Impossible to use $switch, $and and $match for conditional queries or aggregations

[Damien (Damien Frechou)]

I have got some issues trying to use $switch and $match or $eq for conditionnal cases. Here is my collection of 4 test documents:

I would like to query some documents depending on some conditions. But as conditions if/else or switch exist only for aggregation, I use the following aggregation on realm:

  doc_switch = collection.aggregate([
  {
    $set:{
      rm:{
        $switch:{
          branches:[
            {case:req1,then:1},
            {case:req2,then:2},
            {case:req3,then:3}
            ],
            default:0
        }
      }
    }
  }
  ])  

Where each req is a function of 3 arguments: function(arg_c, arg_n, arg_g) where:

• arg_c: code
• arg_n: name
• arg_g: grade

I want to set the rm attribute based on the switch condition req1, req2 and req3. According to mongodb documentation of switch:

$switch
Evaluates a series of case expressions. When it finds an expression which evaluates to true , $switch executes a specified expression and breaks out of the control flow.

That means, if in my aggregation the case req1 is ok, the $switch won’t test req2 and so req3.

I have tried different used cases for my conditions as follow:

// > error:(InvalidPipelineOperator) Invalid $set :: caused by :: Unrecognized expression '$match’
  let match1 = {$match:{"code":arg_c,"name": arg_n , "grade": arg_g}}
  let match2 = {$match:{"code":arg_c,"name": arg_n }}
  let match3 = {$match:{"code":arg_c}}
  
  let and1 = {$and:[{"code":arg_c},{"name": arg_n},{"grade": arg_g}]}
  let and2 = {$and:[{"code":arg_c},{"name": arg_n}]}
  let and3 = {$and:[{"code":arg_c}]}
  
  // > error:(InvalidPipelineOperator) Invalid $set :: caused by :: Unrecognized expression '$match’
  let matchand1 = {$match:{$and:[{"code":arg_c},{"name": arg_n},{"grade": arg_g}]}}
  let matchand2 = {$match:{$and:[{"code":arg_c},{"name": arg_n}]}}
  let matchand3 = {$match:{"code":arg_c}}
  
  let andmatch1 = {$and:[{$match:{"code":arg_c,"name": arg_n , "grade": arg_g}}]}
  let andmatch2 = {$and:[{$match:{"code":arg_c,"name": arg_n }}]}
  let andmatch3 = {$and:[{$match:{"code":arg_c}}]}
  
  let andmatchmatch1 = {$and:[{$match:{"code":arg_c}},{$match:{"name": arg_n}} , {$match:{"grade": arg_g}}]}
  let andmatchmatch2 = {$and:[{$match:{"code":arg_c}},{$match:{"name": arg_n }}]}
  let andmatchmatch3 = {$and:[{$match:{"code":arg_c}}]}
  
  // Erreur d'expression relevée par Realm
  //let eq1 = { {$eq:["code",arg_c]},{$eq:["name", arg_n]},{$eq:["grade", arg_g]}}
  //let eq2 = { {$eq:["code",arg_c]},{$eq:["name", arg_n]}}
  //let eq3 = { {$eq:["code",arg_c]}}
  
  let andeq1 = { $and:[{$eq:["code",arg_c]},{$eq:["name", arg_n]},{$eq:["grade", arg_g]}]}
  let andeq2 = { $and:[{$eq:["code",arg_c]},{$eq:["name", arg_n]}]}
  let andeq3 = { $and:[{$eq:["code",arg_c]}]}
  
  // Chosen case
  req1 = andmatchmatch1
  req2 = andmatchmatch2
  req3 = andmatchmatch3

Depending on the request, I have summariezd the resultas as follow in a tab:

I manually get rid of the _id attributes for readibility.
I hope it’s still readable enough

So basicaly, for all the used cases tested, either there are no match, either the value I have got for the new rm attribute is the wrong and the same for each document.

Is someone know a solution, or know why it doesn’t work.

I hope my issue is clear enough.

Damien

[Tarun_Gaur]

Hello @Damien ,

Welcome to The MongoDB Community Forums!

The reason you are getting below error while using $match in $switch is that, the case field of $switch is expected to be any valid expression that resolves to a boolean . If the result is not a boolean , it is coerced to a boolean value. $match is not an expression (provides document as result and not a boolean value) but an aggregation stage.

Invalid $set :: caused by :: Unrecognized expression ‘$match’

I would recommend you to go through Expressions in MongoDB to learn more about what are valid expressions and how one can use them to their advantage.

Regards,
Tarun

[Damien (Damien Frechou)]

Thank you for your answer @Tarun_Gaur . I didn’t really think about if the $match was doing a boolean response or something else. I think i will try again to do my aggregation by another way with that in mind.