mongoid group by query

stages =  [
   { "$match" => { column_name: "Value" } },
   { "$group" => {  
             "_id" => { 
                  "year" => { "$year" => "$column_name" },
                  "month" => { "$month" => "$column_name" },
                  "day" => { "$dayOfMonth" => "$column_name" },
                  "hour"=> {"$hour" => "$column_name"}
         "get_avg_of_grouped_records" => { 
              "$avg" => "$column_name" 
         "count" => { "$sum" => 1 }


@array_of_objects = ModelName.collection.aggregate(stages, {:allow_disk_use => true})

Query stages
$match stage will apply conditions on query before fetching, so place it before other stages.
$group stage is used to group collection data, “_id” is used to set options on which you want to group records.
Error: A pipeline stage specification object must contain exactly one field. (16435)
As stages variable is array and you need to specify separate hash for each stage in it. Like in above query $match and $group are two stages both are placed in separate hashes and are separate elements of stages array.


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Posted in mongoid

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