How do I exclude a sepcific object from an array in mongodb? - mongodb

Question

I have a collection with the following structure:
{
"_id" : ObjectId("59ef54445134d7d70e1cf531"),
"CustomerId" : "Gym_2",
"History" : [
{
"Created_At" : ISODate("2017-10-24T14:54:59Z"),
"Unit" : 600,
"ReferenceCode" : "1cd15b4d-bc42-4a51-a8b3-307db6dc3dee",
},
{
"Created_At" : ISODate("2017-10-28T00:22:19Z"),
"Sent" : true
},
{
"Created_At" : ISODate("2017-10-29T10:22:23Z"),
"Unit" : 600,
"ReferenceCode" : "998e7fce-8a1c-4f7c-b48c-c02cb5c5ad5c",
}
]
}
{
"_id" : ObjectId("59ef54465134d7d70e1cf534"),
"CustomerId" : "Gym_1",
"History" : [
{
"Created_At" : ISODate("2017-10-24T14:55:02Z"),
"Unit" : 600,
"ReferenceCode" : "d19ebeec-bd81-4a0a-aed5-006f746b50ff",
},
{
"Unit" : 600,
"ReferenceCode" : "a991504f-be1f-4e77-b59f-fba73c59e6f1",
"Created_At" : ISODate("2017-10-26T13:51:14Z")
}
]
}
I'm trying to build a query that returns only CustomerId along with history objects that do not have the "Sent" field set.
The result should look like this:
{
"_id" : ObjectId("59ef54445134d7d70e1cf531"),
"CustomerId" : "Gym_2",
"History" : [
{
"Created_At" : ISODate("2017-10-24T14:54:59Z"),
"Unit" : 600,
"ReferenceCode" : "1cd15b4d-bc42-4a51-a8b3-307db6dc3dee",
},
{
"Created_At" : ISODate("2017-10-29T10:22:23Z"),
"Unit" : 600,
"ReferenceCode" : "998e7fce-8a1c-4f7c-b48c-c02cb5c5ad5c",
}
]
}
{
"_id" : ObjectId("59ef54465134d7d70e1cf534"),
"CustomerId" : "Gym_1",
"History" : [
{
"Created_At" : ISODate("2017-10-24T14:55:02Z"),
"Unit" : 600,
"ReferenceCode" : "d19ebeec-bd81-4a0a-aed5-006f746b50ff",
},
{
"Unit" : 600,
"ReferenceCode" : "a991504f-be1f-4e77-b59f-fba73c59e6f1",
"Created_At" : ISODate("2017-10-26T13:51:14Z")
}
]
}
The closest I could reach is the following query:
db.Customers.aggregate([
{$project:{"Sent":{$exists:false},count:{$size:"$History" }}}
]);
But I get "errmsg" : "Unrecognized expression '$exists'" .
How can I achieve this result?

Answer 1

There is a solution for your problem, and aggregation framework is definitely the right way to achieve what you want. To modify nested collection you have to:
Unwind this collection ($unwind)
Filter out documents where Sent exists
Group by common properties
Project your data to receive it's original form
db.customers.aggregate([
{$unwind: "$History"},
{$match: {"History.Sent": {$exists: false}}},
{$group: {"_id": { "_id": "$_id", "CustomerId": "$CustomerId" }, History: { $push: "$History"} }},
{$project: { "_id": "$_id._id", "CustomerId": "$_id.CustomerId", History: 1}}
]);
As you can see this query is rather complicated and for larger collections you might encounter problems with performance because we're doing a lot more than simple collection filtering. So although it works I'd suggest you should consider changing your data model, for instance having each history item as a separate document like this:
{
_id: "some_id"
"Created_At" : ISODate("2017-10-24T14:54:59Z"),
"CustomerId" : "Gym_2",
"Unit" : 600,
"Sent" : true //can be set or not
"ReferenceCode" : "1cd15b4d-bc42-4a51-a8b3-307db6dc3dee"
}
Then your query will be just simple find with $exists.