Mongodb Aggregation Rows to Columns - mongodb

Question

I have the following dataset. I need to group them by Account, and then turn the Element_Fieldname into a column.
var collection = [
{
Account:12345,
Element_Fieldname:"cars",
Element_Value:true
},
{
Account:12345,
Element_Fieldname:"boats",
Element_Value:false
}
]
This was my attempt to convert rows to columns, but its not working.
db.getCollection('my_collection').aggregate([{
$match : {
Element_Fieldname : {
$in : ["cars", "boats"]
}
}
}, {
$group : {
_id : "$Account",
values : {
$addToSet : {
field : "$Element_Fieldname",
value : "$Element_Value"
}
}
}
}, {
$project : {
Account : "$_id",
cars : {
"$cond" : [{
$eq : ["$Element_Fieldname", "cars"]
}, "$Element_Value", null]
},
boats : {
"$cond" : [{
$eq : ["$Element_Fieldname", "day_before_water_bottles"]
}, "$Element_Value", null]
},
}
}
])
This just gives me null in my cars and boats fields. Any help would be great.
And this is my desired results:
var desiredResult = [
{
Account:12345,
cars:true,
boats:false
}
]

Answer 1

this is a big tricky but you will get what you need :-)
please add $match on the top of aggregation pipeline
db.collection.aggregate([{
$project : {
_id : 0,
"Account" : 1,
car : {
$cond : [{
$eq : ["$Element_Fieldname", "cars"]
}, "$Element_Value", null]
},
boats : {
$cond : [{
$eq : ["$Element_Fieldname", "boats"]
}, "$Element_Value", null]
},
}
},
{
$group : {
_id : "$Account",
carData : {
$addToSet : "$car"
},
boatsData : {
$addToSet : "$boats"
}
}
}, {
$unwind : "$carData"
}, {
$match : {
carData : {
$ne : null
}
}
}, {
$unwind : "$boatsData"
}, {
$match : {
boatsData : {
$ne : null
}
}
},
])
and result
{
"_id" : 12345,
"carData" : true,
"boatsData" : false
}

Answer 2

It is not possible to do the type of computation you are describing with the aggregation framework, however there is a proposed $arrayToObject expression which will give you the functionality to peek into the key names, and create new key/values dynamically.
For example, you could do
db.collection.aggregate([
{
"$match": { "Element_Fieldname":{ "$in": ["cars", "boats"] } }
},
{
"$group": {
"_id": "$Account",
"attrs": {
"$push": {
"key": "$Element_Fieldname",
"val": "$Element_Value"
}
}
}
},
{
"$project": {
"Account": "$_id",
"_id": 0,
"newAttrs": {
"$arrayToObject": {
"$map": {
"input": "$attrs",
"as": "el",
in: ["$$el.key", "$$el.val"]
}
}
}
}
},
{
"$project": {
"Account": 1,
"cars": "$newAttrs.cars",
"boats": "$newAttrs.boats"
}
}
])
Vote for this jira ticket https://jira.mongodb.org/browse/SERVER-23310 to get this feature.
As a workaround, mapreduce seems like the available option. Consider running the following map-reduce operation:
db.collection.mapReduce(
function() {
var obj = {};
obj[this.Element_Fieldname] = this.Element_Value;
emit(this.Account, obj);
},
function(key, values) {
var obj = {};
values.forEach(function(value) {
Object.keys(value).forEach(function(key) {
obj[key] = value[key];
});
});
return obj;
},
{ "out": { "inline": 1 } }
)
Result:
{
"_id" : 12345,
"value" : {
"cars" : true,
"boats" : false
}
}