How to remove duplicate values inside a list in mongodb

Question

I have a mongodb collection . When I do.

db.bill.find({})

I get,

Answer 1

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

db.bill.aggregate([
    {"$addFields": {
        "bill_codes": {"$setUnion": ["$bill_codes", []]}
    }}
])

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

db.bill.aggregate([
    {'$unwind': {
        'path': '$bill_codes',
        // output the document even if its list of books is empty
        'preserveNullAndEmptyArrays': true
    }},
    {'$group': {
        '_id': '$_id',
        'bill_codes': {'$addToSet': '$bill_codes'},
        // 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}}
])