Golang , mongodb can not do select

Question

In my case I have a collection where I have stored a data with a structure that below

{ 
    "_id" : ObjectId("59ad187a0447d3617fb802b8"), 
    "fid" : ObjectId("59ad187a6b9600120bd03a53"), 
    "pr" : [
        {
            "_id" : ObjectId("59ad187a6b9600120bd03a53"), 
            "trashed" : false
        }
    ], 
    "ch" : [
        {
            "_id" : ObjectId("59ad18a36b9600120bd03a57"), 
            "trashed" : false
        }, 
        {
            "_id" : ObjectId("59ad18a36b9600120bd03a99"), 
            "trashed" : false
        }, 
        {
            "_id" : ObjectId("59ad18a36b9600120bd03a98"), 
            "trashed" : true
        }, 
        {
            "_id" : ObjectId("59ad18a36b9600120bd03a97"), 
            "trashed" : false
        }
    ]
}

So I want to get all objects in ch where trashed is false

Here is my query

       type ChildParentsData struct {
       Id      bson.ObjectId `json:"_id,omitempty" bson:"_id,omitempty"`
       Trashed bool          `json:"trashed" bson:"trashed"`
       }
        var tree []ChildParentsData
        err := Connection.Session.DB("cctv_storage").C("tree").Find(
               bson.M{
                  "fid": bson.ObjectIdHex(id), "ch.trashed": false
               }).Select(
               bson.M{
                  "ch.$": 1
                }).All(&tree)

But as a response I am getting all data, but I need only objects in ch

Find returns a whole document that matches the conditions. In your case the conditions are: fid equals to some id, and ch has at least one sub-document with trashed equals to false. You need to either process the result on the client side, or use aggregation framework. — Alex Blex
– Alex Blex, Commented Sep 4, 2017 at 11:41
What's the expected result ? One big array with all ch items where trashed is false ? Several documents with filtered ch array ? you should post the ChildParentsData structure to make it clearer — felix
– felix, Commented Sep 4, 2017 at 11:58

felix · Accepted Answer · 2017-09-04 12:23:09Z

You can achieve this using the aggregation framework, thanks to the $replaceRoot operator introduced in MongoDB 3.4

We first get matching documents for the specific fid, then we unwind the array and remove docmuments where ch.trashed is true. Finally, we remove the ch field by promoting the content of ch as root of the document

Here is the code to achieve this:

type ChildParentsData struct {
        Id      bson.ObjectId `json:"_id,omitempty" bson:"_id,omitempty"`
        Trashed bool          `json:"trashed" bson:"trashed"`
}
var tree []ChildParentsData

pipeline := []bson.M{
    {"$match": bson.M{"fid": bson.ObjectIdHex("59ad187a6b9600120bd03a53")}},
    {"$unwind": "$ch"},
    {"$match": bson.M{"ch.trashed": false}},
    {"$replaceRoot": bson.M{"newRoot": "$ch"}}}

err = Connection.Session.DB("cctv_storage").C("tree").Pipe(pipeline).All(&tree)

if err != nil {
    fmt.Printf("error: %v", err)
    os.Exit(0)
}
fmt.Printf("doc: %v", tree)

yzark · Accepted Answer · 2020-01-02 12:51:56Z

0

An quick way to get the particular fields instead of all the fileds is using projection. An example of golang is here:

    filter := bson.D{{"_id" , bson.ObjectIdHex("59ad187a6b9600120bd03a53")}}
    projection := bson.D{{"ch", 0}}
    collection.FindOne(context.TODO(), filter, options.FindOne())

answered Jan 2, 2020 at 12:51

yzark

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