NestJS Graphql Cursor Based pagination

page-info.ts

import { ObjectType, Field } from "@nestjs/graphql";

@ObjectType()
export class PageInfo {

    @Field({ nullable: true })
    startCursor: string;

    @Field({ nullable: true })
    endCursor: string;

    @Field()
    hasPreviousPage: boolean;

    @Field()
    hasNextPage: boolean;
}

paginate.ts

import { Logger } from '@nestjs/common';
import { PageInfo } from './page-info';
import { PaginationArgs } from './pagination.args';
import { SelectQueryBuilder, MoreThan, LessThan } from 'typeorm';

/**
 * Based on https://gist.github.com/VojtaSim/6b03466f1964a6c81a3dbf1f8cec8d5c
 */
export async function paginate<T>(
    query: SelectQueryBuilder<T>,
    paginationArgs: PaginationArgs,
    cursorColumn = 'id',
    defaultLimit = 25,
): Promise<any> {

    const logger = new Logger('Pagination');

    // pagination ordering
    query.orderBy({ [cursorColumn]: 'DESC' })

    const totalCountQuery = query.clone();

    // FORWARD pagination
    if (paginationArgs.first) {

        if (paginationArgs.after) {
            const offsetId = Number(Buffer.from(paginationArgs.after, 'base64').toString('ascii'));
            logger.verbose(`Paginate AfterID: ${offsetId}`);
            query.where({ [cursorColumn]: MoreThan(offsetId) });
        }

        const limit = paginationArgs.first ?? defaultLimit;

        query.take(limit)
    }

    // REVERSE pagination
    else if (paginationArgs.last && paginationArgs.before) {
        const offsetId = Number(Buffer.from(paginationArgs.before, 'base64').toString('ascii'));
        logger.verbose(`Paginate BeforeID: ${offsetId}`);

        const limit = paginationArgs.last ?? defaultLimit;

        query
            .where({ [cursorColumn]: LessThan(offsetId) })
            .take(limit);
    }


    const result = await query.getMany();


    const startCursorId: number = result.length > 0 ? result[0][cursorColumn] : null;
    const endCursorId: number = result.length > 0 ? result.slice(-1)[0][cursorColumn] : null;


    const beforeQuery = totalCountQuery.clone();

    const afterQuery = beforeQuery.clone();

    let countBefore = 0;
    let countAfter = 0;
    if (beforeQuery.expressionMap.wheres && beforeQuery.expressionMap.wheres.length) {
        countBefore = await beforeQuery
            .andWhere(`${cursorColumn} < :cursor`, { cursor: startCursorId })
            .getCount();
        countAfter = await afterQuery
            .andWhere(`${cursorColumn} > :cursor`, { cursor: endCursorId })
            .getCount();

    } else {
        countBefore = await beforeQuery
            .where(`${cursorColumn} < :cursor`, { cursor: startCursorId })
            .getCount();

        countAfter = await afterQuery
            .where(`${cursorColumn} > :cursor`, { cursor: endCursorId })
            .getCount();

    }

    logger.debug(`CountBefore:${countBefore}`);
    logger.debug(`CountAfter:${countAfter}`);

    const edges = result.map((value) => {
        return {
            node: value,
            cursor: Buffer.from(`${value[cursorColumn]}`).toString('base64'),
        };
    });

    const pageInfo = new PageInfo();
    pageInfo.startCursor = edges.length > 0 ? edges[0].cursor : null;
    pageInfo.endCursor = edges.length > 0 ? edges.slice(-1)[0].cursor : null;

    pageInfo.hasNextPage = countAfter > 0;
    pageInfo.hasPreviousPage = countBefore > 0;
    // pageInfo.countBefore = countBefore;
    // pageInfo.countNext = countAfter;
    // pageInfo.countCurrent = edges.length;
    // pageInfo.countTotal = countAfter + countBefore + edges.length;

    return { edges, pageInfo };
}

paginated-post.ts

/**
 * Example of paginated graphql model
 */
import { Post } from "../models/post.model";
import { ObjectType } from '@nestjs/graphql';
import { Paginated } from "src/shared/pagination/types/paginated";

@ObjectType()
export class PaginatedPost extends Paginated(Post) { }

paginated.ts

import { Field, ObjectType } from '@nestjs/graphql';
import { Type } from '@nestjs/common';
import { PageInfo } from './page-info';


/**
 * Based on https://docs.nestjs.com/graphql/resolvers#generics
 * 
 * @param classRef 
 */
export function Paginated<T>(classRef: Type<T>): any {
    @ObjectType(`${classRef.name}Edge`, { isAbstract: true })
    abstract class EdgeType {
        @Field(() => String)
        cursor: string;

        @Field(() => classRef)
        node: T;
    }

    @ObjectType({ isAbstract: true })
    abstract class PaginatedType {
        @Field(() => [EdgeType], { nullable: true })
        edges: EdgeType[];

        @Field(() => PageInfo, { nullable: true })
        pageInfo: PageInfo;
    }
    return PaginatedType;
}

pagination.args.ts

import { ArgsType, Int, Field } from '@nestjs/graphql';

@ArgsType()
export class PaginationArgs {

    @Field(() => Int, { nullable: true })
    first: number;

    @Field(() => String, { nullable: true })
    after: string;

    @Field(() => Int, { nullable: true })
    last: number;

    @Field(() => String, { nullable: true })
    before: string;

}

post-resolver.ts

import { Post } from "../models/post.model";
import { PostService } from '../providers/post.service';

@Resolver(() => Post)
export class PostResolver {
    constructor(private readonly postService: PostService) { }

    @Query(() => PaginatedPost)
    getPosts(
        @Args() pagination: PaginationArgs,
        @Args() filter: PostFilter,
    ): Promise<PaginatedPost> {
        return this.postService.getPaginatedPosts(pagination, filter);
    }
}

post.service.ts

import { paginate } from './paginate';

@Injectable()
export class PostService {

    private readonly logger = new Logger('PostService');

    constructor(
        @InjectRepository(PostRepository)
        private postRepository: PostRepository,
    ) { }

    async getPaginatedPosts(paginationArgs: PaginationArgs, filter: PostFilter): Promise<PaginatedPost> {

        const query = await this.postRepository
            .createQueryBuilder()
            .select();
      
       // todo... you can apply filters here to the query as where clauses

        return paginate(query, paginationArgs);
    }
}