageSizeBytes" : 48000000, "maxWriteBatchSize" : 100000, "localTime" : ISODate("2019-06-13T08:59:36.884Z"), "logicalSessionTimeoutMinutes" : 30, "minWireVersion" : 0, "maxWireVersion" : 7, "readOnly" : false, "ok" : 1}
Such an operation:
db.users.aggregate([
{ $addFields: { id: '$_id' } },
{
$lookup: {
as: 'items_check',
from: 'items',
let: { id: '$id' },
pipeline: [
{ $addFields: { id: '$_id' } },
{ $match: { $expr: { $eq: ['$$id', '$owner'] } } },
{
$facet: {
matched: [
{ $match: { $or: [{ name: 'Item 2' }, { name: 'Item 1' }] }},
],
all: [{ $match: {} }],
},
},
],
},
},
{ $project: { items_check: 1 } },
])
On such collections:
{
users:[{
id: 'u1',
name: 'John Smith',
items: ['i1'],
gadsden1: 'g1',
gadsden2: 'g2',
gadsden3: 'g3',
},
{
id: 'u2',
name: 'Jane Doe',
items: ['i2'],
},
{
id: 'u3',
name: 'Christopher Gadsden',
items: ['i3'],
},
],
items:[{
id: 'i1',
name: 'Item 1',
owner: 'u1'
},
{
id: 'i2',
name: 'Item 2',
owner: 'u2'
},
{
id: 'i3',
name: 'Item 3',
owner: 'u3'
},
],
gadsdens:[{
id: 'g1',
user1: 'u1',
user2: 'u2',
user3: 'u3',
},
{
id: 'g2',
user1: 'u1',
user2: 'u1',
user3: 'u3',
},
{
id: 'g3',
user1: 'u1',
user2: 'u2',
user3: 'u1',
},
],
}
returns such a result:
[
{
"_id": "u1",
"items_check": [
{
"matched": [
{
"_id": "i1",
"name": "Item 1",
"owner": "u1",
"id": "i1"
},
{
"_id": "i1",
"name": "Item 1",
"owner": "u1",
"id": "i1"
}
],
"all": [
{
"_id": "i1",
"name": "Item 1",
"owner": "u1",
"id": "i1"
},
{
"_id": "i1",
"name": "Item 1",
"owner": "u1",
"id": "i1"
}
]
}
]
},
{
"_id": "u2",
"items_check": [
{
"matched": [
{
"_id": "i2",
"name": "Item 2",
"owner": "u2",
"id": "i2"
}
],
"all": [
{
"_id": "i2",
"name": "Item 2",
"owner": "u2",
"id": "i2"
}
]
}
]
},
{
"_id": "u3",
"items_check": [
{
"matched": [],
"all": [
{
"_id": "i3",
"name": "Item 3",
"owner": "u3",
"id": "i3"
}
]
}
]
}
]
As we can see, the first document in the pipeline gets duplicated items. However, if we swap the $addFields and $match operators in the $lookup pipeline, everything works fine. The same goes for a situation in which the $addFields operator is removed. If I exclude the document with _id equal to u1 then the document with _id equal to u2 will get duplicated items in the items_check field. If I remove the $facet operator from the $lookup pipeline then no documents are duplicated.