removing duplicate array values from mongodb

Question

In mongodb I have collection where arrays has duplicate entries like

{
    \"_id\": ObjectId(\"57cf3cdd5f20a3b0ba009777\"),
    \"Chat\": 6,
    \"string\":          


        

Answer 1

Mongo 3.4+ has $addFields aggregation stage, which allows you to avoid explicitly listing all the other fields to keep:

collection.aggregate([
    {"$addFields": {
        "string": {"$setUnion": ["$string", []]}
    }}
])

Just for reference, here is another (more lengthy) way that uses $replaceRoot and also doesn't require listing all possible fields:

collection.aggregate([
    {'$unwind': {
        'path': '$string',
        // output the document even if its list of books is empty
        'preserveNullAndEmptyArrays': true
    }},
    {'$group': {
        '_id': '$_id',
        'string': {'$addToSet': '$string'},
        // arbitrary name that doesn't exist on any document
        '_other_fields': {'$first': '$$ROOT'},
    }},
    {
      // the field, in the resulting document, has the value from the last document merged for the field. (c) docs
      // so the new deduped array value will be used
      '$replaceRoot': {'newRoot': {'$mergeObjects': ['$_other_fields', "$$ROOT"]}}
    },
    {'$project': {'_other_fields': 0}}
])