Mongolite group by/aggregate on JSON object - json

Question

I have a json document like this on my mongodb collection:
Updated document:
{
"_id" : ObjectId("59da4aef8c5d757027a5a614"),
"input" : "hi",
"output" : "Hi. How can I help you?",
"intent" : "[{\"intent\":\"greeting\",\"confidence\":0.8154089450836182}]",
"entities" : "[]",
"context" : "{\"conversation_id\":\"48181e58-dd51-405a-bb00-c875c01afa0a\",\"system\":{\"dialog_stack\":[{\"dialog_node\":\"root\"}],\"dialog_turn_counter\":1,\"dialog_request_counter\":1,\"_node_output_map\":{\"node_5_1505291032665\":[0]},\"branch_exited\":true,\"branch_exited_reason\":\"completed\"}}",
"user_id" : "50001",
"time_in" : ISODate("2017-10-08T15:57:32.000Z"),
"time_out" : ISODate("2017-10-08T15:57:35.000Z"),
"reaction" : "1"
}
I need to perform group by on intent.intent field and I'm using Rstudio with mongolite library.
What I have tried is :
pp = '[{"$unwind": "$intent"},{"$group":{"_id":"$intent.intent", "count": {"$sum":1} }}]'
stats <- chat$aggregate(
pipeline=pp,
options = '{"allowDiskUse":true}'
)
print(stats)
But it's not working, output for above code is
_id count
1 NA 727

Answer 1

If intent attribute type is string and keep the object as string.
We can split it to array with \" and use third item of array.
db.getCollection('test1').aggregate([
{ "$project": { intent_text : { $arrayElemAt : [ { $split: ["$intent", "\""] } ,3 ] } } },
{ "$group": {"_id": "$intent_text" , "count": {"$sum":1} }}
])
Result:
{
"_id" : "greeting",
"count" : 1.0
}