Mongodb 3.2 and 3.0 $unwind aggregation - mongodb

Question

I have created a query and check it in robomongo and it's working fine for me in mongodb 3.2
db.post.aggregate([
{$unwind: {path: "$page_groups", preserveNullAndEmptyArrays: true}},
{$group: {_id: "$page_groups",
page_names: {$addToSet: "$page_name"}}},
])
But unfortunantly I need to get same data in mongodb 3.0
Can anyone tell me how to get data with empty array in mongo 3.0 and get results by array key?
Without $unwind I get objects where pages have two or more groups and I don't need it.

Answer 1

Thank you for answere, I wanted to use $project at first, but I think I have found easier way using $match and array $size to ignore results where array gets more than one element:
db.post_summary.aggregate([
{$match: {$or:
[{page_groups: {$size: 1}}, {page_groups: {$size: 0}}]}},
{$group: {
_id: "$page_groups",
page_names: { "$addToSet": "$page_name" }
}},
])
In my case "page_groups" have this structure:
page_groups:[
0 =>[_id, group_name]
1 =>[_id, group_name]
]

Answer 2

To mimick the preserveNullAndEmptyArrays $unwind option in 3.2 for 3.0 aggregation pipeline operations, generate an initial $project pipeline stage that creates the array field if it's null or empty (using the $ifNull operator):
var pipeline = [
{
"$project": {
"pg": {
"$ifNull": [
"$page_groups",
["Unspecified"]
]
},
"page_name": 1
}
},
{ "$unwind": "$page_groups" },
{
"$group": {
"_id": "$page_groups",
"page_names": { "$addToSet": "$page_name" }
}
}
];
db.collection.aggregate(pipeline);