Set and concat to string field with arrayFilters in MongoDB

Question

My MongoDB document looks like this.

{
_id: '556ebf103a8b4c3d067bb6466df7c0651c53a1ff79fa348edff656586c24ad0c',
Log: [
    {
        ScriptName: 'UpdateScript',
        ScriptLogs: [
            {
                ID: 'f5bd1fefb2a5c4712ea1d35eb5bd309074984c5932db3723cc29f8d92258a3fa',
                StdOut: 'Hello'
            }
        ]
    }
]}

I have the MongoDb Query

db.getCollection("ServerEntitys").update(

    {_id: '556ebf103a8b4c3d067bb6466df7c0651c53a1ff79fa348edff656586c24ad0c'},
    {
        $set: {
            'Log.$[i].ScriptLogs.$[j].StdOut': { "$concat": ["$Log.$[i].ScriptLogs.$[j].StdOut", "World"]}
        }
    },
    { arrayFilters: [ { "i.ScriptName": "UpdateScript" } , { "j.ID": 'f5bd1fefb2a5c4712ea1d35eb5bd309074984c5932db3723cc29f8d92258a3fa'} ] }
)

After running the query I get

StdOut: {
            $concat: [
                '$Log.$[i].ScriptLogs.$[j].StdOut',
                'World'
            ]
         },

instead of

StdOut: 'HelloWorld'

Can someone tell me why mongoDB uses $concat as value and not as a $concat instruction

Takis · Accepted Answer · 2022-06-06 21:12:56Z

1

When you are using normal update, you should use ONLY update operators

After MongoDB 4.2, we can also update using a pipeline update, using ONLY aggregate operators (no update ones, no arrayfilters etc)

We dont have an update operator that concat strings, so looks like you need pipeline update like the bellow.

Query

nested $map
outer to match the updateScript
inner to match the ID
in both cases if no match is found we keep the old array member

Playmongo

coll.updateMany({},
[{"$set": 
   {"Log": 
     {"$map": 
       {"input": "$Log",
        "as": "outer",
        "in": 
         {"$cond": 
           [{"$ne": ["$$outer.ScriptName", "UpdateScript"]}, "$$outer",
             {"$mergeObjects": 
               ["$$outer",
                 {"ScriptLogs": 
                   {"$map": 
                     {"input": "$$outer.ScriptLogs",
                      "as": "inner",
                      "in": 
                       {"$cond": 
                         [{"$ne": ["$$inner.ID", "2"]}, "$$inner",
                           {"$mergeObjects": 
                             ["$$inner",
                               {"StdOut": 
                                 {"$concat": 
                                   ["$$inner.StdOut"," World"]}}]}]}}}}]}]}}}}}])

answered Jun 6, 2022 at 21:12

Takis

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1 Comment

MeD Over a year ago

thank you very much it is working now. I really hoped for a easier and more compact query.

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