MongoDb aggregate: Select all group by x - mongodb

Question

I am trying to convert the following SQL-like statement into a mongo-query, using the new aggregation framework.
SELECT * FROM ...
GROUP BY class
So far I have managed to write the following, which works well - yet only a single field is selected / returned.
db.studentMarks.aggregate(
{
$project: {
class : 1 // Inclusion mode
}
},
{
$group: {
_id: "$class"
}
}
);
I have also attempted play with the $project pipelines exclusion mode, by adding a field name which never exists, in order to trick MongoDb to return all the fields. While the syntax is correct, no results are returned. E.g:
db.studentMarks.aggregate(
{
$project: {
noneExistingField : 0 // Exclusion mode...
// Attempt to trick mongo into returning all fields
// sadly this fails - empty array is returned.
}
},
{
$group: {
_id: "$class"
}
}
);
The reason why I need all fields to be returned, is that I will not know (in the future) what fields are going to be present or not. E.g. a "student" might have field x, y, z or not. Thus, I need to "select all", group the results by a single field and return it, for a generic purpose - doesn't have to be for "student marks", can be for any type of dataset, without knowing all the fields.
For the mentioned reason, I cannot simply project all the fields which must be returned - once again - because I will not know all the fields.
I hope that someone knows a good way of solving my problem, using the new aggregation framework.

Answer 1

Simply don't use the project operator in the pipeline.
db.studentMarks.aggregate({
$group: { _id: "$class" }
});

Answer 2

You can try group by all the fields push each to the array.
Try below.
Option1
`db.StudentMarks.aggregate(
{
$project: {
class : 1 // Inclusion mode
}
},
{ $group : {
_id : "$class"
,field2: {$push: "$field2"}
,field3: {$push: "$field3"}
,field4: {$push: "$field4"}
}}, $sort{field2:-1})`
I have not tried above, but below works for me.
Option2
`db.StudentMarks.aggregate(
{
{ $group : {
_id : "$class"
,field2: {$push: "$field2"}
,field3: {$push: "$field3"}
,field4: {$push: "$field4"}
}}, $sort{field2:-1})`
Result
`{
"result" : [
{
"_id" : 59,
"field2" : [
59,
59,
59
],
"field3" : [
ObjectId("51e7072086eretetterr0001"),
ObjectId("51e7076786cb343453535354"),
ObjectId("51e716598435353534345335")
],
"field4" : [
11111,
11111,
11111
]
},
{
"_id" : 58,
"field2" : [
58,
58,
58,
58,
58
],
"field3" : [
ObjectId("51e469ad3843534535353535"),
ObjectId("51e7178f86cb349845667775"),
ObjectId("51e71df286cb349842456gtg"),
ObjectId("51e71ea686cb345646466466"),
ObjectId("51e71eac86cb346546464646")
],
"field4" : [
11111,
11111,
11111,
11111,
11111
]
}
],
"ok" : 1
}`