Display all data in a row using mongodb - mongodb

Question

I have a collection myCollection,
ID DateTime myID Cost Flag
1 '2016-07-01T00:00:00' 2048 1 'O'
2 '2016-07-02T00:00:00' 2049 2 'O'
if I write sql query for it
"select DateTime, myID,Flag, min(Cost) from myCollection group by DateTime, myID"
it will display all the fields in data e.g
DateTime, myID,Flag, min(Cost)
but in mongo aggregation framework I can group like
db.myCollection.aggregate([
{
$group: {
_id: {
DateTime: '$DateTime',
myID: '$myID'
},
minCost: { $min: '$Cost' }
}
}
])
which will return me
DateTime, myID, min(Cost)
but I need "Flag" field also in single query. I tried out $Push but it works only for an array.

Answer 1

In SQL Server, the query
select DateTime, myID,Flag, min(Cost) from myCollection group by DateTime, myID
is invalid in the select list because the column Flag is not contained in either an aggregate function or the GROUP BY clause.
In MongoDB, to be able to include the field Flag in the aggregate query, you must apply an accumulator operator on the field and in this case you may either use the $first or $last accumulator operators to return the field value within the aggregation.
db.myCollection.aggregate([
{
"$group": {
"_id": {
"DateTime": '$DateTime',
"myID": '$myID'
},
"minCost": { "$min": '$Cost' },
"Flag": { "$first": '$Flag' }
}
}
])
in the above, the $first accumulator operator applied on the Flag field will return a Flag value from the first document for each group. Order is only defined if the documents are in a defined order.