Project field in embedded document within embedded array

Question

I have the following query:

 cursor=self.postCol.aggregate([
      { \"$graphLookup\" : {
        \"from\": \"pCol\",
        \"startWith\": \"$parent\",
                


        

Answer 1

You can replace the else condition with below using $let operator.

 {
    $let: {
        vars: {
            obj: {
                "$arrayElemAt": ["$parents", 0]
            }
        },
        in: "$$obj.pValue"
    }
 }

Answer 2

You can use the $switch operator and the $let operator to $project your field.

The $let variable operator let you assign a value to a variable which can be used in the "in" expression as shown in my other answer here.

With the $switch operator, we can perform very clean case-statement.

db.postCol.aggregate([
    { "$graphLookup": { 
        "from": "postCol",  
        "startWith": "$parent",  
        "connectFromField": "parent", 
        "connectToField": "_id",   
        "as" : "parents"           
    }}, 
    { "$project": { 
        "pValue": { 
            "$switch": { 
                "branches": [ 
                    { "case": { "$gt": [ { "$size": "$pValue"}, 0 ] },
                    "then": "$pValue" }
                ], 
                "default": { 
                    "$let": { 
                        "vars": { "p": { "$arrayElemAt": [ "$parents", 0 ] }}, 
                        "in": "$$p.pValue" 
                    }
                }
            }
        }
    }}
])