MongoDb aggregation delete duplicates according to parameter

Question

I am trying to find out how I can make a mongodb aggregation to delete duplicate documents according to a specific parameter. When I say duplicates I don't mean that the whole document is the same. For example I have the following documents:

{
  'event': 123,
  'element': 'element1'
},
{
  'event': 456
  'element': 'element2'
},
{
  'event': 789
  'element': 'element1'
},
{
  'event': 111
  'element': 'element3'
}

I would like to make an aggregation that if I specify the field "element" will return me all documents without element duplication. I know that the typical question is, ok but if there are two documents with 'element1' Which one do I want? the answer is that I dont mind if it returns me one or the other, this is why I dont know if this is possible

return example:

{
  'event': 123,
  'element': 'element1'
},
{
  'event': 456
  'element': 'element2'
},
{
  'event': 111
  'element': 'element3'
}

It is also OK if it returns me:

{
  'event': 456
  'element': 'element2'
},
{
  'event': 789
  'element': 'element1'
},
{
  'event': 111
  'element': 'element3'
}

Does this answer your question? How to find duplicate records based on an id and a datetime field in MongoDB? — turivishal
– turivishal, Commented Jun 4, 2021 at 14:37

Alex Blex · Accepted Answer · 2021-06-04 13:18:35Z

2

It's not deletion by any means. It's selection of documents with unique value in the field.

Such things are done by grouping documents by the field in question. Since you need any of the matching documents you can use $first accumulator:

db.collection.aggregate([
  {
    "$group": {
      "_id": "$element",
      "doc": {
        "$first": "$$ROOT"
      }
    }
  },
  {
    "$replaceRoot": {
      "newRoot": "$doc"
    }
  }
])

answered Jun 4, 2021 at 13:18

Alex Blex

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2 Comments

Gabriel García Garrido Over a year ago

It works!! tahnksMay I for example delete the duplicate accoriding to the maxium of some parameter

Alex Blex Over a year ago

If by "delete duplicate" you mean select specific documents from the group - yes. You can $push all documents into an array and add an extra stage to pick the one you need before the $replaceRoot stage.

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