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1

when i tried : db.inventory.find({item:“postcard”}, {_id:0, instock:{$slice:1}})

returned:

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i think it should return instock element 0 only

2

Thanks for finding and reporting it! You’re making MongoDB better for everyone :smiling_face_with_three_hearts: Here are the steps for reporting bug for MongoDB projects: Submit Bug Reports · mongodb/mongo Wiki · GitHub

3

Hi @kuku_super,

I don’t see a bug here.

Collection:

db.inventory.insertMany( [
  { item: "journal", status: "A", size: { h: 14, w: 21, uom: "cm" }, instock: [ { warehouse: "A", qty: 5 } ] },
  { item: "notebook", status: "A",  size: { h: 8.5, w: 11, uom: "in" }, instock: [ { warehouse: "C", qty: 5 } ] },
  { item: "paper", status: "D", size: { h: 8.5, w: 11, uom: "in" }, instock: [ { warehouse: "A", qty: 60 } ] },
  { item: "planner", status: "D", size: { h: 22.85, w: 30, uom: "cm" }, instock: [ { warehouse: "A", qty: 40 } ] },
  { item: "postcard", status: "A", size: { h: 10, w: 15.25, uom: "cm" }, instock: [ { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ] }
]);

The first element of the array instock is { warehouse: "B", qty: 15 } for the postcard.

When you use this query:

> db.inventory.find({"item":"postcard"}, {_id:0, instock:{$slice:1}})
{ "item" : "postcard", "status" : "A", "size" : { "h" : 10, "w" : 15.25, "uom" : "cm" }, "instock" : [ { "warehouse" : "B", "qty" : 15 } ] }

You get that same first element of the array so the result looks good to me. You also get status and size because you never specified anything for them. The _id has a special treatment in the projection so you never chose “inclusion” or “exclusion” anywhere in the projection here so the fields are included by default.

But if you run:

> db.inventory.find({"item":"postcard"}, {_id:1, instock:{$slice:1}})
{ "_id" : ObjectId("6048f80bb8352c2198e23bb8"), "instock" : [ { "warehouse" : "B", "qty" : 15 } ] }

Then it’s different because you have explicitly said that you wanted _id and not the other fields not mentioned so you only get _id and instock.

If you only want instock, you will have to do:

> db.inventory.find({"item":"postcard"}, {_id:0, status:0, size:0, item:0, instock:{$slice:1}})
{ "instock" : [ { "warehouse" : "B", "qty" : 15 } ] }

I’m surprised that the instock:{$slice:1} doesn’t trigger the “inclusion” though and removes automatically all the other fields except _id. That’s what I would have expected to be honest.

Cheers,
Maxime.

4

thx, @MaBeuLux88

if instock:{$slice:1} followed the inclusion would make the inclusion and exclusion mechanism much better.

that gave any other field a 0 to get a array element inclusion was a ugly way.

so i suggest to fix it

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5

I agree that this would be more logical. But maybe there is a deeper reason that I ignore.
Please open a ticket and link it back here so we will have an official answer from the query team.

6

i report bug here : https://jira.mongodb.org/browse/MONGOSH-636

1 Like