Concat two aggregate query MongoDB

Question

I have a collection that looks like this:

{'start': '2011-01-29',
  'target': [0.0, 2.0],
  'cat': [0, 0, 0, 0, 0],
  'state': 0},
...

I had to do some aggregations in this collection using db.collections.aggregate([]), and because of performance issues I had to separate into two queries.

The first one returns me:

{'_id': {'state': 0, 'id': 0},
  'start': '2011-01-29',
  'cat': [0, 0, 0, 0, 0]}
...

The second one returns me:

{'_id': {'state': 0, 'id': 0},
  'target': [0, 2, 1]
  }
...

What I would like to know, if there is a way to join again the results based on _id?

Or is there a way to declare both query independently into the same one ?

db.collections.aggregate([
query1: {},
query2: {}
concat(query1,query2) ])

My desired output:

Combine again both outputs

{'_id': {'state': 0, 'id': 0},
  'start': '2011-01-29',
  'cat': [0, 0, 0, 0, 0]}
...

{'_id': {'state': 0, 'id': 0},
  'target': [0, 2, 1]
  }
...

Into this

{'_id': {'state': 0, 'id': 0},
  'start': '2011-01-29',
  'target': [0, 2, 1]
  'cat': [0, 0, 0, 0, 0]}
...

My solution so far was using pymongo:

x1 = db.collection.aggregate(query1)

x2 = db.collection.aggregate(query2)

for i, j in zip(x1, x2):
    j['target'] = i['target']

I would suggest reviewing available aggregation pipeline operators and/or revising your question to say what you ultimately want to achieve. — D. SM
– D. SM, Commented Apr 28, 2020 at 19:45

Joe · Accepted Answer · 2020-04-28 21:13:12Z

1

You can get that behavior with a $facet stage, and then use $group to bring them together

db.collection.aggregate([
   {$facet:{
       query1:[pipeline1],
       query2:[pipeline2]
   }},
   {$project:{
       result:{$concatArrays:["$query1","$query2"]}
   }},
   {$unwind: "$result"},
   {$group: {
         _id:"$_id",
         document: {$mergeObjects:"$$ROOT"}
   }},
   {$replaceRoot:{ newRoot: "$document"}}
])

answered Apr 28, 2020 at 21:13

Joe

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D. SM · Accepted Answer · 2020-04-28 20:25:13Z

1

To answer your question as stated, it is not directly possible to send your individual result sets to MongoDB to have it perform more operations on them. It doesn't make sense to do so because to construct such a request would require having both result sets in memory, at which point you should simply merge them in the application instead of performing this same operation in the database AND moving the data back and forth over the network.

Technically you could create two temporary collections, insert each result set into its respective collection, then perform another aggregation...

answered Apr 28, 2020 at 20:25

D. SM

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1 Comment

William Over a year ago

Thank you @Oleg. So, I will use pymongo.

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