Conditional push to the array in Mongodb test

Question

I have a collection of documents with the "messages" array field:

{
   "id": 2,
   "messages": [
      {
         "id": 1,
         "text": "foo"
      },
      {
         "id": 2,
         "text": "bar"
      }
   ]
}

I want to update documents: add a new "message" document to the "messages" field only if "messages" doesn't contain a document with such id. For example:

{
  "id": 2,
  "text": "bllllllll"
}

should not be added

{
  "id": 3,
  "text": "foo"
}

should be added

How can I do conditional append? I know that I can filter documents in the Find part via $elemMatch operator, but it will only work for Update, with Upsert it will not work anymore.

Vaibhav Patil · Accepted Answer · 2017-05-18 09:56:19Z

5

To push data into array based on condition you have to use $ne in your query.

  var data = {
      "id": 3.0,
      "text": "bllllllll"
    }

suppose you want to update data into array if the id is not in that array.

db.getCollection('local').update({
    _id: ObjectId("591d6d2091b1fda9dcb6ef69"),
    "messages.id":{$ne: data.id}
    },{$push:{messages: data}})

$ne match that if data.id is present in array then it ignore the update otherwise push the data into array.

answered May 18, 2017 at 9:56

Vaibhav Patil

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2 Comments

Georgy Savva Over a year ago

Thank you, It works, but the condition is in the "query"(filter) part of the update. If I pass upsert: true I will get a new document with this message in the messages field, I want the condition to be in the "update"(modification) part and the "query"(filter) contains only _id condition. Is it possible?

Vaibhav Patil Over a year ago

I didn't get exactly what you are trying to say.Can you elaborate with putting your code here?

nurulnabi · Accepted Answer · 2017-05-18 12:42:56Z

1

In order to push an element in an array on certain condition matches $addToSet is a good way. If the condition matches it will push the element into the array ensuring no duplication with this push, meaning if the element being pushed is already there it will not push it. This helps in the case with upsert:true.

db.getCollection('nndemo').update({_id:ObjectId("591d9517faa19dd56df09502")},
{$addToSet:{arr:{a:2}}},{upsert:true})

answered May 18, 2017 at 12:42

nurulnabi

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1 Comment

Stephen Oberauer Over a year ago

This won't work in this example, because $addToSet will prevent the entire message from being duplicated. The question is how to prevent a message with only a matching id from being added.

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