How to put First Document of a collection inside $cond for custom pagination in nestJs and mongodb - mongodb

Question

I'm Implementing Keyset pagination in a nestjs and mongodb project. And I'm trying to customize the original pagination solution. I want to make a condition if the startId document is the first document present in the collection.
This is the Code I'm trying. AND THE ISSUE WITH THIS CODE IS THAT IT RETURNS ALL THE DOCUMENTS WHATEVER THE ID YOU GIVE IN THE QUERY. I KNOW MY LOGIC COULD BE WRONG BUT ONE THING ABOUT I'M SURE AND THAT IS I'M WRITING THE SYNTAX, IS WRONG, AS THIS IS MY FIRST TIME EXPERIECE WITH MONGO QUERIES
async findAll( page?: number, documentsToSkip = 0, limitOfDocuments?: number,
startId?: string,
): Promise<userDocument[]> {
return await this.userModel
.find({
$cond: {
if: { $first: { _id: startId } },
then: { $gte: startId },
else: { $gt: startId },
},
})
.skip(documentsToSkip)
.limit(limitOfDocuments);
}
Explanation of the above code. For example I'm dealing with Users
1- If the startId(passed in query) is equal to the _id of the first document present in the user collection then the below then should be executed
then: { $gte: startId },
2- If the startId(passed in query) is not equal to the _id of the first document present in the user collection then the below else should be executed. Lets's say the pagination limit is set to 10 documents per page. And If I'm providing the id of the 11th document then else should be executed
else: { $gt: startId },
REASON FOR All THIS
The keyset solution present on the internet uses this
_id: { $gt: startId } and with this the document of the startedId is automatically skipped. SO I'M TRYING TO DO IF THE PAGINATION STARTS FROM THE FIRST DOCUMENT THEN THE FIRST DOCUMENT ITSELF SHOULD BE PRESENT AND VISIBLE. BUT THEN IF USER MOVES TO THE SECOND PAGE OF THE PAGINATION THE LAST DOCUMENT OF THE FIRST PAGE SHOULD NOT BE VISIBLE AT THE SECOND PAGE, AS THE LAST DOCUMENT ID BECOMES THE STARTING ID FOR THE SECOND PAGE DOCUMENTS

Answer 1

I've made the solution for my particular scenario. The issue was I hade to customize original pagination according to the first document present in the collection. But $first wasn't working for me. AS $first returns the first element of an Array and not the first document itself.
SOLUTION
1- To find the first document use findOne() method without any parameter. And it will simply return the first document of the collection.
let firstDocument: userDocument = await this.userModel.findOne();
2-For simple customization of the Pagination
if (startId == firstDocument._id) {
return await this.userModel
.find({
_id: { $gte: startId },
})
.skip(documentsToSkip)
.limit(limitOfDocuments);
} else {
return await this.userModel
.find({
_id: { $gt: startId },
})
.skip(documentsToSkip)
.limit(limitOfDocuments);
}
3- I had to make other changes as well so my original function now look different In case If someone want to know
async findAll(
page?: number,
limitOfDocuments?: number,
startId?: string,
): Promise<PaginatedUsersDto> {
let firstDocument: userDocument = await this.userModel.findOne();
let count = await this.userModel.count();
let totalPages = (count / limitOfDocuments).toFixed();
console.log('First Doucment in User Colelction', firstDocument);
if (startId == firstDocument._id) {
this.paginatedUsers = await this.userModel
.find({
_id: { $gte: startId },
})
.sort({ _id: 1 })
.skip((page - 1) * limitOfDocuments)
.limit(limitOfDocuments);
} else {
this.paginatedUsers = await this.userModel
.find({
_id: { $gt: startId },
})
.skip((page - 1) * limitOfDocuments)
.limit(limitOfDocuments);
}
let paginatedObj = {
paginatedUsers: this.paginatedUsers,
totalPages: totalPages,
count: count,
};
return paginatedObj;
}