How to know that aggregated group has previous/next values? - mongodb

Question

Suppose I have the following aggregation pipeline:
db.getCollection('posts').aggregate([
{ $match: { _id: { $gt: "some id" }, tag: 'some tag' } },
{ $limit: 5 },
{ $group: { _id: null, hasNextPage: {??}, hasPreviousPage: {??} } }
])
As a result $match and $limit stages would result in a subset of all the posts with a tag some tag. How can I know that there're posts before and after my subSet?
One of the possible ways, I guess, is to have expression (with $let) inside hasPreviousPage and hasNextPage that would search for one post with _id less than "some id" and greater than $last: "$_id"respectively. But I'm not sure how I can reference my group as a variable in $let. Also, maybe there're some other more effective ways.

Answer 1

You can use below aggregation:
db.posts.aggregate([
{ $match: { tag: 'some tag' } },
{ $sort: { _id: 1 } },
{
$facet: {
data: [
{ $match: { _id: { $gt: 'some id' } } },
{ $limit: 5 }
],
hasPreviousPage: [
{ $match: { _id: { $lte: 'some id' } } },
{ $count: "totalPrev" }
],
hasNextPage: [
{ $match: { _id: { $gt: 'some id' } } },
{ $skip: 5 },
{ $limit: 1 }, // just to check if there's any element
{ $count: "totalNext" }
]
}
},
{
$unwind: { path: "$hasPreviousPage", preserveNullAndEmptyArrays: true }
},
{
$unwind: { path: "$hasNextPage", preserveNullAndEmptyArrays: true }
},
{
$project: {
data: 1,
hasPreviousPage: { $gt: [ "$hasPreviousPage.totalPrev", 0 ] },
hasNextPage: { $gt: [ "$hasNextPage.totalNext", 0 ] }
}
}
])
To apply any paging you have to $sort your collection to get results in deterministic order. On a set that's sorted and filtered by tag you can run $facet which allows you to apply multiple subaggregations. Pipelines that are representing previous and nextPage can be ended with $count. Every subaggregation in $facet will return an array so we can run $unwind to get nested document instead of array for hasPreviousPage and hasNextPage. Option preserveNullAndEmptyArrays is required here cause otherwise MongoDB will remove whole document from aggregation pipeline if there are no prev / next documents. In the last step we can just convert subaggregations to boolean values.