Mongodb - find query based on an element of an array

Question

I have the following type of document where I have to find instock elements based on a given value. i.e Return all instocks where ele = 5.

{"item":"journal",
 "instock":[
    { "warehouse":"A", "ele":[2,4,5] },
    { "warehouse":"C", "ele":[8,5,2] },
    { "warehouse":"F", "ele":[3] },
    { "warehouse":"K", "ele":[2,8,4] }
    ]
}

I tried to use $elemMatch but it just produces the first element.

db.inventory.find({"item": "journal"}, {"_id": 0, "item": 0, "instock":{$elemMatch: {"ele": {$in: [5]}}} })

But it just gives:

{ "instock" : [ 
    { "warehouse" : "A", "ele" : [ 2, 4, 5 ] }
]}

And the expectation is

{ "instock" : [ 
    { "warehouse" : "A", "ele" : [ 2, 4, 5 ] },
    { "warehouse" : "C", "ele" : [ 8, 5, 2 ] }
]}

How should I get the expected result?

turivishal · Accepted Answer · 2020-09-09 19:08:09Z

1

The $elemMatch or instock.$ will return first match document in find()'s projection,

You can use aggregation expression in projection from MongoDB 4.4, for your example use array $filter operator in projection,

db.collection.find({
  "item": "journal"
},
{
  instock: {
    $filter: {
      input: "$instock",
      cond: { $in: [5, "$$this.ele"] }
    }
  }
})

Playground

For the old version you can use aggregate() using above same operator.

edited Sep 9, 2020 at 19:08

answered Sep 9, 2020 at 18:48

turivishal

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4 Comments

KTB Over a year ago

I think what you are suggesting is to use aggregation. Because $filter is not supporting in "find". So the working answer I created based on your answer is db.inventory.aggregate({ $match:{"item": "journal"}}, { $project: {instock: { $filter: { input: "$instock", cond: { $in: [5, "$$this. ele"] }}}}})

turivishal Over a year ago

yes its perfect, what is the version of your mongodb?

KTB Over a year ago

I'm using 4.2.1

turivishal Over a year ago

updated answer, its support in mongodb 4.4, glad you have resolved your problem with aggregate().

Hussein Awala · Accepted Answer · 2020-09-09 19:34:00Z

0

I suggest you to unwind the instock array, then filter the data, and finally if you want you can group the result by item to obtain the desired result.

Here is an exemple: solution

answered Sep 9, 2020 at 19:34

Hussein Awala

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