Need a proper way of expressing Pipeline for the following aggregation query

[R. Martinez]

Hello,

I've been doing program conversion from (PHP) to go-lang for the past 4 days just so I can learn "golang" in a short period of time 

while still being productive. So forgive me if my question is leaning towards how to express MongoDB aggregation concepts in "go".

I have this aggregation query:

db.apps.aggregate(

  [

    { $match: { "vh.pt_id" : {$gt: 0}} },

    

    { $group: {

        _id: {$concat: [{$substr: ["$vh.pt_id",0,-1]}, "_", {$substr: ["$vh.vdid",0,-1]}, "_",  "$vh.vvl"]},

        yids: {$first: "$vh.yids"},

        yvls: {$first: "$vh.yvls"},

        pt_id: {$first: "$vh.pt_id"},

        mkid: {$first: "$vh.mkid"},

        mdid: {$first: "$vh.mdid"},

        sbid: {$first: "$vh.sbid"},

        eid: {$first: "$vh.eid"},

        leid: {$first: "$vh.leid"},

      }

    },

    { $out: "tmp_ptvh_map_stage2"}

  ]

);

and my current code is:

  pipeline := []bson.D{

    bson.M{"$match" : bson.M{"vh.pt_id"} : bson.M{"$gt" : 0}},

    

    bson.M{"$group" : bson.M{

 

      "_id" : bson.M{

        "$concat" : []bson.D{

          bson.M{"$substr" : ???????}

        }

      },

 

      "yids" : bson.M{"$first" : "$vh.yids"},

      "yvls" : bson.M{"$first" : "$vh.yvls"},

      "pt_id" : bson.M{"$first" : "$vh.pt_id"},

      "mkid" : bson.M{"$first" : "$vh.mkid"},

      "mdid" : bson.M{"$first" : "$vh.mdid"},

      "sbid" : bson.M{"$first" : "$vh.sbid"},

      "eid" : bson.M{"$first" : "$vh.eid"},

      "leid" : bson.M{"$first" : "$vh.leid"},

      }

    },

                 ....

I really don't know how to express the following lines to go/mgo/bson:

        _id: {$concat: [{$substr: ["$vh.pt_id",0,-1]}, "_", {$substr: ["$vh.vdid",0,-1]}, "_",  "$vh.vvl"]},

I'm stuck with these:

      "_id" : bson.M{

        "$concat" : []bson.D{

          bson.M{"$substr" : ???????}

        }

      },

With my limited go-lang knowledge, I know that arrays and slice will only accept one data type. But I'm sure how to fully express 

      {$substr: ["$vh.pt_id",0,-1]}

into go/mgo/bson:

      bson.M{"$substr": []?????}

 

Thanks in advance!

Regards,

Raul

[Gustavo Niemeyer]

Hi there,

Slices are fully supported by mgo/bson, so you can use slice of structs, slice of maps, and even slice of plain values.

For example, this would work with your example:

    bson.M{"$substr": []interface{}{"$vh.pt_id", 0, -1}}

The double braces may look strange if you're new to Go, but the reason is more obvious if you consider that the above is equivalent to this:

    type anyValue interface{}

    type someList []anyValue

    bson.M{"$substr": someList{$vh.pt_id", 0, -1}}

Please let us know if you need anything else.

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gustavo @ http://niemeyer.net

[R. Martinez]

Hi Gustavo,

Thank you for replying and thanks for the alternative type definition, it will really help make things readable for Go-newbies like myself.

Regards!