How to build an array of "values" from a nested mongodb {'key': value} pair?

Question

Below is a database that i have created by mapping one database to another using dot notation:

for ladders in season_ladders_db2.ladders.find({},{'tier.division.ladder_id':1}):
    db_ladder_id.ladders.insert_one(ladders)

The output:

{'_id': ObjectId('5baa8d09a4918a1cc0ed16e2'),
 'tier': [{'division': [{'ladder_id': 198440},
                        {'ladder_id': 197477},
                        {'ladder_id': 198936},
                        {'ladder_id': 197279},
                        {'ladder_id': 199169},
                        {'ladder_id': 197611},
                        {'ladder_id': 197365},
                        {'ladder_id': 197803},
                        {'ladder_id': 198683},
                        {'ladder_id': 197062}]},
          {'division': [{'ladder_id': 197053},
                        {'ladder_id': 198094},
                        {'ladder_id': 199170},
                        {'ladder_id': 198417},
                        {'ladder_id': 197792},
                        {'ladder_id': 197342},
                        {'ladder_id': 197507}]},
          {'division': [{'ladder_id': 197310},
                        {'ladder_id': 197620},
                        {'ladder_id': 197968},
                        {'ladder_id': 198774},
                        {'ladder_id': 197405},
                        {'ladder_id': 198366},
                        {'ladder_id': 197065},
                        {'ladder_id': 199163},
                        {'ladder_id': 197522},
                        {'ladder_id': 198550},
                        {'ladder_id': 198132},
                        {'ladder_id': 197780},
                        {'ladder_id': 198992}]}]}
{'_id': ObjectId('5baa8d0aa4918a1cc0ed16e3'),
 'tier': [{'division': [{'ladder_id': 197921}]}]}

Using Python and PyMongo how can I build an array of simply the 'ladder_id' values?

The output need is something like

[198440, 197477, 198936, ...]

is there a way i can pass these values within mongodb? perhaps to another collection? I want to pass these values to an API call.

Thank you!

score 1 · Accepted Answer · 2018-09-26 00:31:33Z

1

In this case, you need to unpack($unwind) values and project($project) ladder_id to the root of the query result.

db.getCollection('test').aggregate(
[
  {$unwind: '$tier'},
])

This query will literally unwind tier, meaning all division will have it's own tier.

{
    "_id" : ObjectId("5baace3569cb14ccb9e5dfc5"),
    "tier" : {
         "division" : [ 
            {
                "ladder_id" : 197921
            }
        ]
    }
}, {}, {}, .....

Then, you need to unwind those divisions so all those ladder_ids will have it's own tier and division.

db.getCollection('test').aggregate(
[
  {$unwind: '$tier'},
  {$unwind: '$tier.division'},
])

Result:

{
    "_id" : ObjectId("5baace2769cb14ccb9e5df34"),
    "tier" : {
        "division" : {
            "ladder_id" : 198440
        }
    }
}, {}, {}, .....

Finally, you can $project those ladder_ids.

db.getCollection('test').aggregate(
[
  {$unwind: '$tier'},
  {$unwind: '$tier.division'},
  {$project: {ladder_id: '$tier.division.ladder_id'}},
])

Result:

{
    "_id" : ObjectId("5baace2769cb14ccb9e5df34"),
    "ladder_id" : 198440
}, {}, {}, .....

edited Sep 26, 2018 at 0:31

answered Sep 26, 2018 at 0:25

user10356004

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1 Comment

FLAV10 Over a year ago

Thank you!! It took me way longer than it should have to realize that '$unwind' needed to be in quotes, but once I fixed that, this got me there. Thank you!!!

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