How to avoid adding duplicate objects to an array in MongoDB

Question

this is my schema:

 new Schema({
    code: { type: String },
    toy_array: [
      {
        date:{
        type:Date(),
        default: new Date()
       }
        toy:{ type:String }
    ]
   }

this is my db:

{
  "code": "Toystore A",
  "toy_array": [
    {
      _id:"xxxxx", // automatic
      "toy": "buzz"
    },
    {
      _id:"xxxxx", // automatic
      "toy": "pope"
    }
  ]
},
{
  "code": "Toystore B",
  "toy_array": [
    {
       _id:"xxxxx", // automatic
      "toy": "jessie"
    }
  ]
}

I am trying to update an object. In this case I want to update the document with code: 'ToystoreA' and add an array of subdocuments to the array named toy_array if the toys does not exists in the array.

for example if I try to do this:

db.mydb.findOneAndUpdate({
  code: 'ToystoreA,
  /*toy_array: {
    $not: {
      $elemMatch: {
        toy: [{"toy":'woddy'},{"toy":"buzz"}],
      },
    },
  },*/
},
{
  $addToSet: {
    toy_array: {
      $each: [{"toy":'woddy'},{"toy":"buzz"}],
    },
  },
},
{
  new: false,
}
})

they are added and is what I want to avoid.

how can I do it?

[
  {
    "code": "Toystore A",
    "toy_array": [
      {
        "toy": "buzz"
      },
      {
        "toy": "pope"
      }
    ]
  },
  {
    "code": "Toystore B",
    "toy_array": [
      {
        "toy": "jessie"
      }
    ]
  }
]

In this example [{"toy":'woddy'},{"toy":"buzz"}] it should only be added 'woddy' because 'buzz' is already in the array.

Note:when I insert a new toy an insertion date is also inserted, in addition to an _id (it is normal for me).

In this example [{"toy":'woddy'},{"toy":"buzz"}] it should only be added 'pope' because 'buzz' is already in the array. --> Are you sure about this statement ? Do you mean woddy instead of pope ? If yes, your code should work as expected.. — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Jun 16, 2020 at 18:28
@whoami it adds so many elements of objects enter without doing validation. I'll update the code. each time an insert is made a unique _id is generated automatically for each toy, so for this reason I think that each new element seems to be unique. — yavg
– yavg, Commented Jun 16, 2020 at 18:38
Does this answer your question? Stop Mongoose from creating _id property for sub-document array items Yes $addFields is failing because each object has unique _id I guess you might be using mongoose if yes, then you can change your schema like mentioned in that link !! — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Jun 16, 2020 at 18:52
@whoami actually I do want it to be generated, also in my real code a date is also generated automatically. so i keep looking for a way to insert an object if it doesn't exist. in this case enter a toy if toy is unique. — yavg
– yavg, Commented Jun 16, 2020 at 19:00

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-06-16 19:30:47Z

2

As you're using $addToSet on an object it's failing for your use case for a reason :

Let's say if your document look like this :

    {
      _id: 123, // automatically generated
      "toy": "buzz"
    },
    {
      _id: 456, // automatically generated
      "toy": "pope"
    }

and input is :

[{_id: 789, "toy":'woddy'},{_id: 098, "toy":"buzz"}]

Here while comparing two objects {_id: 098, "toy":"buzz"} & {_id: 123, "toy":"buzz"} - $addToSet consider these are different and you can't use $addToSet on a field (toy) in an object. So try below query on MongoDB version >= 4.2.

Query :

db.collection.updateOne({"_id" : "Toystore A"},[{
    $addFields: {
      toy_array: {
        $reduce: {
          input: inputArrayOfObjects,
          initialValue: "$toy_array", // taking existing `toy_array` as initial value
          in: {
            $cond: [
              { $in: [ "$$this.toy", "$toy_array.toy" ] }, // check if each new toy exists in existing arrays of toys
              "$$value", // If yes, just return accumulator array
              { $concatArrays: [ [ "$$this" ], "$$value" ] } // If No, push new toy object into accumulator
            ]
          }
        }
      }
    }
  }])

Test : aggregation pipeline test url : mongoplayground

Ref : $reduce

Note :

You don't need to mention { new: false } as .findOneAndUpdate() return old doc by default, if you need new one then you've to do { new: true }. Also if anyone can get rid of _id's from schema of array objects then you can just use $addToSet as OP was doing earlier (Assume if _id is only unique field), check this stop-mongoose-from-creating-id-property-for-sub-document-array-items.

edited Jun 16, 2020 at 19:30

answered Jun 16, 2020 at 19:25

whoami - fakeFaceTrueSoul

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17 Comments

yavg Over a year ago

if i wanted to do the same but to add for the first time, would it work the same way?

yavg Over a year ago

Are you sure it works with updateOne? with this code it never updates.

whoami - fakeFaceTrueSoul Over a year ago

@yavg : In that case (first time) if you don't have toy_array itself you need to add a prior $addFields stage in update operation like this :: (mongoplayground.net/p/k6wWVBWZU4W), with this code it never updates ? Any error ? Did you replace db.collection with your collection name ?

yavg Over a year ago

Yeah right. that's what i did. I have a suspicion. in my actual example each array position contains a country_id property whose value is an` ObjectId` belonging to an id from another collection. The comparison is between a String and an ObjectId . for example arrayfilter.id_country == db.country_id is done. I'm not sure if it's not compared because db.country_id is an` ObjectId`

yavg Over a year ago

In this example, if it works only with .update, in my real code I don't know why it doesn't work .. I think there must be some comparison problem between ObjectsId. thanks, again

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