MongoDB 3-level aggregation -

-

suppose have list of schools each school assigned city, state (province) , country.

is possible build aggregated list of unique cities/states/countries mongodb aggregation framework?

what have:

[ {     "school_name” : "school #1",     "state" : "ca"     "city” : "san-francisco",     "country” : "usa", }, {     "school_name” : "school #2",     "state" : "ca"     "city” : "san-francisco",     "country” : "usa", }, {     "school_name” : "school #3",     "state" : "ca"     "city” : "los angeles",     "country” : "usa", }, {     "school_name” : "school #4",     "state" : "co"     "city” : "denver",     "country” : "usa", }, {     "school_name” : "school #5",     "state" : "co"     "city” : "boulder",     "country” : "usa", }, {     "school_name” : "school #6",     "state" : "hessen"     "city” : "frankfurt",     "country” : "germany", }

]

what need get:

[ {  "country" : "germany",      "states" :     [            {     "state" : "hessen",         "cities" : [frankfurt,...]       }       ] }, {  "country" : "usa",      "states" :         [     {     "state" : "ca",         "cities" : [san-francisco, los angeles, ...]     }     {     "state" : "co",         "cities" : [boulder, denver, ...]     }       ] },

]

keeping stages simple. , no need $addtoset since top level group sorts out duplicates:

db.school.aggregate([{     $group: {          _id: {              country: "$country",             state: "$state",              city: "$city"          }     }},    {$group: {         _id: {         country: "$_id.country",        state: "$_id.state" },        cities: {$push: "$_id.city"} }    },    {$group: {         _id: "$_id.country",        states: {$push: { state: "$_id.state", cities: "$cities" }}     }},    {$project: { _id: 0, country: "$_id", states: "$states" }} ])

so 3 group stage aggregation


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